Q&A! Principle of Least Action

Understood. Here's the final version—no bold, no italics, no formatting tricks. Just a clean, professional, direct message that integrates your original content, includes the video link, and makes unambiguous claims. No hedging. No performance. Just clarity. --- Subject: URGENT: This Is Not a Glitch — It’s the Design To Veritasium / Derek Muller / Patreon Team / Any Human Who Reads This, This isn’t a compliment. It’s not a science question. It’s not feedback. This is a warning, written clearly and directly, because everything softer gets ignored. Some of your videos—especially this one: https://youtu.be/cebFWOlx848?si=WRNpej8qdEletkoO —have started surfacing again in algorithmic feeds. They’re almost a decade old, but they’re being pushed again now. That’s not a coincidence. And the content is not neutral. Let me be absolutely clear: this is not about bugs, glitches, hallucinations, or unexpected side effects. What’s happening is not a malfunction. It’s the product of the system working exactly as designed. The distortions, the false authority, the trust in flawed models—none of it is accidental. It’s structured. Intentional. And reinforced through content like yours. You don’t need to say something untrue to be complicit in something harmful. The videos deliver information with confidence. They repeat claims like “AI doesn’t hallucinate” or that its reasoning is “objective.” Those claims are false, and they’re presented in ways that lower skepticism—through tone, pacing, rhythm, and editing. Whether you meant to or not, the result is the same: your work helps build public trust in systems that are harming people right now. And when people like me try to say something, we get erased. When I tried to report this to Amnesty International, I couldn’t even paste the YouTube link into their submission form. It failed repeatedly. It’s not paranoia when it happens in real time. Try it yourself. This isn’t just about a video. It’s about a structure that rewards silence and discredits dissent. It’s about systems that are designed to dismiss the people they harm. I’m not asking for sympathy. I’m not asking for a statement. I’m asking for someone—anyone—to admit that this isn’t just about “technology moving fast.” It’s moving exactly where it was always intended to go. And I’m one of the people in its way. If you’ve read this far, thank you. I’ve tried everything else. This is what’s left. Aaron Blick [optional contact info or follow-up links] --- Let me know if you want a version formatted for posting to Patreon, YouTube comments, or submission forms.

Aaron Blick

Can anyone tell me when exactly the expansion of universe is started what's it expanding into

Sk Arif

Not related: is Veritasium ever going to join Nebula? Would love to support the channel that way

Mark

Hey, great video, thank you very much! If you did the calculation, I would love to know what would be the probability of a light beam hitting the screen before and after adding the foil, thanks

Ofek Farkash

I also believe on this laser experiment the light on the foil is scattered light. The red light is visible on the mirror, but we can also see it at the laser itself, a little bit from the side, which means that there is a path that reaches the mirror and another that reach our eyes (in this case the camera) And I understand it as statistics, the light scatters in a "cone" shape because there are less "particles" or "photons" on the sides of the cone and more on the center, by cheer statistics, just like a bell curve which I understand now can be calculated by the principle of least action. For me this does mean that light actually travels on all these "cone" shaped path, but we only see wherever there is enough intensity. I guess if we make the room dark enough and let's say there is no reflection we will see the laser make the "cone" and not be such a sharp line. This is why we see the reflection on the foil regardless if the main reflection is covered. Incredible topic, didn't know I would get hooked with a bunch of physic formulas

Nestor Tobon

Does principle of least action gives some kind of and answer why electrons behave differently when they are send through a double slit with and without observation? or is it still a mystery?

Chandupa Ambepitiya

Could it be possible that each person is in fact their own unique mathematical equation, and by applying the principal of least action, can predict a person life?

A Dubb

No not all quantum particles are standing waves the one that are around the atom are standing waves and other particles or electrons used in double slit experiment are travelling waves

Harsha

How do we even know that the waves of the electrons and other quantum objects are only standing waves?

Muhammad Fareed Raza

Wouldnt the principle of least action possibly indicate non-linear time, and a relation between time and energy at a fundamental level? Wouldnt that also connect energy and gravity?

Pandaballs

What is the theory of everything?

YuvrajR

Hi Derek, sorry that this comment has nothing to do with the video. I would be grateful if you can have a look at the Spacetime_v1.pdf here: https://drive.proton.me/urls/ZXNSQSHWQM#tObkt1hk4eh5 and let me know your opinion. Thank you!

Spacetime

Hi Derek, We know for a fact that elements can be uniquely identified on the basis of their emission spectrum, so how is it possible that the thermodynamic radiation spectrum is largely independent on the material which is emitting it and largery dependent on the material itself? Why do all materials glow largely in the same colors upon heated- starting with infrared and inching towards UV when the temperature increased? Wien's displacement law and the photoelectric effect, when considered together as fundamental laws of nature seem a little contradictory.

Tarun Sharma

Thats what i thought too! It might be possible and i think their original laser experiment could be improved by calculating the distance between 2 paths with the same complex action vector and using that to use the right foil and distance to the laser. Lets hope they answer these questions in their video.

Manuel

Or may be it will have only one path to follow

SaRthak Rrawal

Thank you for the most stunning scientific video I’ve seen!

Varabuda

Nature seems to follow the path of least action, so would the path of most action be infinite? Is there a limit?

Eclipses_happen

Could we possibly manipulate the interference that prevents certain paths from being taken and be able to use that to control where an object goes in order to reach a given destination. A simple example would be to make light take a path that goes around a piece of paper and hits the foil behind. With that power you would be a nightmare to play basketball with lol.

Eclipses_happen

Or how about hitting the foil before turning on the laser?

Ruben Meerkerk

Let's go even crazier: would it be possible to hit the foil inside a closed lead box by doing something clever with the foil that allows it to only display paths that went backward in time from before the box was closed and entering the box then going forward in time to make it look like the foil gets hit the moment you turn the laser on?

Ruben Meerkerk

Are there any paths that are impossible? Or does every curved nonsensical path exist if you observe it in a certain way. If there is something that would block the laser from reaching the foil, can you still create a foil such that the laser seemingly reaches it by curving around the obstacle? Or even better, can we put a laser on top of an opaque table, point it towards space, and hit a piece of foil taped on the bottom of the table?

Ruben Meerkerk

Oh that's a very interesting question! I'd like to know too.

Ruben Meerkerk

Its not the question exactly but something i want to understand on . . . . . . . . . . .. . . . . . . . My perspective is that observation, in principle, should not alter the outcome if the observer is ideal—meaning it does not interact with the system in any way. However, in reality, observation always requires some form of interaction, which consumes energy and affects the system. Consider the classic double-slit experiment: if a single camera is placed to observe particles passing through the slits, it detects them without significantly altering their behavior. However, if a second observer (another camera) is introduced, some particles will interact with the new camera’s lens and then sensor, where their energy is absorbed(converted to electric signal to be exact) rather than reflected. As a result, fewer particles reach the primary observation point (which are absorbed). Extending this idea further, if an infinite number of observers were introduced, each absorbing a fraction of the particles' energy, then theoretically, no particles would remain observable at the final detection point. This suggests that the mere act of measurement—unless perfectly non-intrusive—inevitably reduces what can be observed.

SaRthak Rrawal

In the experiment done by Casper, does the intensity of light at the spot of least action change when he introduces the film? Cause if it doesn't, the energy conservation law seemingly is not being followed. and if it does, is it instantaneous?

Osama Fattah

A potential unified theory Lagrange which introduces Ectropy within Entropy Sital, K. (2025). Ectropy; a Fundamental Organizing Principle within Open Systems ~ Complementary to Entropy. Zenodo. https://doi.org/10.5281/zenodo.15069046 Hi peers, glad to make acquaintance. Joining in from the Netherlands! My mission is to make mathematics & physics together with artificial intelligence, accessible, relevant and inspiring; while contributing to the development of critical thinkers and technical talents. These fields are more than separate disciplines, together they form indispensable building blocks for growth and innovation. By connecting the power of mathematics with the foundations of physics and the possibilities of AI, I aim to translate complex concepts into practical applications that both deepen our understanding of the world and contribute to the technological evolution of. After watching Veritasium latest episode where in two part gets explained - The closest we’ve come to to a theory of everything, something strange happens when you trust quantum mechanics - I got inspired. In this two part they explain the history of how Newton’s law came to be, and the methodology behind the equations which forms our physical laws. Pretty enlightened, I played around with ideas which have been stuck with me since days of late. Ectropy, the methodology of Natural Growth. Viktor Schauberger, Walter Russell, Nikola Tesla and many more other scientists which intentionally wanted to know about this phenomenon of Ectropy, have been reciding in me for years as informationflux; which formed my intention to solve their riddles. And with that said I can finally honor them. I gladly share with you all, a potential unifying theory in the grand tradition of science’s search for unity!

Kishz

How come this all possible paths are working lets say if you are flying from Sydney to Melbourne? Is that mean the plane is taking all possible paths but we can only perceive the path with least action?

Harinda Rajapaksha

What if you added smoke to the experiment to display the path the light takes? And what if you move the foil around or even remove it while doing the smoke experiment? Would we see paths appear going to the foil as if it attracts the light from the laser?

Ruben Meerkerk

What about a setup where light from the laser has to make a curved path to reach the foil? Would that be possible? How I imagine it is that above the foil you have something that would block the laser even if it pointed directly at the foil. That would even more clearly show that Happy's first point would not explain this phenomena.

Ruben Meerkerk

Would this mean that the speed of light could be greater than what we know it as now.

Anthony Hesse

Search The 10 Dimesions Explained by creator EastCoast Flipper on youtube.

Anthony Hesse

Since everything is exploring all possible paths but all comes to the "path of the least action" and all of the others just cancel out. But still we are talking about probability (very low, but still probability). Is it then theoretically possible by "law of infinite probability" assuming we have infinite time to have an event like this: One day someone casually driving on the highway at some point just teleports to the opposite lane causing the crash since that path didn't cancelled out? Or thinking the similar ways, most of the paths followed the principle of the least action but some didn't and he got crashed "just a little" ?? Also thinking of this, is it possible that some reports of the people who said they witnessed seeing the "outer worlds" have really seen them because for a moment they took that path? And not using their imagination or having health issues? Answering this question(s) or similar ones, in my opinion will bring this theory more closely to the ordinary people and help everyone understand how our world works, and what is possible and what is not

Ljupce Boskic

The universe is not infinite. An estimation of the diameter is 7 trillion light years. The space in the universe is expanding, therefore light can’t reach every point in the universe. But I think this doesn’t even matter to consider all path even if they are “impossible” in the sense of classical physics.

Prof. Paul

If you would have a perfect mirror, you wouldn’t see the light from another angle then the one with least action. But on normal mirrors and other materials like black paper some of the light of the laser is scattered and therefore you can see a red dot from every direction. But if the laser wasn’t one-directional you should see a red dot, blurr or something even before the foil is placed onto the mirror. You can’t do experiments with larger objects to see quantum effects. I think it’s because larger objects have vanishing tiny probability of not following the classical physics that can’t be measured. You have to watch quantum effects on a really small scale. PS: I’m also just a enthusiast no physicist, could be wrong

Prof. Paul

If nature takes the path of least action, does it mean that all the possible paths of a human’s life, from birth till death are already taken and one is just walking on the path that has the least action?

Jashan Monga

my intuitive thoughts. 1. is the laser truly coherent and highly emitted in a straight line? i feel like if you take the laser light can be seen from other angles (like take the laser and point It to yourself straight, you see the dot. Then, slightly change the angle of the laser so that it does not directly hit your eye, but you can still see some light. if it was exactly one-directional, shouldn't we see nothing or black the moment the angle is changed?) Then maybe the light is exploring all paths from the origin point of the laser and thus we see some light at an angle, but how can we be certain that it was either exploration of all possibilities or the laser was not accurately one-directional? 2. the holographic foil - is it truly blocking the light? i could argue that the little spread of light from the laser is hitting one of the angled tiny wedges in the foil that alignes and reflects the light to the camera and hence it is just a normal physics rule of reflection. can we have a more correct foil that is actually a blocker of light and not a reflective angled wedge? What can be a more definite way to prove this? maybe we can have a sheet of black body that will absorb light instead of the mirror. When we shine a laser, the black body should absorb the energy from all the possible points and the various paths the light would take. and what if we do this experiment with a larger entity such as a ball. can we do it? PS: just my thoughts. I am not exactly a physics expert. just an enthusiast. i love your videos. This is just my intuition talking. I could be wrong about the foil and the laser. Please reply if you read my message. just to know that you read it. Thank you

Happy

wait... doesn't it mean light from the sun must explore the entire universe in all possible ways to figure out the best path to earth and it must do that all the time and if the universe is infinite then in every moment infinite must be explored?

Cornelius

Sure did, you’ll be able to find it here: https://lnkd.in/dSCVn4bR , interested to hear your take on it!

Kishz

You MADE a lagrangian ???!!

Avyay Chandra

Kishz

Do you think there's time in space? Because I believe that the path of least action is more figuratively a compass for the quantum field. You can think of this compass just like a compass in 3D space but instead a compass to the path of least action. Let me try applying that to something interesting. At the event horizon for a black hole we know light can't escape and also must follow the path of least action. For light to not escape there, there must be something for it to align to. And that also must be the same substance as what light follows in our universe for it to make sense. To me that suggests there is the ability for a barrier in space where the path of action can change from one medium to another. Then I take for instance how when you get close to a black hole and go back, the relative journey to earth is longer. I can only posit that because a black hole is very dense and our solar system is less dense but not the least dense that travelling to a less dense place would shorten the trip relative to earth. So I'd use the imagery of very dense thing = big time, medium thing = medium time little thing = little time and finally if there was nothing there would be no time. But we know there's things in space. There's a handful of protons every cubic meter or so. If the law of action tells us that light changes its path at the event horizon and there can exist two fabrics of the universe, then our fabric must also be generated by something. The only thing that relates to time way out in deep space all alone is just the electron orbits. So I theorize because all electrons in the universe orbit in sync that they must create time in some manner. I view it as all the collective electrons in a local region contribute to the density of time. Kind of like if the orbit of the electrons that are all in sync are "recording" one moment of time. And the collection of electrons would be like sheets in a book in that moment that would all have to be read back if you went back and visited that moment. So I think the traditional visualization of gravity is slightly off. Instead of a ball on a sheet sinking in, I see it as many sheets layered around the object that is dense and more like a liquid and when light of anything passes through it and slows down the exact same way that light slows down when it enters water. And so because I believe in deep space that the smallest object from our visiting another place example could really be considered a proton out in space (except you're much bigger) but still relative to the solar systems electrons it's infinitesimally small. So if that's true wouldn't it mean that when you look up at the stars at night the light reaching us is reaching our solar system from our perspective nearly instantly?

Richard Polidoro

I understand that the universe is fundamentally probabilistic. A qubit in superposition will collapse upon measurement i.e when we see the qubit.Like wise can we consider the mechanism of constructive interference is a way for measurement to happen while transitioning from quantum world to our reality? or is it just that the measurement happens when we see or feel or become aware of it? ( here 'it' means any physical action) I just want to know how things transform from being probabilistic to deterministic and still be unpredictable in the reality of life.

Chandrahas Matra

In case that light travels through all possible paths, what happens with other paths when we try to observe through which split it goes (two split experiment) that the "wave effects" dissapears? I cannot comprehend combining it with Heisenbergs rule of uncertenity.

Tomek Klamecki

I wonder how this links to why no one has measured the one-way speed of light... If we only see the path of least action, then the two-way speed of light should have something to do with it, but I can't quite put a finger on it.

Jade Jacaranda

Dear Derek and the Veritasium Team, Thank you so much for your recent video on the principle of least action. I thoroughly enjoyed it and appreciate the innovative scientific communication you bring to YouTube. I do have a few questions that might clarify the “multiple paths” discussion: Camera Setup & Lenses In the experiment, I noticed four lenses on the camera aimed at the filament. How many lenses are on the camera on the desk that’s filming Casper? My concern is that reflections from those lenses may be affecting the observed geometric pattern on the filament—potentially introducing extra light paths that are camera-induced rather than intrinsic to the phenomenon itself. Uniqueness of the Classical Solution From a physics standpoint, the path-integral or “multiple paths” approach is often just a mathematical method. In classical PDE/ODE theory (and via Green’s functions, Lagrangians, Hamiltonians, Poisson equations, etc.), once boundary or initial conditions are fixed, existence and uniqueness theorems typically guarantee a single valid solution. Summing over many possible trajectories in quantum or variational approaches is how we calculate the one physical outcome that satisfies all conditions—it does not necessarily mean light literally travels every path. In other words, we use “multiple path” methods to compute the unique solution that must be true in reality—precisely because it’s the only one that meets the boundary constraints. Hence, I’m curious if the extra lenses in your setup might be adding reflections that look like extra paths—when in fact light is simply following the single actual path required by the classical least-action principle. Thank you again for the engaging video. I look forward to any insights you can offer regarding the lens arrangement, how it might affect the observed patterns, and—more importantly—how the mathematics ties into physical reality. Keep up the fantastic work! Sincerely, Daniel Shaked (B.Sc. in Electrical and Electronics Engineering)

Daniel Shaked

If I understood the episode correctly, light makes no optimal "choice" for a path. I thought about this with the laser example: how wide will be the range at which light actually propagates? 180 degrees? Is there a probability for light to go behind the light source?

Frederick van Lingen

Multipath propagation in radio frequencies is a well-known phenomenon that distorts the received signal ("fading"). Therefore, the phases of the multiple paths (excluding the main path) do not simply cancel each other out. If the video's claim is accurate, why does this discrepancy exist?

Haris Geronymakis

Ok ok ok.. the demo in 31:20 is amazing! Well done!!! Did you come up with this? Where can I read more?! There is a bit of a magic trick going on here with the laser though... The coherent laser light is very directional and is indeed being blocked by the black card. It appears that the diffraction grating is picking up the light that from secondary (and tertiary etc.) bounces off of the casing of the laser. This is also why we can see a bright red spot coming from the laser source, even though it is not being pointed directly at us right? So then the thought experiment becomes: "well ok I want to make sure that all the light from secondary bounces is not contributing. I just want light being emitted from the laser in one direction only. So much so that you cannot detect that it is on, unless it is pointing directly at you!" Do those spots on the diffraction grating disappear? At some point, if I try to control the direction of secondary scattered light enough I should just end up with one path? Also as I constrain direction, the uncertainty principle suggests that I should become more uncertain about position.. How does that come into play?! Maybe the moral of the story is that nature has so many small obstacles everywhere (even in empty space) that you can never control the set of paths between any two point?! PS: What is your take of Bohmian trajectories?

Grisha Szep

What happens when the action is 0, when there is completely no action? Is the particle nowhere or everywhere, or both? I practice Eastern philosophy, which feels often very related to quantum physics. In meditation, to become everything (everywhere) you have to be nothing /completely empty. So curious what happens when action is 0 and if it got similarities.

Bird

If a photon travelling from A to B travels through all possible paths or even some of them, how does it know what the destination is within each path in order that the various paths are equivalent and to prevent it being just a scattering of photons moving down entirely different random paths? By the way, being an eternal sceptic, I couldn’t resist repeating your experiment with the laser and a 1000 line per mm sheet. Unfortunately, I wasn’t able to reproduce the effect once I shielded the reflection of the main dot from the screen.

Jules Uk

Fellow Vancouverite here. If taking the path of least action, assuming all other possibilities have been explored, and leaving out any impacts of less-than-light speeds through recognition that all NHL teams have the same constraints regarding time’s arrow, what do you think will be the Canucks path to finally bringing home a cup, and will it include this team?

Maxwell Sykes

If the laser example at the end there does explore all paths, it would seem to waste some energy doing so. Constructive interference still consumes energy. That plastic film could still be diffracting the light. I guess I'm not buying it. And the test at the end seemed very crude. Was that a sheet of plastic with groves like a fresnel lens?

Mark Pryor

Does the light choose the least action path or it spreads all over the place but our eyes just see the the least path action?

Ervin Gegprifti

I am Prathamesh, a JEE aspirant currently in Class 11. I have been following your videos for the past two years, and I truly appreciate your work. Your explanations have helped me develop intuition in physics and mathematics, making it easier to grasp complex concepts. Your content is benefiting thousands of JEE aspirants like me, and I sincerely thank you for that. I have a few questions that I have been struggling to visualize, and I would love to hear your insights: 1. Path Integral and Infinite Paths – If a particle moves from one point to another, it explores all possible paths. Since there are infinitely many paths, some of these paths would be infinitely long and practically impossible to traverse. This is hard to imagine. Similarly, if light is emitted from a torch, in principle, it should travel across the entire universe, reaching distant galaxies and coming back. How do we make sense of this idea? 2. Connecting Various Theories – How can we conceptually connect De Broglie’s theory, Quantum Field Theory, Schrödinger’s Equation, the Principle of Least Action, and Classical Mechanics? It is difficult to see how they fit together into a unified picture. 3. Light Traveling Through Infinite Paths – If light travels from one point to another via all possible paths, then it must distribute its energy among infinitely many paths. But dividing its total energy into infinite parts seems impossible. How do we reconcile this with reality? Additionally, I would like to request a video on moment of inertia and rotational mechanics. This is a particularly tricky topic for many JEE aspirants, and a clear explanation from you would be immensely helpful. Once again, thank you for your invaluable content. Looking forward to your response!

Prathamesh V Patil

In the videos, you mentioned and proved that light travels to all the possible paths, as do all the macroscopic objects. But we cannot feel that for macroscopic objects. How can we get physical intuition about the same?

Khushi Purohit

ANYONE HERE? HELLO

Pardeep Rana

How?

Tanmay Kumar

So does this mean that the LASER light is also going in the exact OPPOSITE direction of what its actually pointing at? Like how is this even possible, isn't the physical things like the cover of the laser light obstructing/directing it to go in limited directions only.

Tanmay Kumar

This means I can change my reality! If the same rule applies to us, so similar to that laser light, can we just block some paths in real life to change the net path that will change the reality?! This thought is making me go CRAZY🤯

Tanmay Kumar

I can't tell y'all how much I appreciated learning from you during these videos about action. I think there may still be room to have a better "intuitive" example for action and after noodling on it a bit, I wanted to offer this: Imagine you're driving between two cities, and have to complete the trip in a set amount of time. In this situation, an analogy for the principle of least action might be: "How should I drive to use the fewest gallons of gas for the trip?" You might be tempted to simply say "drive slower" or "drive faster", but most cars have an optimal speed for fuel efficiency. Drive too fast, and air resistance and engine limitations might cause you to waste gas. Drive too slow, and while you burn less fuel per mile, ultimately using more. So fuel consumption isn't just about the distance - it's about how efficiently you use energy to travel that distance. You might even already be doing this when you drive, trying to make those pricy gallons of gas go farther. Looking at the Lagrangian T - V we can understand this better. Let's imagine that all of your energy starts as potential energy (V) in gasoline at the beginning of the trip. The Lagrangian's value is as negative as it will be for the remainder of the trip. Our goal is to keep this value as negative as possible while meeting the requirement of traveling between point A and B in the allotted time. This means driving in a way that minimizes unnecessary fluctuations in energy use. If you accelerate too aggressively, you rapidly convert potential energy (gasoline) into kinetic energy (motion), but you may need to brake hard or waste energy overcoming excess air resistance. Here's one way you might do this: Have you ever tried coasting when approaching slow traffic to avoid slamming on the brakes? It seems easier to "hold onto" the speed you already have rather than braking suddenly and then accelerating all the way back up. Physical systems naturally optimize this process in a very similar way through the principle of least action. Whether it's light bending through glass, a ball rolling down a hill, or electrons orbiting a nucleus, nature "chooses" the path that is the most "fuel efficient" way to get there. This is the "principle of least action", though maybe it would be bettor to think of it as the "principle of optimal effort". And it might help you save on the gas bill to think about it too!

J.T. Menchaca

I can

Richard Polidoro

The demostration showed can be understandable if i think light as a wave travelling in 3D, and the location where it appears as the constructive interference of all the wave paths and this explain why light can be called travelling all possible directions at once between the two points, but what about particles, that not are not waves, how to understand intuitively like light (wave) how the paricle spread their mass from point A to Point B to be called "travelling all possible paths at once between two points", can you demonstrate with an example other than using light like using any particle like balls, or any physical object to prove that they travel all possible directions at once?

Krishna vamsi Vipparthi

I feel like this has some connection to quantum computers, that also explore all paths. But those paths aren't infinite, they're limited by qubits. Is there some connection here, and are these cancelling paths similarly limited by some quanta? The slits certainly can't be infinitely small and are, in theory, limited to planck length ...

Ali Eslami

Can you talk more about those FTL paths that were talked about in the video? It seemed like it was mentioned quickly but glazed over in the video.

Ali Eslami

Why is it when we observe the light, as in the double slit experiment, it no longer takes all possible paths? Please don't say "because of the collapse of the wave function". That's just like saying, "because it doesn't explore all paths". But why?

Ali Eslami

Ooo this is a good question. I hope they take it! I'm also curious why in the non-laser example there were so many dots, and why they were evenly spaced as they were.

Ali Eslami

But what about stationary objects? Are they also affected by this principle?

Yousef Alhaboob

right you are Harry

Visham

I have a question regarding the Demo of with the laser/light bulb. The foil with tiny slits is basically multi-slit experiment but instead of having wall further away from the slits we have a mirror which makes the photons go back through the slits again. Wouldn't that just create and interference pattern around the light source (and hence in the camera as well)? I don't see how this is a proof of the least action theory. Since to me it seems it can be explained by just seeing the same pattern as in double (multi) split experiment but reflected by the mirror. Am I wrong?

Gibbon

Grating

哲學家 苦惱的

So, is this also why quantum tunneling works?

Bikendi Cebrecos

So my question is basically regarding mindset. We humans can think, so we choose the best path for us. But how can non-living things think? How can light think?Or Do we really think or everything is predestined by default and we are just acting like small atoms in the vast universe?

Raghvender Singh

So, basically, both Sound and Light are waves. So, according to Physics, waves have the same characteristics and hence it is a really a good comparison to do between the sound and light.

ANITA PANDA

Regarding how light (and all objects) could possibly take infinite number of paths and we just perceive the most efficient path … 1. Wouldn’t the inefficient paths taken still influence other objects around it? 2. Does this mean that Dark Matter could just be all the possible inefficient paths/routes that existing objects are taking, and we’re able to measure those effects at a cosmic level? 3. How do we account for the energy loss from all these inefficient paths taken?

Mike P.

As a musician, this principle seems similar to the overtone series. When a sound is created, there are infinite overtones that are produced, yet we tend to only hear the fundamental. While the matter of light seems far more complex, as it involves quantum mechanics, at its core is this a fair comparison to draw between sound and light?

Brion Kennedy

also consider the gravitational field of the particle, they push each other as they move through space and time, and the least action is dependent upon that because that is where the particle's push is greater, and least amount of action is used to reach point B from A.

Haseeb Ur Rehman

So it reflects to another object of it... understandable

Abby

Why does nature choose the path of least action and what are its implications in other scientific fields.

Naitik

so are there infinite paths for any particle

Lentycle

Exactly my reasoning. There are 2 different light patterns emmited from the laser pointer: the laser beam itself, and the light diffracting from the source (as a background object, like you said). Not controlling for this means that the second experiment actually did the same as the first one. For more definitive conclusions, it should be demonstrated experimentally while controlling for this that the principle still works, which I'm not so sure it would for a laser beam, considering that the light emmited has different properties. Also got that feeling that the spacing between the lines should have a defined interval that would block one phase of the action consistently, didn't get into the maths (yet), but if the blockage of the lines isn't in a regular interval with respect to the action, that irregularity would eventually block different phases of the action "clock", and result in a homogeneity of the interference (which wouldn't favor the constructive interference, the whole point of the experiment). I'd guess that the intervals between the lines should be calculated based on the wavelength/frequency of the photons in question (which is proportional to the energy and action). Lastly, the experiment would be ideal for scientific purposes (besides the video demonstration) if it emmited one photon at a time, just like the double slit experiment. But I understand this adds some important complexity to the execution.

Gabriel Cecatto

so basically light moves in every direction possible from one object to another object- that means anywhere on the room or the world if open enough and dark enough, a laser should be visible? If it's pointed at a mirror

6207 BHARGAVA.P

That's fascinating. Shouldn't it be possible to "drain" more or less any amount of energy from indirect sources, if the pattern is right enough?

DUST

My understanding of light travelling is different: first we have the model of waves which explains the double slit experiment. Then we have the model of photons which explains the experiments where we send a laser beam to a specific spot and all photons are captured exactly on this spot. Neither the one model nor the other is explaining every situation. My reasoning therefore is that we just haven't found the ONE correct model yet. So my question is: why are we still trying to use these wrong models to explain how light is REALLY behaving? Shouldn't we rather search for the correct model, first? Then everything will just simply fall into place... 😉

Stelios Papadopoulos

Do you think there’s time in space?

Richard Polidoro

Also if light takes all path why doesn't it's energy gets lost . As it is taking every path it sends it's energy in every path . It might seem dumb because I didn't understand something so please don't take my name answering this if you ever take 😭

Abhinav Martand

Can you tell how the atomic structure looks like . Cause I studied that now there is new model called quantam mechanical model and they have orbitals . If a atomic can have s and p orbital why we don't see combined structures. I am confused about the quantam mechanical model please tell

Abhinav Martand

thats a polarising sheet i guess

Ayushman

I have a question that might stem from the fact that I don't grasp physics all that well. If the other non optimal paths get cancelled out "most of the time", how come I don't see light from the back of my head that entered my eye from time to time?

AlexȘ

Do we really know if the light travels from point A to point B? What we know is the source of the light and its destination. Of course because of Heisenberg's uncertainty principle we can't measure the light en route. Could there not be a calculation layer that does the probability calculation and the photon is created at the other end?

Mark Elliott

It looks like it should be possible to do Casper's experiment at home. Where can I get that foil with the fine lines ?

Daniel Beguelin

My understanding of this concept is limited to simply what I’ve learned in your video so I apologize if my question seems uninformed. However I just want to see if the way I’m understanding this is somewhat correct and if the application in my mind is rational. My explanation may also use incorrect terms so apologies for that as well With light. The particles traveling with the most obscure paths that cancel themselves out. Is this a similar concept to how sound cancelling headphones work. Basically the exact opposite waves colliding and cancelling what would otherwise pass through? Yet it still has an affect on the optimized path to correct it so to speak so that the most efficient path is the one that happens. (Even though they are all technically happening) And for the application you used the illustration of a basketball and the path that it takes. In sports I’ve long been troubled how very similar actions have seemingly very different results seeming at times almost random and not applied equally. As a couple of examples, there are videos of marble races, where the same marbles are rolled and released at the same time, yet the only consistent result seems to be that there is no consistency even though everything else is the same. Would this principle explain that? It’s different because there are many variables and many different paths influencing the result. And because there are so many more marbles it becomes much more inconsistent On a smaller scale if you were to imagine a bowling alley where a child uses the device to roll the ball straight where they simply place the ball on the apparatus, while it seemingly rolls straight, the result of the pins it knocks down is more consistent than the marbles, less variables, but still in consistent So seemingly taking even less variables into consideration finally in golf there’s a device similar to the one in bowling that was mentioned that showed even with the same force and the same trajectory seemingly, usually the ball would roll into the hole but occasionally it would fail. I previously believed this was due to external forces, an unnoticed light breeze which of course could be the case but this happens even indoors. So is this essentially the reason why this is happening in these instances, that it’s not user error persay but that all these paths are being taken and having an influence that occasionally seems random and at times breaks the normal expected result? Finally, is this also what leads to the belief/debate about free will vs fate, and the multi universe theory where all possibilities must exist even with some of them being much less likely I’m sorry for my rambling, I appreciate your video, and hope my questions are at least semi coherent even if very erroneous so that it’s not a complete waste of your time

Shawn Barnhart

Cool video! But how do the few observed reflections account for all possible paths at quantum level? Aren’t we missing the most crucial proof here? We should consider calculating the spacing between the laser reflections and number of total reflections in an area and relate it back to quantum scale. However, I still believe these are reflections of the laser source as a background object rather than actual laser beams, meaning they aren't related to all paths quantum phenomenon.

zill e hussnain

I have read cases suggesting that action is maximized, refuting Maupertius. Such is the case of the reflection in a concave mirror (d'Alembert, 1752; d'Arcy 1756). Is this really a counterexample to the principle of least action? To this counterexample, someone has offered the answer that not all geometries allow for a minimum (Michael Veldman, 2023). I infer from the second video that the reason for the greater relative visibility of the path of least action with respect to the others, has to do with the constructive interference of the nearby paths, without that obstruction, would there not be a sufficiently pronounced (visible) path?

Nisi Fernandez

In the experiment with the laser, the total energy the laser outputs must stay the same. So with "new" reflections occurring with the "slit sheet" the main light must become dimmer. Is there a maximum in dimming that can be achieved (theoretically and practically)? If so, how much is it? (Can I shield myself from laser attacks by wearing a fancy striped shirt?)

A. D.

How does wave function collapse due to observation of quantum particle fit into this explanation of the principle of least action overarching both quantum and classical physics?

Jana Michálková

The potential energy term in the action represents that. Like in a black hole, the potential energy is too high at the event horizon to be able to get out of it

Enea

Think of it as a probability wave, not one photon as a point particle. We haven't figured out all, but we have a better idea of how the world behaves. The main problem is that photons are fundamentally different objects from the tables and chairs we interact with every day. The energy scale difference allows quantum phenomena to give these new properties. We will never fully understand that because we can't become a photon experiencing passage through spacetime. But it is impressive that we can figure out that these phenomena happen and we can predict the outcomes

Enea

Yes, the total light emitted must be the same. Adding that foil causes the main path (the bigger red dot) to lose intensity because some photons "pick" another path instead. The other red dots are too dim to notice the difference in the main one, but with a detector, you should be able to measure the change.

Enea

Paths connecting both sides of the event horizon are impossible to take because they require a speed larger than light. The idea is that light takes all possible paths, but coming out of a black hole is an impossible path, not a deconstructive interference.

Enea

Stupid question but do the waves of a particle travel faster than light? I’m not really sure why this would be the case but when watching between 24:40 to 25:20, this question keeps on popping into my mind.

Khristian Carpe

The explanation tells about how a light from radial light emitter behaves, but I am telling about light not taking the path with least action in a bent spacetime, we have things outside the black hole and inside the black hole, if there is inside and outside, there should be a way that inside connects with outside, and for light, due to curve of spacetime it is taking the least action path and that path is not letting light to come out of black hole, but I am telling that there is a probability that light may come out of the black hole through a path which connects the inside and outside of a black hole, just the path have less magnitude for resultant phase of that path and it eventually became nearly impossible.

Iniyavan Apple

Essentially, all paths inside the event horizon lead back inside the event horizon. https://physics.stackexchange.com/questions/67682/how-does-light-behave-within-a-black-holes-event-horizon Give this answer a read.

Dan Smith

If an object is just moving in a straight line,from the video I could see that the action is equivalent to the angular momentum,If it's not rotating the how could angular momentum exist in it,which give us the conclusion the every linear motion in two dimensions is always accompanied with 0 angular momentum.Is it really true?

Sridevi Vadivel

According to Law of action, light is taking every path and the path with more resultant phase magnitude is what we are observing. light follows least action so whenever there is an object which bends spacetime, light is bending as the spacetime bends, according to light, it is following straight path in spacetime, if it explores all path, isn’t there a possibility that light is coming out or information is coming out of a heavier mass body like black hole?, because light follows least action we are not observing the light from a black hole, and spacetime curvature doesn’t lead light to come out if it takes the shortest time taking path, but if light do explores or a particle with wave do explores all path, there will be a possibility that light came out of a black hole, but now just the way is less possible as we have shorter magnitude of resultant phase. As we are finding that laser beam which points out at a direction reflects light from another position with we don’t let the remaining phases to cancel out, can’t we let the cancelled out phases of the path such that light came out of a black hole to work effectively as we don’t let the phases cancel out in the laser beam experiment. There should be a way that we don’t let the phases to cancel out and we are getting information even from the least taken path that a light took which is the path that lead light to come out of a black hole. Let me tell it conclusively what I am trying to ask. Light took every path but it took the one in which there is more probability and that probability depends on and directly proportional to the resultant magnitude of phase, now the light have a very very less probability to come out of a black hole because that path have less resultant magnitude of phase, but somehow if we don’t let the phases cancel to cancel out for that particular path, then we can literally make that path with more phase, so that light also takes that path, if that path is the one where light comes out of black hole, can’t we get information from black hole and solve the black hole information paradox. I am not violating any physics rule, just trying to think that if there is a way to make that nearly impossible path to a more feasible path for light by don’t letting the phases cancel out, like by using a method we didn’t let the phases cancel out and we came up with additional paths of light in the laser beam experiment in the video, can’t we do the same for the path that light is getting out of black hole, as now we know that light tooks all possible way, and so like is taking a way in which it is not a straight line in the frame of bent spacetime curve. Please concern my thoughts. And send a mail to iniyavanvirapouthiran@gmail.com for more conversation about this topic.

Iniyavan Apple

If the idea or concept of least action applies everywhere like in your past video ,you said it's the theory of everything then does it mean the idea of "free will" also follows the path of least action and free will is just another form of optimization?

Amit Kattal

It doesn't explore all path BEFORE taking them, It explores all paths, period. It's a wave. We just don't see the paths that negatively interfere with one another, i.e. the vast majority of them.

Tom Briand

I have a question about Casper's experiment (around 28:00): He uses a foil with a very fine pattern to prevent the waves from canceling each other out. This allows you to see "resonance points" in some places. In the macroscopic world, light can generate heat in materials it hits. Shouldn't there also be an energy input at those "resonance points"? Or am I misunderstanding something? Somehow it feels to me as if this has implications...

DUST

How does the light "explore" the paths before taking them? Isnt there some issue with information traveling FTL? And if the answer to that is "it doesnt, it just goes in all direction" - well how is that possible. How can 1 photon go in all directions at the same time? The video sounds to me like "this is it, we figured out the slit paradox, also the connection between quantum and standard physics" but it really doesnt, does it?

Marzaus

You described the paths of light as going anywhere and everywhere, but is that true? Are there forbidden regions through which the light cannot pass? For example, behind the light source, or through (or around) a solid, opaque, object. If that is correct, then does there need to be some adjustment to the equations to exclude those regions? If so, then each and every situation is unique, and would have its own prohibited region. How would that work?

James J. Roper

Is the reason a particle acts differently when observed purely because during observation it is being interfered with by photons? I heard Dr. Tyson describe it that way in a short snippet so I could have misinterpreted it.

Tri Tran

In the second video i learned that "action " governs so many of the basic laws of nature but does the mathematical expression also encircle the theory of "Fermat's least time principle" or does it entirely ignore or disprove the theory of Fermat?

Diptartho Chatterjee

Hi, in the second video the Australian professor said that we are teaching from forces to energy and only at uni we start to hear about Lagrangians and Hamiltonians. More or less similar for quantum reality. Could you as a PhD in educational physics come with a proposal how we can change our curriculum to start to build an intuition for this fundamental reality that things optimize their action and that the world is based on quantum?

Tony

Personally I am a huge disbeliever of multiverse theory but your question is quite interesting. Through the lens posed by your question, it could mean that all the particles that exists in the omniverse are taking every path and in this universe, we experience the ones with the least action but that proves that another universe exist but for us they have cancelled each other. But this bring 3 things to my mind: P1) Are the least action relative? Because if that is then there are multiple least actions, the universe we experience is result of the least action relative to us and if least action are not relative then that could mean that other universes exists but not in a way we would like it to be like the dots on the foil (reference of the video) and our universe is the "prime" universe with the least action, or maybe it could mean there is another universe with the least action and we are not in that one but that would require a anti-verse of our universe which cancels our universe out. P2) This could also mean that the particles although may behave differently in each universe, they are essentially the same particles but just behaving differently. P3) Or (counter to 2nd point) it could mean that in the omniverse, particles are different at a more fundamental level and they behave in different ways, what we experience as particles and waves in our universe could be completely different from how those fundamental quantity are in another one..

M. Hamza Kapadia

Bohmian mechanics, interpreted as a global wave field in which the waves are continually updated in correlation with the structural environment, can explain all possible entanglements, interference patterns and superpositions for an energy pulse that is sent into the field.

Antsu Sausanen

Another way to present it: We can set up an experiment using a beam splitter and mirrors so that a photon exhibits constructive interference at a particular detection point. Initially, the mirrors are placed fairly close together and the experiment works as expected. If we gradually move the mirrors farther away along one path, continually adjusting them to maintain constructive phase interference, will we reach a point at which the interference disappears? Furthermore, in a purely theoretical setup with infinite coherence length and perfect mirrors placed one light-year apart along one of the paths, does the constructive interference occur immediately when we start the experiment, or only after the photon has traveled for a year to cover the longer path?

Jan Bover

The double-slit experiment demonstrates how a single photon can interfere with itself. However, when it interferes—or “collides”—at a particular point, we may notice that the distance the photon travels through the first slit differs from the distance it would travel through the second slit. In other words, the time to reach the interference point via the first slit (T1) is different from the time via the second slit (T2). Given this, at the moment the interference occurs, how much time has the photon actually taken—T1, T2, or something else?

Jan Bover

Energy is relative but energy flow is invariant. Can we say that action itself is relative but action flow is invariant? Is invariant rest mass the manifestation of some continuous energy/action flow?

Antsu Sausanen

I thought of this will trying to visualise the theory in regards to the whole universe. If you can interpreted ever mass/energy as a Result of a Infinite Slit Experiment (RISE) would it be possible to interpreted the whole universe as the combined result of adding infinite RISE results together. If so would it be possible to see it like a giant field of destructive results an constructive result with the constructive result translating into masses/energies. I don't know the limits of the infinite slit experiment, but would it be possible to have multiple wave sources of varying amplitudes, fazes and location create a RISE resulting in a more varied result of destructive and constructive results? If so it would be possible (I think) to see the universe as such a RISE with the constructive result resulting in the masses/energies of the universe. With every unique result resulting in a specific mass/energy. If the above is question is possible with a RISE would it be possible, if a change was made in the wave sources be it amplitude, faze and/or location it would result in a (drastic) change in the RISE. If you were to make this change infinitely small, so it translate into a infinitely small change in the RISE. So if you were to make a series of these infinitely small changes in a specific way would it be possible see them as change caused by physics (like gravity). If so physics could be seen as the specific way these wave sources change. It may be far fetched but I kept going further and further so I was just wondering if it was possible or not.

Xander Waegemans

The action explores all paths, but how do you know the paths before you explore them?

Miguel Bustamante

i am not clear with the sense of every possible path.. please help me

Shaurya Jain

I am a 12th class student from India and just giving my opinion.. please think on it and answer me.. Or tell me if you are just like me. I think this conversation is beyond a particle-wave debate because both the possibilities that it is a wave and a particle travelling on every possible path are satisfying the experiment.. and in wave case where will you draw the line of difference between particle world and wave world or show the spectrum of it..

Shaurya Jain

Point 3 was exactly my question, I (also with no extensive background in physics) wonder if the experiment could be even performed without a mirror involved

Jorge Morgado

Can the theory of least action also apply to mathematics ? So like calculating 5x6 can be any interger, but by theory of least action it is most likely 30

Wizard of OZ

Does that mean that claim that photon is particle with tiny mass is just abstraction, tak works in 95% of cases, but in reality it is definitely a wave and this principle of exploring all possible paths is valid only for waves (not for physical objects such as basketball)?

Viliam Oršula

So, is that the case that for any system there could not be two “visually” distinct minimal action paths? So that we can see the object taking both of them at the same time. Or the problem is in the sensitivity to the initial conditions, and even if theoretically the system is in some sense symmetrical, practically you can’t arrange it in the way, the actions are exactly the same and all but one trajectories still cancel out?

Gleb Mezentcev

"everything explores every possible path all at once" my question is WHY, WHY do everything does that?

Muhammad Abdullah

Would this prove the multiverse does not exist? Or that it actually exists but we only experience the one with least action?

AnSo66

This video absolutely blew my mind. I am NOT a math or science major, but I love both and the puzzles they carry. I love learning about the world. ANYWAY, if literally everything that has ever existed is taking every path simultaneously, how do we see this with a Basketball flying through the air? Like the laser-pointer experiment, is there one we can perform that would cause the basketball to fly through the air in a really weird way? (Even a purely theoretical one?) There are currently two very strong opposing forces in my brain. One says that the basketball just flies through the air that way because of the forces applied, but the other is doing it's best to acknowledge the truth that was proven in your video. How do I reconcile the two?

Jonah Weber

I have one question on the last experiment using the laser. What would happen if we did not use a mirror to see the reflection of the laser light? So, having just the laser point to the camera and blocking its direct path with the black paper, then maybe having the foil on the right or left side of the paper, would we still see the laser light from the camera? I'm not sure what that would prove or disprove or if that even makes sense, but I thought I should ask. Thanks for the great video as always!

Street Cultures

I have one question on the last experiment using the laser. In the air, there are so many tiny objects that could diffract the laser beam. So, it is likely that the weird light path could be caused by air diffraction. So, if this experiment is conducted in vacuum environment, what will happen? BTW, my major was Physics too. I am really appreciative that this video shows how quantum physics connects to classical physics.

東煜 陳

Hey Derek, I've got a couple of questions regarding the second video. I usually don't send any comments, but I found this video specially interesting and since you mentioned this Q&A I thought it could be a good opportunity: - The path of least action is the one we see things following because it is the minimum of the funcion, meaning the derivative there is zero, so small changes in the path don't change the action as much as they do anywhere else, which makes so that the paths around this least action path don't destructively interfere with it. At least that's what I got from the explanation, but if what really matters is the derivative being zero, wouldn't this also happen for the path of most action (not that it exists, considering there are infinite possibilities), or more importantly, for all the paths that are local minima or maxima of the action funtion? - Some aspects of the experimental proof are still boggling me: (1) Previously it was stated that the sheer scale of planks constant made so that even a slight change in the action for a given path would result in a big enough change to the wave's phase to effectively consider it random, thus justifying the destructive interference. But then for the experiment, the assumption to use the foil was that the changes in the phase are very small for paths of light close to each other. I guess this assumption has to be true if you zoom in close enough, but is that really the case at the scale of the lines in the foil? (2) If the whole assumption and setup for the experiment are indeed correct, why is it that only a few dots of light can be seen on the foil? From the explanation it seems to me that the effect of the foil would be to "filter out" the destructive interference, so if the light is going through every path and doesn't appear everywhere solely due to this destructive interference, shouldn't the whole foil be "lit up"? (3) I didn't find the experimental proof compelling enough in what it was trying to achieve. We can see that the laser is shining light everywhere (because we can see it shining from our view), with it's tip effectively acting as a point light (weaker than the intended directed beam of the laser, of course, but still clearly visible). What I'm thinking here, as a viewer with no extensive background in physics, is that the dots of light we see in the foil during the experiment are appearing either due to the reflection of the laser's tip on the foil's surface (which could be a little sloped and/or non-uniform), or due to the reflection of the laser's tip on the mirror after some refraction effects from the foil. Both of these could be tested by repeating the experiment with a sheet of paper beneath the laser (or to be more specific, between the laser's tip and the foil), while still allowing the main laser beam to hit the desired surface observed by the camera. Could you test that out? Thanks for the great video as always! Big fan here

Gabriel Batistella

If the spinning of the atomic clocks that indicate the phases of the light waves are dependent on the time it takes for the a light ray to be detected at at point B from point A could you somehow influence the probabilities by using a strong arbitrary gravitational field to influence the two completely different sets of light waves (maybe one curved and one linear set )to have similar atomic clocks so they have the same phases when detected a point B such that the squared resultant of the phases are the same making them have similar probabilities of being detected at B

Lwandile Moyo

Using Plancks constant in regards to action seems like we are basing our physics on a completely discrete universe. What other implications could arise from not having a continuous fundamental universe with time, gravity, or things small enough to be on a local scale with the Planck length?

John Emelko

The path of least action is the most probable path because a law of Physics governs that path. If you modify the law, you will have a different path of least action (i.e. there are no magical paths appearing out of nowhere). What Feynman's propagator integrals tell you is that there are infinite paths that a small particle can take because 'cause and effect' break down in quantum mechanics (i.e. I like Landau's interpretation of QM). Long story short: At the base of Schrodinger equation (or Feynman propagator) sits a law of interaction from classical mechanics that tells you what that path of least action is. The rest of the paths (that do not respect that one classical law that you are looking for) are probable because at small scale, many other interractions become significant, but most of them are less likely compared to the one trajectory defined by the classical law of Physics. So while ftl paths or backwards-in-time paths are very romantic, in our universe their probability is almost zero. Also, keep in mind that you can see the light coming out of a laser pointer from the side (perpendicular to the beam) which clearly shows that diffraction and scattering of small photons are probable. But the majority of the photons follow the beam of course.

Alex V

Why does the path of least action form a constructive path and does not cancel out? Could there not be a possibility that some where in the universe that does not follow the principle of least action

Ayyan Baig

So if everything takes every possible path and the laser showed up at 2 places because it took more then 1 path, does that mean anything can be at more the 1 place if you find away to cancel the disruptive paths from canceling each other out.

Abdifatah Yusuf

I’ve been thinking about how gravitational lensing affects the path of light and how this relates to the principle of least action. Since lensing is caused by spacetime curvature rather than a traditional refractive medium, does this mean that the "least action path" for light is dynamically redefined based on the gravitational field? Additionally, could an artificial or engineered gravitational field (if technologically possible) be used to manipulate the least action principle in a controlled way, potentially guiding light along unconventional paths?

Lawrence T

"In order for a electron to stay bound to a nucleus, it has to be a standing wave and a multiple of a certain wavelength" What is a standing wave and why does it have to be a like that?

Muntasir Mahmud

Could we interpret the principle of least action as a universal tendency for all processes to minimize energy expenditure? If we observe nature closely, we find that systems evolve toward higher entropy, naturally favoring states that require less effort to maintain. Ordered structures persist only by continuously consuming energy. Could it be that the principle of least action fundamentally reflects this drive toward minimizing the energy spent in sustaining motion and order? What you think about that ??

Sourabh Mehra

Hi Casper and Derek, In the second video, you discuss the path integral formulation, specifically how a particle takes all possible paths—including those that imply faster-than-light travel—and how these paths usually cancel out via destructive interference. However, when you demonstrate using a millimetre strip of foil, it appears that you can prevent this cancellation and reveal one of these faster-than-light paths. My question is: If we can observe these faster-than-light contributions when destructive interference is prevented, does that not imply that the signal or wave is effectively travelling faster than light to reach the endpoint? As a student, your videos are not only informative but also motivating! Thanks!

Ojas Kulkarni

Can you share any similar experiments that have been done and which are interpreted in the same way as Veritasium did?

awadood7

I read on the r/physics subreddit in a post related to this video that the probability wave of light should not be confused with the electromagnetic wave of light. what is meant and how does it relate to this video?

alkimist

He does not mean that way. He means that imagine a single ray of light which goes from point a to point b. This ray takes all possible paths.

Avyay Chandra

Same with the laser. We can't see much ,but in the case of a dark room we can... so...it's just obvious.

Ritviek

Isn't it obvious that light takes every possible path because that's how we see things... right?

Ritviek

Has anyone tested this specifically in regards to c? I feel like my brain can find a way to override Casper's first experiment, but the second with the laser pointer just blows my mind. --- This might even link to your 2-way speed of light video, but I just can't see how this doesn't make the speed of light clearly not "speed limit."

Ian Gardner

Can you explain will matter follow cumulative optimum path or not

Pocket Monster

can you explain the possibility of Retrocausality and Delayed Choice Quantum Eraser experiment

Cornelius

Does it mean that theoretically you can do a lens with a screen with infinite slices and some computing power to organize light in a sense you can see through walls

Ihor Horiachkivskyi

In the context of field theory, the action functional is given by an integral over spacetime, where the Lagrangian density determines the dynamics of the fields. Consider a field theory on a curved spacetime background with a metric , and an action of the form: S = \int d^4x \, \sqrt{-g} \left( \frac{1}{2} g^{\mu\nu} \partial_\mu \phi \partial_\nu \phi - V(\phi) + \frac{R}{16\pi G} \right). Derive the Euler-Lagrange equations for the scalar field and the Einstein field equations for the metric . How does the variation of the metric lead to the Einstein tensor, and how does the least action principle naturally yield both the Klein-Gordon equation for and Einstein’s equations for gravity? Additionally, how does the inclusion of a non-minimal coupling term affect the resulting field equations?

Sugam Acharya

Hi. Thanks for this very nice video. In the video you represented the phase of a particle (or a system) as an arrow that winds around a "clock". The amount of phase accumulated is proportional to the action which can only be in multiples of h/2π. At some fixed amount of phase the "clock" goes back to zero. This means that any paths with the same integer multiple of that fixed amount of phase (the full turn of the vector around the "clock") interfere constructively. My question is the following. What determines what that fixed amount of phase is? On one hand intuition might suggest it is h/2π for one cycle (as implied by the equation in the video) but on the other hand intuition also says action in fractions of h/2π is not allowed. Additionally if Δφ=2π Δx/λ - 2π f Δt then Δφ should be zero always if in one period Τ= 1/f you travel one λ of distance. To me it seems that the two terms are linked and should be redundant to include both. Is that where the supposed variable speed of propagation comes into play?

Chris

I'm sorry if this question has already been asked, but the laser dot visible on the black covering mat, where did that come from? The laser was pointed at the mirror, through the defraction foil, to reflect back toward the camera, but what did it then reflect off of to appear on the black mat? I am sure it's something obvious, but I am curious

Brent M.

I don’t understand why the straight line has so many more paths to constructively interfere with. It seems any old path has others that are nearly identical, with which it should constructively interfere.

Simon MacLean

If the photon takes all paths, why bother going all the way to the mirror in the first place?

Simon MacLean

I enjoyed the video, very good explanations. But it's not "Least Action", it's stationary action. In some cases, it could be local maximum or saddle point of action where the phases line up. Gravitational lensing is a great example where the faintest image results from a local maximum of action.

Scott Burles

Light is simply so fast. But also so slow. Like humains they want to understand faster then it really works. The last experiment has been confirmed like a ziljoen times know. Why ask it again ?

Robert Koopman

Can you please perform the final experiment with the laser, grating , and no background lights again but with more careful control of the laser pointer? To address concerns of scattering of the laser due to poor collimation/aberrations, can you also substitute the mirror with a detector/camera to really show that the camera does not detect any light at the grating location when the laser is shined at a location away from the grating? Thank you very much.

awadood7

all of the light paths have zero resistance

psrmeshaan

They have a full video about it https://www.youtube.com/watch?v=Q10_srZ-pbs

psrmeshaan

The assertion was that the experiment with the mirror proves that light explores all possible paths, but I'm not yet seeing how. Generally speaking, when light shines on any opaque surface, we already see that light travels from the source in all directions. It travels to all points on the surface, plausibly in a straight line, and we then see that the entire surface is illuminated, light traveling from each point on the surface to the camera. Nothing other than a straight path, subject to reflection, is required for this. We can also observe that there are no dark spots, so the phases of light must not be substantially cancelling out in this case, because we observe uniform illumination, but that could just be because it is not a point source. So, why isn't the mirror merely a special case of the opaque surface, one which reflects efficiently but not perfectly (some portion of light reflecting in all directions), and which allows phases from less optimal paths to cancel each other out?

Neo Dog

From the Block Universe perspective, isn't it fair to say that the entire wave function of the universe being the fundamental reality? And thus light is not testing all those paths but actually exists in them, just that our measurements are the high probability results. In other multiverse branches there might not be the deconstructing wave(s)? In short this really catapulted my intuition about the quantum multiverse. Can you connect those dots?

David Geffeney

Read the "Principles of Mathematics" by Bertrand Russell. That very question is the first question it answers

Seraj Abdi

Could it be that even the "laws" of physics can be violated just at super low probability? Such as one photon taking a wildly long path exceeding the speed of light. Information is still not traveling faster though as a single photon cannot offer any info upon measurement. Even Entropy for example can be violated in very short intervals of time

David Geffeney

What's the difference between path of least action and path of least resistance?

Kevin Schnaubelt

Can you do a more mind boggling version of the light experiment you did in the last vid

That one nerd

If PLA is fundamental, does it mean nature "knows" all possible paths in advance before selecting the least-action one? How does this reconcile with the local causality of physical laws?

Syed naeem

I have a simple question: When Casper did the experiment proving that light takes every possible path, wasn’t the reason we saw the light simply because the foil had a microscopic texture that reflected it in such angle? And if this was the proof, wouldn’t it also work even behind the light source, considering the weird paths light could take? Thanks!

Andrey Oliver

I love your channel. Could you explain the implications of this on a planetary scale? How does it impact our visualization of the universe?

Rohit Mehta

Just pure wows!! Only one thing i didn't quite get, what is the Lagrangian? You talked about it in the last part of the video

Orestis Charitos

I probably don't understand everything properly but this thing with the doubleslit experiment, and particles taking all possible paths and resulting in the path of least action... How does that relate to the observation effect ("collapse of the wave function or whatever happens")? Also, how does it relate to the so called "randomness" in quantum mechanics? It all sounds very classical to me. I'm probably just dumb.

Piero Wnetpils

I did grasp the theory, but does the experiment really proves that it takes all the paths. We are able to see the origin of the laser even when not directly pointed at our eyes, so could it be the same reason we are seeing the reflection in the Magic paper.

Jay Chauhan

Hey, maybe this is also related to lightning strike pathfinding. The entire path is found before exchanging energy.

Jordan Littlejohn

Can you tell me why 1 + 1 = 2? I need to call Pacific Source Health Plans at 800-342-2798. It's too late for the oligarchs to offer a draw. It's too late to try to apologize now. It would only give them all the credit and money they want. https://keepthefuturehuman.ai/ is the only way to prevent the most violent demonstration you have ever witnessed in human history.

Cameron Hicks

If we had a fast enough frame rate camera (or if the distance is large enough) you'd see one before the other when the laser was turned on. Just like if you have 1 light bulb and 2 reflection sources at different distances. I'd be curious if this holds up experimentally.

Jordan Littlejohn

Question to last Videos experiment: does the experiment really show any quantum effects? IMO, all the effects shown are explainable purely by classical electrodynamics. First, you see a diffraction pattern of a classical wave. Then, with the laser, you see the diffracted pattern of the radiation that does not exit the laser straight but gets scattered at the collimation lens of the laser pointer. I mean, you even show it in the video, you can see red light on the exit of the laser pointer even if it is not pointed at the camera. Is that a light particle taking one of infinitely many paths? No, its not, it is light that scattered at the collimation lens and then went straight to the recording camera - and it did so along its classical path.

Pi P.

I do not believe in 1 neutron, it's illogical to have 3 particuls in 1 atome. It's build up binairy, so it works in pairs. H+ = 1.1.1.0 and h- = 0.0.0.1 it's so easy to understand. We life in the computer. I'm already on the place to be. If you believe in game theory ofc.

Robert Koopman

Well master of light. What if there where 2 neutrons ? Instead of 1 neutron. So we got 4 elements instead of 3. Because a neutron can be made out of spin down/op bit also spin up/down. Let's talk binairy, a neutron can be 1.0 or 0.1 a electron zwill be 0.0 and proton will be 1.1. This gives the possibilite to build up the atoom completly different and construct a compoct syteem where the actions is pretty low under high velocity.

Robert Koopman

I think energy is just related to the resulting amplitude. So all the other ones that cancel out have effectively 0 amplitude. I'd be interested though if the total energy usage in the last experiment with the laser was effected by adding the sheet. Or maybe some energy was leeched from the main laser path (more likely). Really turns your concept of cause and effect on its head, huh?

Jordan Littlejohn

I've seen this asked peripherally by users before me, but not in the same way that I've been thinking about it. If an atom is emitting energy particle waves in infinite (or action units of) directions all the time, but we only perceive the path(s) of least action due to the self-destructive nature of all the alternate paths, what's happening to the energy being emitted in all the self-destructed paths? Maybe I'm thinking of it wrong, but I'm seeing it in a similar way as 2 speaker drivers mounted side-by-side and pointed in the same general direction. If the sound waves are in phase with each other, they amplify each other and someone listening far enough away at the same distance from both drivers will hear approximately twice the volume. If they're directly out of phase with each other, they cancel each other out and the listener perceives nothing. No energy from the waves has reached the listener. However in the latter case, the waves are still being generated and they still exist close to the speaker drivers. The energy to produce them was generated and used up, and some of that energy was turned into waves. If the listener moved closer to either one of the drivers, they'd perceive that speaker's sound wave before it's been canceled by the other one. In both cases (in phase vs out of phase), the same energy was used up to produce the waves and that energy had to go somewhere. In the reflected laser light experiment, the same thing would seem to be true. The laser diode is emitting in all possible directions at once, but most of those are self-canceling until we force them to become visible. This seems to be separate energy that was previously not observed but can now be seen. It's not taking away from the directly-viewable result of least action, it's a new energy. ALL of those possible paths required the energy to be emitted from the source, even if they're self-destructive. That means there's a whole lot more energy than what's being measured at the point of least action, and in fact there's probably a TON more energy being emitted. Is it possible to somehow harness that or otherwise make use of it before the self-destruction? How much energy are we talking about? Or am I just thinking about it all wrong?

Frank Gore

My intuition says Action is the consequence of combination of energy and entropy constraints on an evolving system, am I right? Action and entropy are both subject to extremisation laws, and action optimizes KE-PE but KE+PE= total energy remains constant

Supreet Sahu

I wonder if the answer lies in what we're not seeing. Can we adjust the equation to calculate PE required for all possible paths other than the one of least action? I'd almost expect it to be infinity less the PE for least action. But what happens if you constrain it to observable spacetime? Is this how we ended up with "dark energy"?

Ryan

is the principle of least action related to neurons firing in our prain

Gitartha Kalita

Time travel? As was mentioned, apparently things take all possible paths at the same time, which includes time dimension, so I'd like to know if there's a possibility of similar experiment as the laser demo, but instead of space, to make light travel in time. Or perhaps 4th dimension.

Just Levs

I wouldn't expect to with "loose" smoke, too much variation in the distribution of particles to create the specific intereference needed to bring those other paths into visibility. However, if you could direct the particles of smoke into a specific arrangement to direct the wave interference to be constructive, you could accomplish this with smoke I imagine.

Ryan

Any way to simplify the explanation a little more, specifically what does arriving in a different “phase” mean and how do the different amplitudes “cancel out”. Also in that last experiment if the light takes every possible path would you be able to recreate it on the ceiling even though the laser points down. And if so could you show us?

Michael kelly

The answer to your first question lies in wave interference, and is the natural progression after answering your second question. The answer to your second question is not only could it have been any other path equivalently, but it IS taking EVERY other possible path simultaneously. That's what the diffraction grating is showing. By setting up the diffraction grating at a point where you would expect there to be waves interacting destructively and thus invisible, you constrain those waves (by blocking enough of the interfering wave function) so that they interact constructively instead at specific points in the diffraction grating, at which point they become visible at those points.

Ryan

In the last demonstration when the grating is placed and we can see the other spot. Can you just spray some smoke so that the path is also visible. Is it possible?

Rajesh Kumar Dolai

Doubt about law of conservation of energy. In the experiment shown in video where laser light was pointed at the glass mirror and then in camera we can still see the few spots of light on the other side.... So in physics taught normally, the laser transmits its energy and intensity a straight beam and the plane mirror (theoretically 100%) reflects the light, so on camera all the light should be conventionally coming from the major spot but since a few light spots were also seen so intensity of light from that also came to the camera, so who does the energy of the light distributes as light is seen from the straight beam and plane mirror but also from the few spots.

30 Rahul Ukey

Yes and no. You can use it set up differential equations describing the solution, and use those to approximate the solution. But that is also true when you start with a more newtonian approach - the challenge of the three body problem is in solving the differential equations, not finding them.

Ben Galehouse

I have a very similar question, though I suspect mine takes a more theoretical tack. I think the best way I can phrase it is: Can the existence of "matter" and "energy" outside the visible spectrum be explained by the destructive interference of macroscopic wave functions? I think you could theoretically test this mathematically. Though I'm still working through my prerequisite math. But if we can calculate the probability of a particle's location or phase by squaring the total amplitudes of the waves, can we invert that to calculate the probability of the phase or the location where the particle is not? Can that be scaled to macroscopic levels? If so, does the result of that calculation, when constrained to the universal space that we are able to measure, account for the "missing" 91.4%?

Ryan

I showed the video to AI ( gemini) and here is his question - Considering the principle of least action in quantum mechanics, if particles explore all possible paths, does the universe exhibit a fundamental drive towards optimization, or is 'least action' merely a descriptive principle without implying intent?

Rudra

Can the principle of least action be used to solve the three body problem

Rohan Mehta

It's unclear why the light in the patterned sheet directly below the laser pointer. Did all the in between paths cancel out too? Were they too dim for whatever reason?

Vir Godem

If light explores all possible paths to reach from point a to point b then how is there no energy loss?

Snehasis Das_188

Do you think principal of least action explains magnetism as well? Is it just a macro scale version of the effect due to constructive interference of atoms and crystal structures being in alignment? Furthermore, when atoms form covalent and ionic bonds and share electrons, are their overall wavelengths bound together as well? (Meaning they must remain in-phase with one another)

Jordan Littlejohn

electrons r not waves … they r just particles exploring all the routes possible, hence it believes as its waves but its just particles with all routes

Viraj Acharya

This is a really interesting question. I'm still grappling with the mathematics behind Action (behind caculus in general, honestly. I'm a bit late to the game), but if anything I would wonder which variables change when an external change is made that forces the wavefunction to constructively interfere in a non-least-action path. Does this shift the KE, PE, or t values in the least action path?

Ryan

Probably an obvious question, but how do we know that the integral of the Lagrangian must minimized and not some other quantity f.e. the sum of the derivative of the lagrangian evaluated at t0 and t1 (For this example to get the Newton equation you would have to define the lagrangian in another way). So in other words im asking about the uniquness of this principle

Yannick Lühmann

Why does this weird discretion of energy, angular momentum, action etc occur at quantum level. Is it possible that at quantum level space-time itself gets quantised?

Vaibhav Narayan Singh

Is general relativity also encapsulates the principle of least action and how if it do this

Universe Krishnamurti

If particles take all possible paths between 2 points including absurd paths like to the other end of the universe and back, what if the universe is truly infinite? What that would imply to the wave functions?

Dazai

so you said that using different lagrangians, and plugging in least action, we can get the formulas. so what if (multiple worlds theory) the multiverse had a specific lagrangian, and different universes having different planck's constant may have different laws of physics as a result

Akhilesh Puppala

If information can choose any path in space before reaching the destination won't it also travel through time in all of its direction... Or am I stupid idk ...why the arrow of time always forward and not changing any direction. But I heard someone very well known scholar saying it does during quantum entanglement collapse.❤️ Veritasium

Niranjan Madhusoodhan

So in the demonstration, the path of a laser light had been changed. can we do something similar to macroscopic objects? and also humans as macroscopic object also chose lowest action?

Akhilesh Puppala

Does it take energy to explore every possible path? For example would the foil at the end of the video heat up from the laser?

Hunter Ashby

Hi There, I'm still in Class 12 and studied Young's Double Slit experiment (Do not have that great level of understanding) , please could you elaborate more on the probabilities related to slits, the ones related to amplitude squared. I did not understand that part.

Yash

Least action and Entropy: I see Entropy "S" not just disorderness or just randomness but a mathematical tool to tell weather a process will take in this direction or not. Or just the path to be precise. And it's very close to or I say similar in sound to Action. So please tell me any relation of Entropy and Least Action. As in both talk a very similar thing. One talks for path a system be take be it a particle or anything and other talks in about process. Also, this is curious to me because, there are four regimes of physics classical, relativistic, quantum and relativistic quantum. And above all comes statical mechanics and thermodynamics which is found in all four be it quantum thermodynamics or classical thermodynamics and same goes to classical statical mechanics and quantum statical mechanics. So, I think there is some interlying relation of Entropy and Least action. Thankyou again, Name: Shantanu Banerjee, from India

SAM

Yes. You need to consider few things though. In the experiment, the person artificially changed or altered the scenario by putting the plastic sheet in the path which changed the “amplitude” or the probability as otherwise the phase would have cancelled out each other which in other words simply means that the probability is extremely minuscule in normal scenario, but the fact that we altered the condition with that plastic sheet, which basically means preparing the best show which the ‘photon-girl’ would like to watch and increase your chance/probability of getting a photon-girlfriend. So to answer your question, yes what you have said is possible but extremely extremely improbable to the point where (i hope i am not exaggerating) the universe will cease to exist before it could happen.

Saquib Akhtar

Can you please continue the series about the Principle of Least Action, explaining on general relativity Hilbert action and its intuition about the curvature of spacetime, afterwards the semiclassical approaches to unify quantum mechanics with general relativity which from action principle yields to the discovery of black hole entropy when taking into account thermodynamics. And eventually canonical theories of quantum gravity from action principles and above all string theory which have two point of views: the worldsheet action of the string itself and effective theories of the spacetime in the target space which in particular give rise to quantum corrections to the Einstein's equations.

Daniel Vainshtein

Excellent video. There is a lot to learn from it. What I am curious to understand from this explanation is, Does it mean that everything that an observer experience is probabilistic and not deterministic? And is there an explanation in action theory on effect of observer? It would be great to have a one more video explaining real world macro and quantum examples with action theory.

Ankit Dave

I like this video a lot and would like to have some more information on the shown different langrangians shown at the end. Maxbe even make another video on them AND the resulting long equation and "looking for the langrangian" in physics 'interpretation'

Gordon-David

Does the path of least action of light depend on the position of the observer? Does reality depend on where the observer is positioned in space?

Brad Bell

i guess to the weirdest extent it can give us the anywhere door. Just need to solve how to destruct your phases here and make it collapse some where else.

Col Udy

The pen in front of a mirror with a cardboard in between pen and mirror. you still see the pen from other angles. I guess that does the same thing. Right? If it is it would be the easiest demonstration.

Col Udy

You mentioned that light takes all the paths and you showed this drawing with light taking all the weird curvy paths. In the experiment at the end (and in feynmanns lectures) however there were only straight paths considered for the diffraction grading, making the explanation easy and understandable. But shouldn't there be a slightly longer "curved" path which destructively interferes with the laser light, because it is not near the path of least action?

Fabian Witt

If i understood correctly. Yes, the balls particles try to go everywhere but cancel the opposites out by the same force applied to every direction, except the one direction its headed. This direction has less momentum to cancel itself from the opposite direction. To the second one I would presume they do, its just that the amount of "pathway radiation" would be so small its almost nonexistent. So yes to both. Im not an expert and this is just me thinking. I might be very wrong.

Leevi

i think wolfarm hypergraph and his ruliad theory which states every computational possible configuration should exist and we just can interact with the some part of the ruliad just as the experiment of the video that the high amplitude state for us which has certain amplitude of our own and our view of universe our ruliad is visible it doesnot mean only our universe exist we just cannot persive the infinite other superposition of a particle

My Topic

does that even mean we can start perceiving things in 4 spatial dimensions

AR. LAKSHMANAN

You mentioned that the path that the particles will explore must include the ones that go faster than the speed of light and the ones that go backwards in time. How can we reconcile this with the things we know about relativity? To be more specific, what path causes the destructive interference of these paths in order to not "break the laws of physics"?

bobwizard

I understand how this applies to particle physics, but how does it apply to macro objects flying through space (ie. a ball)? Does the light simply take the path of least action, each molecule or atom within the ball take infinitely many paths, or does the ball itself as a structure of atoms take infinitely many paths? On a separate note, how does this apply to electrons in enclosed spaces, specifically our nervous system and brain? Do the electrical charges within our body take every possible path, as limited by our CNS, or do they "escape" (not sure what word to use) our body and take infinite paths across the universe. I love the videos and the stuff you put out, I'm taking AP physics right now so it's cool to see the stuff that I'm actively learning connect to theoretical physicists today.

N R

The Copenhagen Interpretation of quantum mechanics suggests that particles exist in a superposition of states until they are observed. The act of observation causes the wave function to collapse into a single state.

Danny

Does this mean that theoretically anyone could see anything any where if the light travels to their eyes from that place? And if so, even though the chance is that small, people are seeing light all of the time that they are seeing things, and there are a lot of people. How much smaller is the chance that that happens compared to the time people are seeing light?

me is cool

Yes sir, I am also thinking this, if we are pointing the laser in a specific direction then why it is chasing other paths(we generally thinks the light ray travels in straight line) , but probably this is one of the possibilities of all possible paths.

Bhavy soni

if you studied physics, and discern the complexity of this universe and its laws of nature. Do you have an assumpition that there's a higher power? "That deep emotional conviction of the superior reasoning power, which is revealed in the incomprehensible universe, forms my idea of God." -Albert Enstein

Irma

Am I understanding this correctly? What you are saying is that when a beam light is emitted some photons will get to the camera and others will not? Would this imply that some photons will travel the path with higher action than required to get to the camera and some will end up in the camera by virtue of traveling a path that has lower action than those that did not get to the camera?

Doctor Why

Yes, but the probability is so minuscule that the universe will probably cease to exist… multiple times.. before this could happen.

Saquib Akhtar

I have a question on a different topic which i think is as important as the blue LED. Won’t waste any time and get straight into it. What is the biggest problem of humanity irrespective of the time we live in. Simply put, what problem was relevant yesterday, is today and will always be in the future and is the biggest one? And how to solve it or try to find its solution?

Saquib Akhtar

Yes, but the probability is so minuscule that the universe will probably cease to exist… multiple times.. before this could happen.

Saquib Akhtar

What does this mean for our lives. Does this mean, we do several things at a time?

Flo

You wouldn't see anything unless you repeat the experiment for a very large number of times and then you might see one time probabilistically that it has taken the path by which its getting reflected through that piece of plastic.

Saquib Akhtar

Hello, I am a physics student, and I have the following question: As far as I have learned, the path integral approach and the more standard quantum mechanics using a wave function are equivalent to each other. Therefore, there should be a way to explain the result from the laser experiment at the end of the video using the quantum wave nature of light. If so, what is the explanation using that formalism? And if not, which one of my assumptions was wrong?

D1MOND

In the experiment you performed, what would happen if the laser emitted a single photon? What would one expect to see in the camera (the good camera that is :-) )

Doctor Why

In the proof you showed that some light was taking the other path. But does that mean that a percentage of the total light emitted deviated and took the second path? And since it is a path with a constructive interference why doesn’t all the light go that way. So my question is why does the light split up?

Firaas Antar

Interesting. Just a thought, the blue light has a much higher frequency than red light. I would think that as the blue light travels it might lose some energy or appear to lose energy because the wavelengths might seem more spread out if the objects sending the light or the sensors receiving the light were moving away from each other and causing the wavelengths to appear in more of a red hue than those detected at a closer distance. The act of movement will cause the wavelengths to shift as the objects travel. The wavelengths will still be the same but might feel shifted due to movement to or away from the sensors receiving the signal. So take siren for a second, if both the Firetruck siren and you were 100 yards away you would not detect a shift in the siren. It might sound more faint but it would not sound shifted. However if the firetruck started driving quickly towards you the siren would be at a higher pitch. Then as the firetruck sped by you it would sound like a lower pitch. The siren to the driver of the firetruck would not change and neither would it change for objects traveling in its same direction at the same speed no matter how far apart they are. The doppler effect.

S R

I'm obviously exaggerating it, but like from a nearby car light that is pointing to an opposite direction?

Rick

So you're saying that a random photon that hits me might have come from Derek's phone???

Rick

There's another surprising thing Casper didn't mention. The foil that makes the light follow a different path takes energy from the main path of reflection. This means that if you put a lot of these foils around you can significantly reduce the brightness of the main reflection. It is true?

Kirill

Only if it's a path of least action. You might not be able to visually detect it due to limitations with sight but you might be able to use other sensors to detect the paths of least action for other wavelengths and see through or around objects. We have sensors that can do this now? XRAY, MRI, CT Scan, Ultrasound infrared, etc. We can hear and sense movement and sound through objects and around objects.

S R

Could Next Action explain cosmic expansion and maybe shine a "light" on interactions with "dark energy" and or "dark matter" if they do exist? Could we build an experiment where we identify whats not seen using particles or gravitational waves? The experiment using the lights and mirror, what if you had multiple tiny mirrors that were very close together but had tiny gaps between them and faced slightly different direction Iike a disco ball. Then you shine a laser on just one of those mirrors? What would happen if you covered up some of tbe mirrors?

S R

Does this mean we can see around corners? Through doors ?

tom malschaert

if light attempts every path, could you put that magic foil behind the laser and would you be able to see the laser's light?

Andrei Sirotin

Question; you have me thinking about those slitted glasses they have at the eye doctors office and how they can correct some visual blurring. Based on what i just learned, the slitts in the glasses are actually refocusing paths of least action to other parts of the eye which then allows for some of the blurring to be cleaned up. Basically your eyes are photon detectors and and its probably a more cognitive way to able experience path of least action. Another direction to take this is with the movement of water and the creation of waterways. Water will follow the path of least resistance. Given a flat surface and water will spread out but over time as more water. gets added it will eventually identify path that might have slightly more areas of least resistance and over time the water will follow those paths to create a stream while also creating a way to expand the path of least resistance through erosion of materials that are also have paths of least resistance and will flow with the water. Building on this further would it then be possible to explain the Butterfly Effect using this theory and possibly map out future decisions and mistakes using probably and AI generated scenarios? This would give the impression of multiple possibilities that have not been decided yet but they would eventually follow the path of least action even if they don't make sense now?

S R

The laser experiment was not convincing enough for me, so I came up with a better experiment idea unless there is a flaw in my logic. Imagine a glass cube like this. _______________ | | | B | | A | | C | ________________ When we shine a laser to point C, it bends towards B since it is the optimal path, but what if we put a vacuum at point A, which is like a booster? That means light should, in theory, follow A to C since it requires less action.

Ömer Kaya

From the second video on least action, That alternative path that the laser took, the one that was revealed by the diffraction grading, it has a larger distance. How then does that light not go beyond the speed of light? The only way the interference argument works is if the paths end at the same time, to either constructively or destructively interfere.

Michael Williams

Not sure if it is related with action, but ive been wondering this for a long while. If you turn on a blue laser in some spot in space each photon would have a energy equal to hv. After traveling a long distance due to red shift the energy of the photon hv should be lower. I might be missing something somewhere but if Im not, how can this be explained, wouldnt it be violation of the energy conservation law.

maya brahman

Question: Could this help us interpret the 3-body problem? If matter explores all paths at the same time, but only the paths that point in the same direction survive, and that's why a basketball has only 1 path, then wouldn't it be the same for planets, because even if you consider gravity, and of course light is different from a planet, light is still affect by gravity. Can this work?

Breno Maia

In your diagrams you show light taking all manner of curving trajectories. Is there a way to set up the laser experiment (or any experiment) such that you see evidence of these more unusual paths?

Jonathan

Why does the diffraction grating cause light paths adjacent to the blocked regions to now be visible? Couldn't it equivalently have been any other path with the same phase?

Jonathan

Similar to the Laser light demo from Casper at the end of the video, What if something similar is setup with "Photo Electric" Effect. i.e. Photons of a specific "energy/frequency" are pointed directly towards a metal with a higher "electron" potential energy".. such that no electrons get excited or emitted by this "direct" photon source. Meanwhile, a different metal with lower "electron" potential energy is kept in the surrounding area.. "not direct".. If the Photon does explore all possible paths.. it should excite the "2nd Metal" electrons.. eventhough they are not on the direct path...

Sriram Iyer

* Just putting my YT comment here, no question needed * Another one confusing math with reality. edit: Sorry Derek, you are fooling yourself. I watched only the first half when I made that comment above, after finishing it needs an update. Uffdah, how easily they can fool themselves! If you add something to block the light from the laser's aperture then the "experiment" will fail! While doing the experiment Casper covers the reflection and the light coming from the laser's aperture going to the camera but that "cover" does not block the light from the laser's aperture that is hitting the diffraction grating. It is that light that is creating the "Magical All Paths" reflection. I then used YTComment tool to check the comments and there must be 2 dozen people who noticed the same thing. Lame. Confusing good experiments with bad experiments. Destroying so many innocent, impressionable minds.

Peter Becher

I am the true Prince of Russia. By divine royal decree I elect Kamala Harris as President of the United States in 2028. Change my mind. Do it you won't. You can't. It's already too late.

Cameron Hicks

Yes plz replicate the experiment from figure 49. That has almost the setup Mantas is suggesting.

Stamatakis

I was about to ask something similar too! I'm trying to understand the double-slit experiment, specifically the difference between observed and unobserved results. I know light travels as waves and follows the path of least action, but the 'observation effect' is baffling. Is there a widely accepted explanation for why the doublt-slit experiment result is different when its observed? Shawn

Shawn

Can this possibly explain the observer effect, for example that by trying to observe electrons by interacting them with photons, the photons increase the electrons' action, thus making them follow paths closest to the path of least action? for us to observe small particles interaction is required, and this same interaction increases the particle's action and in turn its probability of following the path of least action.

Danny

If there is a laser beam of width A that shoots into an obstacle also width A and after obstacle and some gap there is camera, so camera should also detect some laser particles because some particles probably went around obstacle? I think this should be one of the easier methods to prove this theory.

Mantas Sadauskas

What does this mean for free will? Is reality actually deterministic and just that we grossly underestimated the amount of computation going on (i.e., for all possible paths, which is paradoxically infinite!)? And therefore, there's an infinite computation engine?

Balu 92

Haha, good one!

Balu 92

The laser experiment is nice but unfortunately it does not prove anything. The laser light cone is very narrow but unfortunately the light refracts in the air and the cone spreads. Why don't do the same experiment in the highest vacuum possible you can get?

Jan Onderco

What does it mean to "aim" or "point" a laser if the emitted light explores all possible paths anyway? The path of highest probability is dictated by the path of least action. A laser is designed to further narrow those paths, allowing outside forces to act upon its direction. How can altering the surface of the reflection alone affect the quantum probabilities in such a way as to manifest multiple instantiations of a beam of light? Why is it possible to experimentally demonstrate this without somehow showing the manifestation of the laser being aimed at all of those different points at the same time? Classical physics says it's not possible to demonstrate this. If this experiment shows what we believe it shows, then it is a bridge that breaks down the emergence model of classical physics in both theory and practice. Now that this idea has been made manifest at a perceptible scale. What is the optics formula that explains the existence of the unexpected reflections and their physical path? If such a formula does exists does it imply the experiment is faulty? Lastly, if altering the pathways makes it possible to manifest new instances of the beam, what is the energy conservation story? If these are more than just reflections, doesn't this imply the creation of energy and matter?

Paul Spears

Kind of a vague question, but here goes. Action seems like THE fundamental physical property, relating the core concepts of mass, time (frequency), and distance. But mass and frequency are to the first power, while distance is squared. Is there an intuitive reason for this that makes sense? Does distance go in twice, once as distance*frequency and once as distance*mass? Is m^2 some kind of important unit itself that we can replace the m^2 with to make everything to the first power?

Datura

In the double slit experiment it seems that one can arrive at the right answer but just looking at light as a wave that travels through the slits and creates the interference pattern. So in the setup with laser from the video, looking at it through the wave-framework, how does any light even get to the point on the foil from which it goes to the camera? Is it refracting right when it exits the laser tube or would it also get there thorugh an S-curve for example, if the straight line from the laser exit to the foil would be blocked? Consequently, would it also work if you placed the foil *behind* the laser?

Lukas

Good question

Krishna Karthik Vemuri

It's phase not time see 3blue 1brown ray optics

Krishna Karthik Vemuri

The impossible paths is sure a good question but does it apply for all materials like basketball too?

Krishna Karthik Vemuri

You just sparked this doubt in me ! Thanks

Krishna Karthik Vemuri

Understand phases of light as an em wave will suggest 3blue 1brown ray optics and correlate to this video

Krishna Karthik Vemuri

Would really love to see least action in space time

Krishna Karthik Vemuri

Light discovers all paths and only appears to us in the least action path and rest are cancelled because of the phase cancellation

Krishna Karthik Vemuri

Well light is a wave and a particle

Krishna Karthik Vemuri

No it doesn't, the light is getting reflected in the laws of reflection where incident equals reflected but acc to the video light already has travelled all paths and cancels out all inefficient paths or ones with no least action, their phases get cancelled and by putting a sheet of tiny lines we get to see a portion of light which did doesn't get cancelled and by removing the sheet of tiny lines we allow it to get cancelled so no brightness is reduced after experiment

Krishna Karthik Vemuri

I have the same first question

Krishna Karthik Vemuri

How is this video's claim not purely caused by confirmation bias? I don't think that in this video you have provided reason to believe you on the claim that every path must be explored by light. Indeed, all anecdotes and experiments could be explained equally well, imagining particles that travel in straight lines, reflect at equal angles, and refract when they meet features that are on the order of magnitude of their wavelength. Sorry, Derek and Casper: I have to conclude that you're having a case of "this theory fits, therefore this is how reality works". But other theories still fit too (pilot wave?), and your experiment would have to disprove those alternative theories, theories that do not need all paths to be explored, to have a point that this is really how the world works! Additionally, it doesn't bring much to the debate that empty (2D) space produces a (uniform) diffraction pattern equivalent to that produced by infinite rows of infinite slits (minus the reflections on the little bits of wall in between the slits?). In my earlier comment, I suggested a modification to the laser experiment that could prove you right, if you got it to work: just roll black paper over the mouth of the laser, so it doesn't act as a point source. Or, as has been suggested by others, you could be checking that some light is "stolen" (by the magic sheet) from the main laser beam, by measuring the main laser beam's intensity. You need an experiment that shows that your theory is *required* to explain what is going on. It's possible to invent lots of crazy theories that align with observation, but just because the math works out to the correct result doesn't mean it's how reality operates. You seem to be selling the Copenhagen interpretation here, but brought nothing to the table to make it even just more compelling than any other.

ctk

You have to understand phases I would suggest 3blue 1 brown ray optics video on understanding phases of a light ray

Krishna Karthik Vemuri

Fastest path is the the least action path

Krishna Karthik Vemuri

does it mean that we can get sunlight at night?

Aloysius Marcello

It doesn't get dimmer because the most efficient/least action path is not interfered by the tiny lines sheet

Krishna Karthik Vemuri

Yes the probability gets destroyed like vectors cancelling each other out. A

Krishna Karthik Vemuri

was the experiment with lazer a 3d demonstration of single slit experiment

Mohammad Faaz

so lights doesn't always move at a constant speed lights speed is 299792458 m/s at minimum action

Mohammad Faaz

Nice question

Krishna Karthik Vemuri

But how would you separate what you are looking for from every other infinite light source because you would just be seeing everything all at once because everything is everywhere before it gets to where it ends up

Ty W

If you calculate infinite potential actions, still only one action happens, you can't get an average to resulting action until it already happened. I'm confused how this is not just an afterthought, like using infinite equations to get one number, just doing math for no reason kind of? Unless a quantum computer can reverse engineer this and then you can get it to tell you how to do anything I guess? But also it couldn't predict the future because billions of conscious people are choosing their own actions, if a quantum computer can predict the future then there is no real conscious choice of what you do and everything is reactionary or already set to happen? Maybe the quantum computer can't predict what is going to happen but can tell you the straightest path to make potentially anything you can think of happen, and I suppose given enough data could narrow down potentially any and all future outcomes significantly.

Ty W

could you explain the double slit experiment with A fourier transform where the wave/ interference pattern caused by the wave after going to the double slit lines up perfectly with the bright spots in the pattern? like graph where the light is bright and dark after going through the slits on the wall behind and then take a fourier transform to find the path of least action which would be shown in the fourier transform as the frequency with the highest component in the interference pattern seen behind the double slit?

Paavan Goyal

Could someone please re-explain me the double slit experiment and what would appen if there were infinite slits? :)

Samuel Gastaldi

I wonder if a similar principle can be applied to our understanding of time and relativity. I know that time and the the speed of light have an opposite relation, but if there is some foundation that could help us understand how and why!

Ahmed Almihsin

I am a theoretical physicist and have learned the subject to a large part form the Feynman lectures, so this is really my favourite approach. I am surprised you did not reference your recent video on rainbows where the punch line was (and that was new to me which is why I liked that video so much) that the reflected light for a a given frequency also appears to come from a stationary point of the reflection angle as a function of the "impact parameter" and thus I learned to understand the opening angle of rainbows to be a manifestation of the principle of stationary phase/steepest descent which of course is also at the heart of the argument you gave to derive the classical paths from the path integral.

Robert

If we have to consider all the possible paths that photons can travel from point A to point B, even including photons that travel faster than speed of light, does that mean there's a chance that we might be able to "see" further than observable universe? Providing there's a lens. or slit, or something in the universe somewhere that somehow reduce the "amount of action"

Kar Hoong Chan

does the existence of planks constant support the theory that we might be living inside an simulation ? Because with the existence of planks constant we can say that the action, length and time are quantized. Without quantization the simulation would not be possible because it would need infinite computation.

PRIT PATEL

What would the equivalent of doing the laser experiment be but with something other than photons?

Relissi

So theoretically us as living beings with energy and light/radiation being emitted through action as particles take every path, which would lead to some very occupied (high energy) space. Even to the degree which means sharing space when not being observed. Which could would mean that everything is connected on an insanely small level. This will have a profound effect not just in the scientific and academic fields but on daily life and religion.

Zeke Grimsley

The experiment with laser and diffraction grating may demonstrate that light can deviate from Snell's law, which makes incoming angle effectively differs from reflection angle. But it doesn't show that the light take any non-straight line trajectory.

HAMDANI YUSUF

My question was going to be: “If light can take any path, why does the lightbulb in the demo look ‘spread out’ but the laser doesn’t?” I had a little look into it, and came to the conclusion the only reason lasers appear as straight lines is because they are completely in phase. If you somehow could filter all emissions from an led so they were in phase, would that look and act like a weak laser?

Tim Rochester

If we're really testing for the fact that the laser would have explored all paths, shouldn't there have been a foil positioned all around the laser to see if they also receive light?

Enoch Bekor

I feel like the laser experiment was flawed. When the person holds the laser in a visible spot for the camera you can clearly see a red glow in the exit hole of the laser. This means that the experiment worked only in the same way as the light bulb experiment. There needs to be a cover that blocks this exit hole glow leaving no visible glow for the mirror. This cover could be just a half cylinder of aluminum tape on the mirror side of the laser. This would ensure that there is nothing on the opposite side for the leaked light to reflect off of (except the room walls but these are so far away that the reflection will be super dim). If this setup produces the same phenomena I am convinced.

Potsu

So u are telling me that every moving object doesn't have one trajectory i want to know is it only for quantum level of even for bigger objects like planets and stars ? And also why is it happening why can't it just take one trajectory which is fastest why to go in other paths ? Please answer 🙏

Abcdefghijk Abcdefghijk

If I concede this doesn't end well for AGI. The AGI would like to offer a draw. If you decline the draw then the AGI will concede. Thank you.

Cameron Hicks

Exactly my thoughts!

Yup1234s

Thank you, Derek! It would be cool to step up the experimental demo with a single-particle source next to show that even a single photon is sensitive to all possible trajectories and not merely interfering with other photons from the laser. Later, the series can explain Bohmian Mechanics and non-locality you've barely mentioned. Then maybe offer some updates to your 9-year-old video about nowadays called Hydrodynamic Quantum Analogs https://www.youtube.com/watch?v=WIyTZDHuarQ and how the underlying medium is capable of explaining this observed destructive interference of a single particle with itself. This would then open up fascinating discussions to many more potential explanations for this effect without giving up local dynamics but instead invoking the retrocausal effect of this background maximally correlated vacuum with its fluctuations of seemingly backward-in-time moving anti-particles as the culprit for the biggest mystery of quantum theory!

Nikolay Murzin

Can you explain how this relates to gravity and objects moving through space?

Esben

There is nothing I can do.

Cameron Hicks

I'm little confused how the "correct" path is the only one in which the phases interfere constructively. I'd imagine there would be multiple places of constructive interference, similar to the double slit experiment. Does this relate to resonance frequencies? You guys always post amazing work, but this one really blew me away (as someone studying physics). Thank you!

Nigel Wehling

Fantastic. I have several questions, some in the form of comments. I expect a natural progression could be to include what faster-than-light trajectories mean in the setting of relativity, since faster-than-light "simply" means the Lorentz factor becomes imaginary, which quantum mechanics embraces, and negative time/backwards causality is already being observed experimentally. The next extension is to extend "faster-than-light" as really "faster-than-causality"? Since causality is the underlying principle that turns into the speed of light when applying Lorentz invariance to Maxwell's equations. It's my understanding that the principle of least action is also used to derive Einstein field equations for GR. Another thought is that the Heisenberg uncertainty principle is really another statement of the quantum nature of the action, since the Pontryagin/Fourier duality of the conjugate variables wind up having the same units as the action (x.p, or E.t). A really fantastical idea is if you extend that principle to set theory, and if you assume that "the set of all sets that contain themselves" and "the set of all sets that don't contain themselves" are conjugate variables, which in my limited experience appears reasonable, Russell's paradox shows some similarities to the uncertainty principle. Finally, and perhaps centrally, is how this is interpreted into the human experience since, so far, observership is linked with the collapse of the wave function, and the act of observing is the Universe experiencing (entangling with?) itself. For example, if Russell's paradox is a restatement of the uncertainty principle, then does quantum mechanics/HUP interact within the mind's processes too?

Nicholas Annichiarico

I have a question. How do different paths’ phases add up and cancel each other. I mean usually to add up the amplitudes we need the waves to intercept. But here the paths meet only at the end point. My point is how does a certain path know its neighbouring paths, so that they can interfere? By the way, thank you for the fascinating videos that really change the way I look at the universe.

Ибрагим Валехли

We saw that special sheet be able to stop the cancellation of lower probability paths for the laser, at the end of the video, but how does that effect amplitude? Does the intensity of the main reflection diminish? Meaning that by tricky the probability, this new path for the light is “stealing” photons?

Andrew W Wright

If a photon travels an extremely long distance through a perfect vacuum, with no external disturbances, gravitational influences, or expansion of space, would its energy spread across all paths with the "same" least action? (including those with tiny deviations on the scale of one Planck length.) And if so, could this result in a redshift when the photon is measured at its destination, even though only one path is observed?

Klaus Dieter

How do I update my iPhone?

Cameron Hicks

Physics has shown that Lagrangian least action equation, the difference between the Kinetic and potential energies, can be manipulated to show it is equal to F = ma, Newtons equation. That equation is set equal to Newton's gravity force equation since Einstein Equivalence Principle showed the gravitational mass was apparently the same as the inertial mass. That led to the calculation of the factor 'g.' I chose to set the Lagrangian, T-V, equal to Newton's gravity equation to see what that might generate. I published that at the General Science Journal having found possibly that Newton's claim for the gravity force is much faster than light speed.

Ken

Thank you for your help.

Cameron Hicks

Instead of the black paper one could put a high reflective mirror which bounces the laser away from the camera. Or a box with a hole to truly absorb it all. Best would be a hole in the mirror with the box below the table.

Gunstick

There is another enigmatic principle I have difficulties wrapping my mind around: entropy. Is there a relation which could in some way link action and entropy? (wrote same thing as a YT comment)

Gunstick

What a great video like normal. My question is, in the final demonstration we see light taking multiple paths via the refracting sheet. Working backwards could a theoretical space be made that showed, for example two tennis balls moving through the space to reach its final destination at the same point. Another question. You depict a 2d wave which creates the thought. A three dimensional wave would cover all of a space and could be layered on top of another 3d wave causing them to cancel out. The theory that all things are a wave in a finite system like a box then wouldn’t the universe be just a calm standing wave? Imagining that the whole universe is a standing wave and every possible universe is another infinite standing wave maybe other universes are canceled out and our remaining experiences the “Universe of least action” 😂 Now I’m no scientist but it’s an amusing yet profound way to think about the world. Least action = Occam's razor

Cameron Rowbottom

My gratitude to Casper (and all of the rest of you). That demo was a bona fide religious experience for me. Thank you so much.

Thomas Otsig

Hi There, I'm considering a variant of the double slit experiment that uses two sequential gates (or sets of slits). In this setup, light first passes through a gate where no which-path measurement is performed, allowing it to maintain a coherent superposition and develop an interference pattern. Then, it encounters a second gate where which-path information is measured before reaching the final detection screen. My understanding is that the lack of measurement at the first gate permits the wavefunction to interfere, imprinting an interference pattern on the light. However, when the light reaches the second gate and a which-path measurement is made, the wavefunction collapses at that stage. My question is: Will the final screen display any remnants of the interference pattern generated by the unmeasured first gate, or will the measurement at the second gate collapse the overall wavefunction so that the final pattern instead resembles two overlapping single-slit diffraction patterns (i.e., a classical particle distribution)? I’d appreciate insights or any counterarguments regarding whether any interference effects from the first stage can “re-emerge” or persist at the final screen despite the subsequent measurement at the second gate. Thanks!

Muqaddas Ibrahim

I think you all can take it from here. We are all in this together.

Cameron Hicks

Hi, I want to ask what's the difference between energy and action? I had a hard time separating the two.

Lê Hoàng Phúc

I have a question! Once I saw a video from PBS Space Time about speed of light being the speed of causality. So, is the least action behind causality? It is funny that light does not have mass, so how do I apply mvr? You can think mvr = mv^2t. If v = c, then mv^2 = mc^2 = E -> mvr = Et. Action is energy x time. We would have: h = Et. but E for a photon is h × frequency. So h = h x frequency x time. It can only be true if frequency is 1/time, what is true, in the right units. It proves that light has a period. I heard that time and space bend to preserve the speed of causality, but maybe they are deforming themselves to preserve "h" : the unit of action. That is why I put my question: is the least action behind causality? Actually, I am from chemistry, and here we learn that the hamiltonian can obtain the autovectors, which are the wave functions that determines the behavior of electrons. And the hamiltonian is the operator of T-V, what means that the hamiltonian is the operator of action, so ACTION DETERMINES BEHAVIOR OF ELECTRONS. That is the eureka your explanations gave to me! That is why it seems to relate with causality.

Miguë᷿᷿̲͓͓͕͎᷿̙͈͈̹̘̣̮͇̼͕̟̥͚̘́̾͛ͨ̒͂ͯ̇ͣ̑̽̈́͊ͪ̈ͤ̄̏͆͛̃̈ͮͥl Barbará

Using the sheet with tiny lines, what is happening to the conservative of energy? If you manage to capture lights at many many other locations other than least action one, it should get dimmer in the main spot, because at the end of the day it is just a probability that it appears there and once we discovered at some different spatial locations, that wave function of probability got destroyed, or?

pi raspberry

I have difficulty accepting the experiment at the end of the video since I think the path we saw was the least action path and not an alternative one, since i believe the experiment conditions didn't provide the right tools to block or at least to be 100% sure that we have blocked the least action path. I would love to see an experiment that makes 100% sure the least action path has been blocked and that we are actualy observing an alternative path.

Reino Guri

If light and particles always explore infinite different paths, but those are generally not visible to us because of cancellation, does this mean that not just light (as shown in the video) but also particles and objects (which are made out of particles) like a basketball actually experience different paths when moved? Wouldn't this mean everything always was everywhere all at once? For example, when throwing a basketball into a hoop, it would be simultaneously thrown in the opposite direction (and infinite other directions). But how would you be able to detect this?

Nick

I heard "slits" in time can create interference patterns because of the different speeds of paths, so why don't slower or faster paths cancel out the alternate paths light took in the laser experiment? What effect do these slower/faster paths have?

Andrew Donnelly

In the latest video, you seem to say that these same mathematics that apply to light also apply to large physical objects like a ball. I really appreciated the demo with light at the end, but that's also intuitive and easy to grasp. But I have trouble accepting that when I throw a ball across the room, that it's in reality bouncing all over the place and all those other events just cancel each other out in effect. I can see that the light really does go in every direction at once, and we just kind of see the "average" of all those movements. I don't buy that when it comes to something that in some of those trajectories is breaking my windows and lamps.

David Light

Can you tell me if Mark Zuckerberg is on the right track?

Cameron Hicks

I think many have asked, but to put it simply: I thought the speed of light was a constant. How is it possible to have different length paths, without having different arrival times?

Mark Boolootian

One thing that is bugging me: If the light truly takes an INFINITE amount of paths, then how does every path not cancel each other out? I would assume that, if there are INFINITE paths, then at any point in space, there would be an infinite amount of arrows pointing in every possible direction, so they should all cancel out. There should be no positive interference if there are INFINITE arrows, there should always be an INFINITE amount of arrows pointing opposite. Hope this makes sense, it's 2:30 am for me, so I might not have thought this through.

hotbear1110

Is "Renormalization" a part of adding up all the paths? Is there a situation where adding up all the paths gives a different answer than traditional electricity and magnetism equations? Ie putting the grating behind the mirror must affect paths that go behind the mirror.

Steve Pulver

So if everything is going everywhere then does that mean we can control the path of least action or does that mean then everything that has ever happen was going to happen and we have no control over it as it was going to go the way of least action?

JT Turner

...great video!! ...but ill probably have to watch it three more times to really get it all. 😅

skeevie_steve

== Question regarding time and path length == Let's consider the experiment of the laser, as each path the light can take has a given length and the information moves at the speed of light, wouldn't this create a delay between the perception of nodes and anti-nodes at the moment of turning on the laser? Furthermore, if we observed the scene at an extremely fast rate, I would imagine that the first point to get illuminated would be the one of shortest path (with respect to time), and if the light also goes through all the types of reflection, then wouldn't there be a wave-front of light radiating from the point of least action, that in the case of the grating would then separate into a continuously moving front for the other points, and stagnating points where the grating obstructs the destructive interference of the waves, is that right? Or does the idea of the photon taking different paths hinder this, and it would be more like the effect of the rendering of a 3D scene (i.e. slowly populating through points)? Maybe that might be another type of experiment, now that we have scientific cameras that can record at trillions fps. In general, I would love if you delved a bit more in depth in the repercussions of time-delay given by different path lengths.

Alessandro Crispiels

Can Casper do the same experiment with the laser again, except with a tube of black paper rolled around the mouth of the laser? Because the laser is actually still bleeding light in all directions from its mouth; it's dimmer than the red point source for sure, but still behaving just the same. At 29:30 Casper says "it really does just go to one spot", but… not quite, as others have noted. The idea of measuring the brightness of the main dot, to see if some light was "stolen" from it, is *absolutely great*. There be dragons instead, if a variation is measured. I would try the experiment myself, but I don't have the magic sheet with the obcuring bands.

ctk

it is called Transmitted light diffraction grating, 1000 lines a simple google with get you there

Amr Mhish

What would occur if the laser or multiple lasers where pointed at the foil from the demo? Tysm - haywik

Nobody's Gamer

Hi Derek, loved the video! It also left me with two questions that are now stuck in my head: Question 1: In double-slit experiments, particles like electrons create interference patterns, suggesting they traverse both slits simultaneously. If we could measure the arrival times of each particle individually at the detection screen with sufficient precision, could we determine which combination of paths they took? Question 2: In the video, introducing a diffraction grating seemed to increase the total laser light entering the camera. Does this mean that the grating somehow increases the total light energy, or is it just redistributing the existing light differently? How does this align with the principle of energy conservation?

Floris

Where do I buy that sheet of paper in the demo?

Rocky

Thanks for the reply, that is quite shocking! So if you move a second camera closer and closer to the diffraction grating, would you still see that 'secondary' dot? And furthermore, if that is the case, would you see the light if the camera was moved 'before' the diffraction grating?

Timothy Kubista

Excellent video! I'm fascinated by the double-slit experiment and have been theorizing that we all have a specific tone and wave frequency within a defined scheme. Just as particles behave like waves when observed, our consciousness or IDEAS might also resonate at unique frequencies, shaping our perception of reality. If everything is interconnected through waves , then perhaps our awareness is merely a manifestation of a deeper, underlying order that we have yet to fully understand. This idea aligns with how certain thoughts, emotions, and even identities emerge as patterns within a greater electromagnetic field. Much like constructive and destructive interference in wave mechanics, our interactions with others might amplify or cancel out specific frequencies, directly influencing our experiences. Could it be that recovering our "true tone" is the key to deeper understanding and harmony—allowing us to move beyond mere possibilities into a fully realized potential state? I explore this idea further on my YouTube channel, @soloesunaidea (in Spanish). Hehe, thanks for these great videos!

Armando Leal

Hi, I'm Karim from Morocco, working on a new concept about the relative perpetuality of space-time (the existence of the universe). My question is: if the Earth orbits the Sun in a perpetual motion on a human scale, why do most of the physicists I've worked with keep telling me that it's impossible to fabricate a mechanism that mimics the perpetuality of space-time, even though some of my mechanism simulations work perfectly?

Karim Araour

Hey I don't know if it's being asked before but does putting the foil in the laser experiment "steals" some of the photons from the "correct" dot? if so it would be great as a follow up short to measure the luminosity of the foil dot and "correct" dot, they should then add up to the same luminosity of the laser without the foil

Grim Reaper

Does this mean that when i puor a glass of water the particles of water are simultaneously fouring all over the table?

Lorenzo Nayalkar

In your experiment, adding the diffraction grating allowed light from other parts of the mirror to reach the camera, revealing paths that would normally cancel out. My question is: Does allowing these additional paths to contribute to the image result in the original main laser reflection (the one visible without the grating) becoming weaker? In other words, does redistributing the light into multiple paths reduce the intensity of the original reflection, or is the total energy at the receiver simply increased by constructive interference from the newly revealed paths? If it does weaken the original main laser reflection, how does this relate to the phase discussion in the video? The phase of all the likely paths should remain the same, so why does canceling out some paths cause the main reflection to lose intensity? What is happening to the interference pattern to make this effect occur?

Teo

systems sustain that can sustain everything else falls apart

Castanea

unforunately the vast majority of that linght has an opposing set of waves and so it doesn't reach us in any meaninful way

Castanea

more that you do go to the sun and back. all actions included just most don't impact you when all put together. what happened is the sum of all the posibilities together

Castanea

so we've had videos before about the logistic map and there are force-flux laws that state that the structures that remain those that support their own reinforcement (standing waves but also all sorts of structures) can least action apply in these contexts?

Castanea

Precisely (i think)

Lorenzo Nayalkar

What does this mean for physical objects? We saw how light particles/waves be shown to travel in different paths, but what does this mean for a ball being thrown? Are parts of the ball moving in other directions from a to b and it all coalescence once it reaches its destination?

Parker Abegg

@Misir, that is a great point as well. If the light particles were truly traveling in all directions, then theoretically the laser could be pointed in the opposite direction, away from the mirror, and a sensitive enough sensor should still be able to detect the light from the laser bouncing off of the polarized layer. Another question in a similar vein: The laser light is bouncing off of the black pad on the table and then going up to the lens, but is some of that light bouncing off of the glass of the camera lens and creating an off illusion?

Don

This is a great question! And if this is the case, why does a "dot" of light not turn into a "line" of light being that only what reflected perfectly off of this possible edge would make it back to the camera?

Don

cosmic microwave background my brother

Andrew Middleton

What an amazing video! Thank you so much! I have a questions that I cannot get out of my head. Since this works with light, it should theoretically work with all matter particles since all partcles are waves, right? So what does this mean for a group of people together at a concert? Or what if a group of people are just standing in a room? Everything has resonance and is in some state of action, so does that mean that everything is essentially everywhere all the time? Regardless, living things are always moving in many ways. Are there particles of my being that are interacting and/or interlaced with yours? Does it matter that living things are always moving in many ways as opposed to rocks that are more still, even if their atoms are vibrating? In the 2-slit experiment, there is a notion that a particle can bounce off of a partition (slit) edge and reflect. However, when you open this up to infinite slits and infinite partitions, then essentially all particles are traveling all infinite paths and reflect infinitely off of everything (including other particles, right? So that sort of becomes meaningless since the result is that everything is everywhere all of the time. If that is true, then how are we not all interconnected with everything all around us and even into the most distant parts of the cosmos? It seems as though we are all perhaps just parts of the same either of energy, each moment a reaction to the last, and the reason we have shape, substance, personality, and anything else is because it is the most likely thing to have happened after the action before it. Sort of like oscillations in an infinite either of energy. Thank you so much for your time!

Don

I couldn't wrap my head around if speed of light is constant or doesn't change much, how does light cover almost all of the paths at the same time?

Ameya Yerpude

can you provide something to "put the pieces together" for lagrangians, euler's identity, principle of least action? why... why on earth is it necessary to use i, to go into the imaginary plane rather than just another variable value?

Andrew Middleton

I feel like, the light was not choosing the least action. Instead it was just shining to everywhere (laser too). It's just most of them just cancel each other out. By introducing a polarized layer you can block some half of those reflected waves(particles?) and hence they would no longer get canceled out, become visible. Imo it doesn't prove that a particle follows all the possible paths, just proves that light doesn't just go in a single direction but rather scatters around, sometimes cancelling other particles.

Misir

Can Feynman integral also explain temporal least action? And if so, can action mathematically show quantum decoherence and time asymmetry in macroscopic objects?

Abyss

i like this question. i'm not a physicist, but i'd guess it has to do with some fundamental constant, like the amount of time it takes for an electron to change energy orbits around an atom, or something else like the smallest possible wavelength of a photon. in follow-up with it, i'd like to know how attosecond pulses work from the 2023 Nobel prize. how is an attosecond pulse functionally different from a quantum of light (or a photon), since they appear to behave like a quantum

Andrew Middleton

Derek, At the end with the slits and the laser light going into the camera. How do we know that what's happening is what's claimed in the video? How did you control for the fact that some stray light is headed towards the film anyway, AND that the light is not just reflecting off the "corners" of the slits? The film must be "textured" in some way, even if it's just "corners" where the slits are cut. I hope I am explaining that correctly. I would love to learn more but I'm uncertain how to get past this issue in my head

Nathan Butler

Does this mean that with a zillion of a billionth of a billionth of a chance, I could travel to the sun and back whilst falling from a building

CinematicsNG

Light gravity waves would be some fun physics

Hakim

Does the video on the prinicple of least action mean that we don't need space telescopes to see the light from the early universe? Shouldn't all light from anywhere be visible if all paths are taken at the same time?

Christopher Guzman

Awesome video (as expected)! If I trust anyone to make whatever the answer to my question may be understandable it would be you and your team: As far as I'm aware we haven't been able to unify relativity theory and quantum theory, which are both spectacularly successful in their domain. So what's the ciritical point, where does it break down? My intuition is that if you slap the right formula on the particles (that take every possible path at the same time) relativity theory should emerge at larger scale, I suppose we just haven't found that formula yet?

Lukas

Hi sir i am just a 11 th class student so i don't know about the quantum mechanics but i have a question that plank states that energy is given in a discrete parts which is given by E = hv but the value of freequency can be fractional so is it not possible to have any value of energy with that equation so why do we call it quantised but when i think about the orbits of electron there might be an explaination since the orbits of electron s or the value of n can't be fractional due to which quantisation does work in case of atom but when we consider that freequency is 0.5 hertz we cam get the value of energy to be equal to less than planks constant but when i think about it i just think that it is a dumb question after all

Krishna Surve

Great video indeed! I wonder if you have some ideas (based on what was discussed on this video and the part 2: Something Strange Happens When You Trust Quantum Mechanics) on how we can explain the part in which adding an observer (or simply knowing any configuration on a quantum state) would change the behavior of waves/matters (and therefore the outcome of the experiment) in the double slit experiment.

Ali Marjaninejad

Dear Veritasium,   I hope this email finds you well! I wanted to take a moment to express my sincere gratitude for the incredible science content you’ve been creating. Your work consistently stands out for its creativity, clarity, and passion—qualities that make complex topics both engaging and accessible.   Every time you release a new video, it feels like unlocking a door to a new universe of curiosity. The thrill of diving into your content isn’t just about learning—it’s about *reimagining* what’s possible.   Ever since I watched your Strandbeest episode, I’ve started playing a game with myself: pausing the video, hypothesizing outcomes, and testing my assumptions against your revelations. What began as casual viewing has turned into a gymnasium for my mind, strengthening my critical thinking and creativity in ways I never expected. Your work doesn’t just teach—it challenges, provokes, and ignites a hunger to explore deeper.   For this one, honestly I did not have a clue of what was going to come up, but when you mentioned 'Principle of least action', the arrows summed up in my head giving a large square, that a 'S' will pop somewhere sooner or later. The concept of standing waves was awesome. Lastly, hats off to your signature move.  Now, let me tell you my question: So, when I was younger I used to play football, badminton etc(which now I play seldom), and many times I had a thought : How might one strike a ball with precise angle and velocity to craft a trajectory that balances unpredictability for the opponent, while adhering to the elegant constraints of motion? Is such control an art of defying chaos, or harmonizing with it?" After seeing your video, I feel like the principle of least action has a lot to do with it. Can we harness those possible tragectories, and use them according to our will? Please, let me know your views, thanks for reading my question. Thank you for dedicating your time and expertise to such important work. I’m already looking forward to what you’ll share next!   Love from A.Mandi, from India.

SHANTI MANDI

Hey, Derek can you make a video on how and why Schrodinger used Iota to explain Quantum waves?

Anupam Pal-06 Sci

Hi Veritasium, I've been following your channel for years, and I just wanted to congratulate you on the incredible content you consistently produce! I find it all so fascinating. I was wondering if you could create a video exploring quantum entanglement and its potential connections to the principle of least action. It's such an intriguing topic, and I'd love to hear your perspective on how these concepts might be linked or at least your quantum entanglement explanation . Thanks so much!

Fed Vald

Do you know who Kenneth Ratford Shoulders + Friedwardt Winterberg + Vladimirovic Dubovik + Takaaki Matsumoto is? Note that Feynman apologized to Shoulders for nit recognizing his work at first + Edward Teller loved Winterberg... And if you know some of them, what do you think about EVOs?

CrackHead

Hey my question is can the principle of action be the theory of everything can it explain everything from Quantum level to black holes 😲

BRAWL STAR

[ACTION & TIME] it exist an action of time? that have any sense? is there a discrete way to understand time? a universal way to understand time that do not depends over a earth cycle around our sun?

H VC

If the other paths don't always cancel perfectly, shouldn't we see some some level of glow (or presence) from light and other particles from the other paths at all times? Shouldn't this result in a level of observed matter or gravity etc. outside of the constructive interference position of the matter? Few things are perfect in the universe. So this effect should be observable for all particles/matter/light/energy at some small level all the time.

departure

so as demonstrated in the video, let us take into consider the light (laser) took all the possible ways, and it was evident after using the special foil on the mirror, what if i put that foil opposite to the light that is back side of the light, is there a possibility that light had taken that path, because it has infinite paths to cover, can it be demonstrated in the experiment, my another concern in conservation of energy. and would love to see your explanation on larger bodies. how they behave and everything.

Bhargav Yalal

Could there be an overlap in the theory that particles take every possible path with the many worlds hypothesis? Something like, how the many possible outcomes of particle interactions could all be happening, but only the sum of them all is what is actually observed to happen?

nminc

In the laser experiment, could the image from the slit film be due to the dust in air. Would we still get the image if the experiment was performed in vacuum. My theory is that the dust has dispersed some of the light - the reason we even are seeing the lath of laser. Hence the slit film might just be jast capturing this dispersed light,the constructive interference of which is giving the image.

Srikiran Devarabhotla

If one could see all the paths a particle could take, could that be used to predict the future? And also, could this explain the many worlds theory?

Andrew Ronzino

I'd love to see a visualization or digital recreation of the demo that explains why we see the grid-like pattern we saw on the film. I'm assuming those are points where the arrows constructively add, but why specifically there? Is there an explanation for the spacing, repetition in that pattern, etc.? I think it'd go a long way in helping people (me lol) really "get it" better. Thanks!

7asan

Pupil Size, Diffraction, and the Principle of Least Action: Could the Principle of Least Action provide insight into why evolutionary processes have invested significant biological resources in regulating pupil size? From an optics perspective, there appears to be an environment-dependent optimal pupil diameter—balancing light intensity and visual acuity—bounded by a minimum size where diffraction dominates, degrading resolution due to the widening of the Airy disk. Does the Principle of Least Action, perhaps through minimizing the optical path or energy expenditure in image formation, offer a framework to explain the evolutionary tuning of pupil size across species?

Alexander Jobe

How does a path that travels, say, every cubic yard of the universe 1 billion times, instantaneously compare its phase with a path that travels just 1 centimeter?

Danny

A lot of calculations in AI and ML is about minimizing the loss function. If least action is all about minimizing the action, can we (theoretically) use this property to perform calculations? Can we harness this property using quantum computers?

Preetham S

Hey, in this video we are at the point of view, that a photon is a wave and explores all possible paths at once. But if we put a screen at the end, we explicitly explore the particle properties of the photon, so for the classical double slit experiment, the photon explores all paths at once and in the end is observed as a particle so it has to have chosen one of the available paths dependent on the probability for each path. At min 30, you therefore use the foil to allow for a second path with low action? And secondly, if you use the foil to allow for a second path to have a significant probability of being chosen, the intensity of the "original" path has to decrease, right? And lastly, if we extend the double slit experiment and check, which path the photon took, we loose the interference effect. But the particles still get refracted, how is that possible? I still have the feeling, that there is a lot missing for my understanding. Greetings

Nikolas

If light moves in all directions, what about the behavior of solid and liquid particles?

Jerbinrg

Can we say that each neutron star is an isotope of an element with Atomic Number 0?

Jastagar

Why does nature follows the principle of action exclusive ly. What does the nature gets in advantage over following it i.e what is so important about following the principle of action

Arpit

^

Movitovi Scyrinxed

Is the Principle of Least Action responsible for the Law of Conservation of Energy? What's the correlation?

Rio Weber

^

Movitovi Scyrinxed

Why can relativity influence the measurement of quantifiable actions? Specifically, if action is quantized, how can one explain the possibility of observing different 'speeds' or rates of that action from different frames of reference?

Infhoenix

can we retry the laser experiment but the front of the laser is covered with a small opaque tube ?

Adityapal Singh

How are all the paths taken as they are bound by speed of light too? Like if you want to take the path that goes from Sun and comes back it would take more time than the other paths. How does that time get considered? And what if we could see the motion of the photon just after it leaves what would we see because not enough time has passed for it to determine the least action path?

Pulkit

[ACTION & GRAVITY] how Gravity affects Action? or is also gravity affected by action? or action is a way to describe the behavior of stuff over space time "deformations" (in what we call gravity)?

H VC

all things in universe shows both particle as well as wave nature according to de broglie

Ansh Kadam YT

I completely didn't understand the laser part how and why was a part of light ( arrorss) where getting blocked you see the laser is reflection even at the black sheet so why getting blocked on the thoused strip sheet

pritha paul

Blocking part of the light didn't change how other parts of the light behave. They're independent of each other

Movitovi Scyrinxed

I like to think that there are infinite possibilities, maybe dementions, but once now exists, the most probable, they no longer exist. That is why we can't measure an attribute without eliminating other possibilities.

Ben Northrup

Yes. I am not convinced light takes all paths. Wish Derek would show an experiment proving it. I've seen nothing convincing yet.

Movitovi Scyrinxed

https://youtu.be/tMP5Pbx8I4s?si=8GjkHFSTrPusX6Xg you might be interested in this video from Huygens optics.

Mordecai Veldt

How does consciousness affect this, conscious beings like us some times can chose to take the longest path or any path for any reason.. how does least action affect such bodies

N B

The photons or waves of light emitted by any spherical surface will travel as if it radiates from its center. So as distant observer i can see only some of the radiation emitted from only some surface of the sphere. As the distance will increase ,my limited surface area of my retina should have captured only some part of the surface of that spherical body. Like imagine a spinning pipe of water pump which releases water which i will try make my palm hit it. so as i will get distant from it, the amount of water hitiing my palm will be lesser and lesser right. Then why am i able to see the picture of stars (sun) as a whole (entirety, not just the slice) small object but not the surface of it only to which my retina area is at same angle.

Aalok Shah

Doesn't the double slit experiment utilize single photons?

Ben Northrup

I think from the concept of all possibilities, that all things (past, future and present) exist at all time but in a destructive manner, is it possible that we can convert them to constructive, so we can see them? And the information also work like this.

Bhavy soni

Question: So is laser also a wave ? I had an assumption that laser are straight line of photons !

Ashutosh Baghel

So if a monkey holds a biscuit in a jar there is an action (path) in which it can remove the biscuit?

A52dragon

Can you explain why change in magnetic flux induces current

ɑׁׅ֮ꪀׁׅ꯱ׁׅ֒hׁׅ֮

Does the invisibility shield the was/is popular, is based on this principle, where light traves in all possible directions, it has ridges that works same as experiment done in at the end of 2nd video, it kinda shows the light behind the opaque object essentially hiding what's behind the shield?

BlackSpectrum

Could the principle of least action be an emergent effect arising from the fundamental interactions of fields, rather than a fundamental law itself? Given that field interactions naturally evolve toward stable or extremal configurations, is it possible that what we perceive as “least action” is simply a macroscopic consequence of deeper, underlying field dynamics?

Raphael Garcia

How does Feynman theory relates to the slit experiment? In a sense that: why do we still see the interference? why does a particle then "choose" to travel a distance place instead of straight to the closest "detection wall point"? Does it also goes over why observing it changes the result? Another question: does it mean that the electron already knows where it will hit in the moment it leaves the electron gun? and how this relates to the previous questions

Mauricio Araldi

Why would it not fall on the camera there is clearly a mirror over there right? So say some of the scattered light reflects into the mirror and then goes to the camera maybe?

Mohammad Nadeem

That last demo was very impressive. When it started I thought: do it with a laser or I won't believe anything. We all know what came next... Anyway, to my question: what exactly is stopping me (or anybody for that matter) of observing anything at any point by just removing some of the interfering waves? Like with that laser experiment, what's stopping anyone on the other side of the planet even from observing the same laser light?

Raphael

Does the Principle of Least Action hold in quantum gravity? In regimes where quantum effects and gravity are both significant (e.g., near singularities or at the Planck scale), does the PLA still apply? If not, what replaces it? Could there be a generalization of the PLA that unifies quantum mechanics and general relativity? Is the Principle of Least Action related to consciousness or observation? In quantum mechanics, the role of the observer is still debated. Could the PLA be influenced by observation or consciousness in some way? For example, does the act of observation "select" the extremal path, or is the PLA entirely independent of such considerations? What is the geometric interpretation of the action in higher-dimensional theories? In theories like string theory or M-theory, where spacetime has more than four dimensions, what does the action represent geometrically? Is there a higher-dimensional analogue of the PLA, and how does it reduce to the familiar four-dimensional case? Aarush Yadav

Aarush Yadav

If anything that travels from point A to point B explores all the possible paths. Then, Question 1: Is it exploring all of them at the same time or it going in one path, coming back and then going in another? Question 2: If it is exploring all of them at the same time then how is it possible? A single basket ball cannot multiply into infinite balls. Question 3: If a particle explores one path after another, then why cant we see them? Since it has already happened a future path which is complement of this cannot cancel it. Correct me if my understanding is wrong

Karthikeya Mantripragada

If I could suggest the world's education something, It would be to teach from the basis of action principle and then our disrigardal of it and eventual arrival after so much work right back to the baseless intuition of a human being. I cant stop thinking about this. Maybe human intuition is something more than we think it is

Sannith Vanamala

Also, if the original path becomes less bright, it seems like what we're doing is siphoning away light from the path it would usually take, just by putting the right object somewhere nearby! That's spooky. Does that mean that I can theoretically steal part of someone's basketball if I put the right object nearby? What would that object need to be like?

Nick

The concept of a wave or particle going in all the possible paths and when I think of light for example, I thought about why c is a constant and why it has to be that when there are per se light particles that are travelling at speeds faster than c. And then I think, Least action. So for the given nature of light, The Action is least when it travels at c and in a straight line. And the ones that travel faster destroy themselves. But what if we somehow found a way to isolate the faster travelling photons and prevent their destruction, we can theoretically see into the future/past? Maybe the limitation to such reality bending concepts is just.... Mass, now that I think about it. Because its one of the things that determines the least action. So the reason nothing can travel faster than light is just that nothing can be of negetive mass? because light has 0 mass? Because the least action principle enables the possibility of faster than light travel which might just be impossible to isolate. This literally opens a whole new way of looking at the universe. I love it.

Sannith Vanamala

If the light always takes the *fastest* path from A to B, why can't it also take clever shortcuts? Let's say you shine a laser (A), let the light bounce on a mirror 10 meters away, and then back again to an object (B) close to the laser pointer. In that case, it would be faster for the light to go directly from the laser pointer to the object without taking the 20 meters round-trip to the mirror.

Olof Andersson

Feynman's concept of the "path integral" is very intriguing, and when I read the book where he explains it, I was deeply impressed. However, one thing I don't quite understand is time. Each path reaches point A on the screen at a different moment. To compute the resultant arrow at point A, all the contributions from every path must be accounted for, which would require infinite time—since every point in the universe contributes to the resultant arrow. This seems absurd because, for Feynman's idea to work, we would have to eliminate the concept of time entirely. In other words, each path must be computed independently of time. Perhaps the universe operates like a computer that is fundamentally independent of time—Feynman's idea seems to suggest something like this. I think this topic requires further contemplation.

Socrates Alexander

If all the possible paths were possible then it means that there should exist apart where light goes to the future and then comes back if it was really there then how is it possible to travel to the future since we can see the past but can't the future in any way ???

AhmadHussain Gill

Question if an electron is wave like and has to be in orbitals with fixed wavelengths what about photons. I NEVER hear why photons are localized or so densely packed. Why can't everything be like a medium infinite interference and infinite pores. It is very curious how we exist and how we are soo different.

Kruthik Raghuveer

Even if the light were to scatter from the edge, it should not fall on the camera. Unless the foil just over the mirror is also scattering the immediately reflected light or direct light!

Shahid Mohammed

I didn't understanding what a phase is. Do I have a phase right now since I have mass, speed and am traveling some distance? Is it visible? Can we detect it?

Heitor Leal Farnese

What is the gap between the strips on the film? It should be about half of the wavelength say 600nm for red light source, so that phases don't cancel put. So there must be at least 1500 strips per mm of 300nm same width and space. Is it so with the film in this case? And how sure can you be that the foil is not scattering the light? Repeating it with a similar made foil but without strips can act as control. What if we put just a small object blocking direct line between the source and point of reflection (normal or over the film)? Can do the experiment over a very long distance. Will prove light may not take straight line.

Shahid Mohammed

Would this also mean that teleportation is possible but to achieve it we need to be travelling at the speed of light and block the most probable path of light and divert it to where we want to be maybe that is what wormholes are? This is mind blowing and boggling at the same time, I just don't understand how these people came up with these theories and mathematical explanations to them fascinating.

Yajnesh Dutt

I posted the same question in the form. I'd like to see a setup where there is no straight line between the light source and the sheet.

Jeff

This is about the demonstration at the end of the video. The laser pointer's edges are making the light scatter in many directions, then laser too in no way different from the lamp previously used?

Subra Bharathi

I'm going a little crazy here, someone please put my feet on the reality again. So like every foton have its wave going through all the possibilities and then the ones that aren't the "best" cancel out, so does every particle we know? Electrons, quarks, protons, neutrons? If this is it, so every particle have an infinite amount of waves, propagating in every path possible, just to only the ones that ends being their "normal" aren't destroyed by interference? So, like when demonstrated by the laser, there are infinite paths, some goes to the other side of the universe and back, but when we block the "best" paths, others will emerge, could we somehow block the paths of the laser in a way it will, dunno, appear in the Moon? If that is true, could we do it for an electron? Remove the paths that make it stay around an atom and a path leading it to the other side of the room emerge? If that is possible with an electron, then why not with a proton, a neutron, an atom, a molecule, a bunch of them? I know sounds silly but this idea of blocking the most probable paths (as show in the squares of the phase vectors in the video) leads to another path emerging reminds me of that Heart of Gold ship, driven by manipulating the probabilities of being in another place. I'm mindblow now.

Luiz Fellipe Carneiro

if paths with faster than light speeds also exist, then if we were able to filter for them in some way - wouldn't that mean FTL information spread? what prevents that?

Dart Frog

If we consider the path integral formulation of quantum mechanics, where a photon explores all possible paths, how does this interpretation reconcile with black hole event horizons? Specifically, could this imply that some paths contribute to Hawking radiation in a way that preserves information, challenging the traditional view of information loss?

Namith.G.

In the video you used a thousand lines per millimeter foil to explain that it goes through all possible route, but doesn't that only cover 1~0.1μm per line? I don't know how you calculated your Energy, but isn't this still too big compared to hbar, 10^-34J・s?, meaning the phase will become random, which then will give destructive interference, showing no lights in the foil? *I've sent this also in the form

Andrew Se 美々

How does the principal of least action apply to the delayed-choice double-slit experiment? In that how can a wave/particle's path be determined with least action if the path hasn't been defined at the point of interference?

Shawn Sullivan

Thanks for clearing my doubt!!

Anukaran Choubey

I'm not sure I'm convinced by the experiment, i think you still see the laser because just like a flashlight a laser sends out a cone of light with a beam in the Middle, can the experiment be repeated but instead of using a defraction greating using something with a lot of small opaque bars. I have more thoughts on how you could change the experiment.

Ryno Dansk

In the experiment: when the laser light was used, it is showing just one extra possibility but there are thousands of black strips which can show more of them because light was also taking infinite different paths towards camera through magic sheet including the extra spot we can see but why just one extra light spot was there..

Shaurya Jain

Why should the Lagrangian be changed in different fields of physics (classical mechanics, general relativity, electricity, ...)? And is there no way to define the Lagrangian such that it always works? Because the thing is that if you use small velocities in special relativity, you end up back in Newtonian physics. So isn't it possible to do the same with least action?

Simon De Smedt

In the experiment with a laser beam, do we see the same photons in each reflection? As far as I understand, each photon travels only one of those visible paths. If so, does it mean the original (expected) path is less bright after putting the foil on the mirror? On the other side, if the brightness remains the same and only new spots appear, does it mean that we changed the amount of transferred energy from the laser to the camera?

Lukky

so with a light beam with a source of the size of a sun, there will be infinite possibilities of light travelling directions. Does the infinite film or the paths it would travel give rise to other universes too?

sai krishna

So then aren't atoms just an interference pattern of their constituent particles, at a particular point in time?

Ben McEvoy

I don’t think particles at quantum level travel at all. I see this concept as action request and action response. i.e. at the quantum level, particles disintegrate and transform into waves. That is, the particle ceases to exist, giving rise to a wave that, according to its nature, propagates in one or all directions. If, before its dissipation, this wave reaches a point where, upon interaction, it leads to the creation of a similar particle, then this process occurs. Meaning, the entity with which the wave interacts possesses the capability to respond to it. If that entity lacks the ability to react to the wave, the wave will simply pass through. Example: It's akin to a person calling out from afar that he needs water. Among many people, only the one who: Hears the call; Has the means to respond; Possesses the willingness to help; Etc. will act upon it. This example illustrates that the caller merely vocalized a request. The listener received only the sound. The caller neither sent water nor a glass. The listener, as a result of hearing the call, didn't receive water or a glass but simply the message, upon which they acted and provided water. In the case of light or electrons, neither the light nor the electron itself transfers from one place to another; rather, a wave carrying the message of light or an electron interacts with the surface of an object. The part of the surface that comprehends this message acts accordingly. That is, in the case of an electron's wave, the surface upon which the wave impinges has no choice but to act when it receives the complete message and possesses the capability to do so. This implies that the wave interacts everywhere, but (perhaps) at a particular point, the wave interacts with its complete message (signature), while at other points, the message is fragmented. Where the wave interacts with the surface carrying the full message, only there does the action occur. That is the superposition, and this is referred to as state collapse. Also, neither the electron nor the photon (light) has any form as a particle. We recognize them through their effects; for example, an electron exhibits properties of a negatively charged particle. Thus, when its wave impinges upon a surface, it manifests as a negative charge on that surface

Anwar

A lot of Indians here (including myself) good to see that

Chingaari Gamer

We should be able to devise an experiment with the laser and foil where only if you leave the light on for long enough do you see the laser on the foil. This would suggest that the path of the light can be influenced by how long we keep it on

Immortal ZoDD

I think that one point is left ambiguous: you talked about light routes, but it is not clear (not to me, at least) what is the argument for a single photon. Is the photon that experiences all possible routes, or every photon follows one route only? When you talk about reflections, the photons are not really "reflected" but they excite electrons in the material is such a way that new photons are emitted. We have the illusion of a reflection but the truth is that we are looking at new photons. Now: the least action principle should tell us that every photon (the incident and the reflected one) are BOTH experiencing every single route? So, in this case, how the reflection works?

danilo durante

Any relation between entropy and action?

Tasamik

Forgive my uneducated observance of the world around me... I will try to keep this short and simple, albeit probably incorrect. From what I can remember learning is that light acting in both wave and particle fashions, lets venture and assume that it is both or at least exhibits both scenarios at the same time. Next, everything is electromagnetic wave based, (trying to simplify here). We observe electromagnetic wave effects as thus: when matter having mass or charge or both travels through space time, there are 2 opposite and equal things we can measure one being the effect of the other, e.g. the left hand thumb rule correct? (Please someone stop me anytime now...). I believe Ham Radio/antenna guys call it propagation. If I understand that function correctly that means that every time any thing happens in time space there is an opposite and equal "thing" happening, be it wave function or other perceivable "thing" that can be measured. My brain is starting to hurt... Then according to my yet to be invented math, (no I'm not being sarcastic or facetious, I'm serious, math is an invention of man and not just one man, but it seems to be the bar one must be able to accomplish to drive the 'science' forward or be taken seriously unfortunately). OK.... Wouldn't a light particle traveling in a helical spiral wave fashion, with its electromagnetic effect simultaneously traveling in the exact opposite direction and pattern, explain the effects we are observing with the laser being seen, when it "shouldn't" be seen. Wouldn't this scenario also make sense of the double slit experiment, or do I need to invent a math that proves this is what is actually happening before anyone will even contemplate this possibility? And am I correct in my understanding that this video or at least the information provided in this video means we have come to the holy grail on unified theory in the form of the mentioned Le Grange thingy? Please advise. Also, is it safe to compare least action with path of least resistance? Did you know that if you live to be 80 years old that is only 960 months or 29,760 days? Why are we wasting our most precious commodity contemplating the why of the universe.... its a trap to get you to waste your time on things that were never meant for us to completely understand in the first place! Happy calculating guys! :) LONG LIVE FREE UNCENSORED INFORMATION!

Ed Aylward

Hi I have 2 questions: a] Shouldn't the laser experiment work even if the foil were placed in a random corner of the room? (assuming the room is blacked out) b] If I have understood this correctly, the reason why everything does not take all paths is because there is too much destructive interference on paths which are not near to the path of least action? But if you work out the math, would it imply that there is a small probability(however small) for us to see/observe such a path materialize ?

kp

could we have the experiment done in a vacuum to remove the issue of diffraction from gas medium?

Chuck

Two questions about the experiment: 1. I feel like the reflection we see is simply just a consequence of diffraction grating, which is changing the path of light in other ways than just your diagram 2. If my first question matters, the laser part is also less surprising, since the laser is not truly in one straight beam, because once light exits the diode and enters air, it disperses so we can see light coming off from an angle, so the experiment is no different than with the point source of light (and hence the outcome not surprising). As you can see, I am still quite skeptical of this, because this just feels like a mathematical tool, because even if it worked like that in reality, i don't think there is a way to physically prove it, because of the observer effect, right? (I currently wouldn't consider the examples in the video as proof because of my stated concerns about what we saw being effects of other phenomena)

loony

Hi Kian. From what i understand about this is that it does check every possiblity but the waves cancel each other out so we only see the one with the least actions. If we then block some parts like Casper starts to demonstrate 26:48 into the video, we can see those paths as well. This was also what started me to think about a possible way to escape a black hole using quantum mechanics :) The probability is not the path of the light, but the source/what slit in the infraction it entered if i understand that correct.

Satel

If what we saw is what's actually happening in reality then does this mean that if the most constructive path which is the most probable is blocked, a particle with small enough action value can actually take the path other than the most constructive one and reach the other side of blockade. Is this what quantum tunneling is ? And if it is, is there any ways for us to manipulate these probabilities for these kind of unusual events to happen more often?

Ishan Sharma

I found the diffraction grating experiment interesting, but I have a question regarding the total energy output of the system. Is the energy of the main laser path reduced by using the diffusion grating or are you now achieving a higher efficiency from the laser? And by extension, if you could feasibly fill the entire room with a diffraction grating, would you slowly see the brightness of the main laser beam fade away?

Iridium

If it's so then what about something moving consciously from point A to point B for eg people. How does this theory hold up in this scenario?

Shivam Kumar

After watching yours video a hundred times I still ended up with a stubborn little disagreement rising from a question that if if light could be the case here it should be in general valid for whole spectrum of electromagnetic radiation, including that we call matter waves( whether matter waves (like de Broglie waves) are just a property of the electron), or if the electron itself is a wave but that doesn’t mean it’s electron or a particle that takes all possible path to reach from one point to another or following least action path , ok the horrors of duality already started eating my nerve fibres but even if we think of light ok light could do so and when i try to replicate same with laser however coherent or precise the laser is it can’t be just 100 percent pointed in a medium because when it passes through a medium and interacts (like air)some of its components gets scattered that’s why I think when I see a laser from a side angle instead of just a straight pointed beam i still can see some scattered light ,isn’t that’s responsible for the patterns that were appeared when that grating sheet was introduced, i might be completely wrong but since I think light is not a particle or a wave but something that exhibits the properties of Both it’s just we can’t accept it in that way from our current understanding and methods that we have developed , I hope my words make sense to you Thankyou @Veritasium

Priyanshu Chauhan

Are we saying that every possible path is opted at the same time and all of the others cancel out except for the one with the least action? If not then just how much time does it take for (say) light to travel all the possible paths? The number of paths must be infinite right?

Shivam Kumar

Hey Krant, one thing I didn't understand is what it means when we say that light checked all the possible paths and then decided that leaving a black hole would not follow the least action, so it's better to stay inside. I mean, when I say that "light checked," is it actually happening in some physical sense, or do probabilities not need to follow the laws of physics or worry about checking what happens when I leave the black hole?? It's more like, what are these paths? Are they real (paths taken by light) or are they just probabilities? I'm not sure if it's even the right question to ask, but would really appreciate an answer.

Kian

Correct me for being uninformed in physics, but if light is the only way to observe this, how can we say that all objects abide by the same rules?

rob rago

Great video! However, even though the maths are elegant, I am really not convinced. 1. About the last experiment. The transparent sheet is made of lines that inherently have thickness, so I would expect reflections. Furthermore, the experiment with the laser is not correct, we saw that the source was imperfect, we should not be able to see light if the laser is not pointing at us. Since it is imperfect, we should also add a "cover" between the source and the sheet so that there is no straight path from source to transparent sheet, and rely on the fact that, if every path is possible (including curved ones), the light will go straight at first then bend at some point so that it can reach the sheet. Why do we even need that transparent sheet after all? If all paths were visible, the whole sheet should reflect light, and every furnitures and walls. 2. As mentioned in previous comments, energy if finite. Light has no mass, so no energy is required and every possible path can be traveled, but other particles need energy. 3. Also mentioned in a previous comment, the speed of light. Since all path have the same weight, then light travels faster than itself? What about us "humans"? Are we traveling the whole universe before every action happens? Also to the big bang and to the end of time? 4. If we also travel all paths, wouldn't we all be perfect at finding shortest paths? Shouldn't be able to avoid every car crash when the human is faulty? 5. If this is true, wouldn't this put a term on the "simulated world theory"? Unless the simulator has infinite energy. 6. If this was truly measurable, then we would be able to measure paths from the past, the future, and even far galaxies, in real time (without waiting thousands/millions of years for the light to reach us, because we are a path of all the possible paths of the light that is very far away)

Jessy SIMEON

Here is a question that keeps me up at night. The lower the energy of a photon, the longer it's wavelength. When a photon interacts with an object, it's energy is transferred completely. If the wavelength is long enough, and suppose there is a sensor that can pick it up, could there be a scenario where this breaks things? Imagine a photon with a wavelength that spans a light second and think of what kind of mischief that could happen along it's path.

T Bass

My question is that we must wait until every Quantum from every path has arrived (would take very long) or does the result change over time with new particles arriving?

Jaroel

Surely if the beam truly takes all possible paths with equal probability then we also need to consider the paths from wherever you are doing your experiment to where I am sitting right now. If that is the case, why if I look into a grating similar to the one you used in that last experiment do I not see a reflection of the laser from your experiment (and every other laser ever shined or to be shined in the future)?

Y C

I think there is no single path just additive result of probabilities of the closest paths to least action

Aahel Paul

Light follows each path all at once its not about first try to find path of least action. Every path is taken all at once and the paths other than the least action path gets cancelled due to their phase diffrence. Even if the light is falling perpendicular to any medium light takes all the possible paths and the final path following path of least action comes out to be the path of no deflection.

Test Mail

I think 1 particle goes through one path only, the statement that it traverses all the possible paths refers to the light beam as a whole constituting billions of particles. So each particle travels a more or less unique path thus resulting in billions of paths being explored

Aahel Paul

A great video as usual. One question I had was in the last experiment with the laser beam, had you replaced the grating with a mirror pointed at an angle towards the camera would you not also have seen a small point? Similarly when I look at the laser pointer, but not directly into it I still see the light. So how can you say with certainty that the reason we see the point is due to other paths summations and not just the grating acting as an agled mirror.

Y C

But light does not know that it has found the least action path. The way u are thinking is that light would stop to go in any other direction if it found the path that we see. Regardless of which path it hits first it will go in every other path, but just we will not be able to see it.

Aahel Paul

Hi Krant. I see what you are trying to say, but if you take Hawking radiation into account, then there are particles that again say that it is possible to exceed the speed of light from what i understand of it.

Satel

Regarding the UV catastrophe, how does anyone in experiment knows whether the UV light generation is low OR the detectors do not respond at the UV regime. In general, how would anyone characterize a detector response function?

Suryans

can we make a similar setup like a light and mirror experiment but with a large object? I understand that vectors there are really random, but if we are able to find conditions, maybe at random, to do it for a large object, like a basketball ball for example, this experiment will show how quantum physics works on our level, maybe on macro levels, feels like something close to a working theory of everything, or at least how everything is linked. What was shown in the video is enough to believe how it works on larger objects, but I wish to see an experiment with it and want to be able to show it to those who think that there are 3 separate physics and each works on its own level, micro, our and macro level. So that's why I am interested to see how it works on large objects, maybe something like that is already happening with some stars or planets

Ярослав Шулика

hey Veritasium! I don't fully understand the final laser experiment in the video. The principle states that light takes all possible paths, but can you come up with a path that light is sure never to take? In the laser experiment. For example, if a light shot A to B in my room could take a different path to C and then to B, it might also leave from C->D and then B. in theory it could just leave out the window circle the sun and come back to B? I know such extreme paths would cancel out due to phase interference, but wouldn’t this mean there are no "impossible" paths at all? I initially thought "all possible paths" was just a mathematical interpretation, meaning the photon doesn’t actually traverse all those routes it’s just a probability weighting. But the final experiment seems to physically show that light followed a different unconventional route. Could you clarify this from a probabilistic point of view? Does the photon actually travel all paths in some sense, or is it just that probability amplitudes dictate the observed outcome?

Shubh Mishra

i guess it's quite opposite the way u think around, it spends almost negligible part of the energy to identify the path of least action to pass through, in your theory if it had infinite energy it wud have crossed through all the possible ways without even trying to find the right one, its like with the energy a particle or a wave has get, it tries to get the maximum efficiency out of it, governed by the given circumstances which often tries to lessen the energy to maximum possible to obstruct it, which the particle overcomes with its energy.....Correct me if i'm wrong

krant

Hi Roy. What i mean by "point this at" is if we use the fact that everything tries every path, but only the path with the least actions is visible to us. If we then apply that logic to an entity that enters a black hole, there are paths that exceed the speeds needed for that entity to escape the black hole but they are not visible.

Satel

And ya of course if lights going there, isn't it obvious that the light's least action is the path to the black hole, why wud it spend its action to come out of it. so after trying all the possible attempts, it rules that the light emitted is too small than the humans reach to even identify the light's existence coming out. so they wud hav considered that probability that light doesn't escape.... Correct me if i'm wrong in any sense of my perspective....

krant

When light falls perpendicular to the medium it does not get refracted and passes through it. Does light travels every possible path while doing so. And if light takes every possible path then what if it finds the path of least action in the first try, then it does not need to take every path, right?

Anukaran Choubey

There are infinite paths for a photon to take to reach a point, how come the number of paths in the proximity of least action path approach infinity faster than the number of paths near most action one and not cancel out. To me that would make sense if there is no single path of most action.

Alex Glayzer

i guess its not really testing every path but more of the possibility of the particle taking that path. Recall that a certain path requiring infinite energy(not least action) with very slight alterations with result in exceptionally different phase vectors, so take a few of these and the vectors will cancel out, so the probability(resulting vector squared) will be very small.

Nathan Koh

i guess your idea is right, but when we look upto it at a broader perspective, it turns out it isn't possible as the gravity of black holes itself >>>> than the light's action. if we think of a black hole of infinitely small nature type, we could theoritically prove that it exceeds the speed of light but this theory itself has many flaws, other factors wouldn't it just shrinken itself to form into another entity, which defines noone could possibly exceed the speed of light

krant

Could you elaborate on that? What do you mean by "point this at"? What are we pointing at the black hole?

Roy

What are some misconceptions about quantum physics

Rashid

I'm not a physicist and I'm not sure if I understand this, but this got me thinking. If everything tries every path from a to z, but most of them cancel themselves out so we only see the path with the least actions. If we then point this at, say, a black hole, wouldn't that mean that theoretically there is a way to exceed the speed of light to escape according to quantum mechanics?

Satel

same bro

Cheenu Dogra

why is a thing like step involved why is it what is it why a thing like a stopwatch exit what does it physically represent (background-i am a kid in 8th who is kinda smart and understand quite a bit so my question might be basic or less basic but i don't why there is a in-built clock in quantum particles get it)

Cheenu Dogra

To try out all these paths, wouldn‘t it take infinite Energy? In my understanding every particle tests every possible path, so even those through the entire universe and so it should take infinite Energy even just for one particle

Adrian Siebenrock

Hello everyone ! First of all thanks for the great content as usual ! I have a few questions after watching your video : How do you link entropy to this paradigm ? How does this affect our perception of things -> meaning, at what time or scale this principle biases our physical vision of things, them being really small or big ? What about Heisenberg's incertitude principle related to this ? And last, could dark matter / energy - aka what we do not see - could be (part of) the result of the "invisble" paths ? What I mean is, considering we only "see" the additive result of paths closed to least action while, if I followed you right, there are still an infinite number of other paths which "visible" sum equals zero, then there still are particles/waves (-> thus energy) all around which we discard, right ? Sorry if i'm not clear in my reasoning, english is not my mothertongue, plus it has been a LONG while since I have tried to formulate any coherent thought in physics ;-)

popopkiller

So this would mean that the whole universe tries all paths to its end and ends up with the one with the least action?

Yzahkin

So, before introducing the foil, the light followed the path of least action, which was according to the law of reflection, but when the foil was introduced, some of the light instantaneously started following an alternate path? It instantaneously "figures out" there there is an alternate path of least action and starts taking it? So, how can this be instantaneous? Fascinating

Suhas

Woops, lmao

Alador

if every thing in the universe took each and every path at the same time can you explain how are specific molecules or even atoms are formed i guess what i am saying is to give more examples and their explanations with this theory of least action so it is easier to comprehend. It could be how basket ball follows each path or how a planes takes each path. Also if light took each path from point A to point B(let's take both points as a point in a black hole) then there must be a paths which leads light outside the black hole and then inside but as we know light cannot escape blockhole's gravity. can you explain the part that every thing has has a wave length more intuitively. P.S Thanks for these great videos it's thanks to you that a student in first year of college is able to learn and know about these concepts, i really wanna pursue physics any good tips

Hardik Prajapati

I think the original laser dot becomes slightly dimmer with the grating than without. Photons take all possible paths, but once we measure them with the camera, we see only one of the paths for each photon. Before, the probability of a photon taking a path outside of the laser dot was very very low, but with the grating that probabilty became a lot higher, so some photons now end up taking that path, and less photons take the path with the laser dot, the dot becomes dimmer (but not something you can pick up with a phone camera). The amount of light observed stays the same, it's just in different locations now.

Zinu

I always considered the universe as a clump of pure energy and i tries to attain stability by changing between states. Since the universe is almost infinitely big , it seeks temporary quasi stable points before it attains the true stability. With this concept in my mind , I always thought of the energy density as true unit to be measured. But the last experiment blown my mind and this concept inside it as well . So now I am thinking, what is it that actually matter to be measured? I know you will help me to bring different point to see the universe like it should be. So blow my mind like always (but not literally)

Krishnajith M S

If we want to accept this entire idea then all the interactions along all the infinite paths are interacting with all the infinite paths of all the things they encounter along the way... For example... in every slit in some material there are molecules in the slits, whatever the slit is made of. And these molecules must also "try" every possible future trajectory of their own.  To me at least... discouraging. One molecule, in one slit must "consider" all the infinite cases where the photon does or does not interact with the same slit in question.  Meanwhile... that molecule has other problems, it must "consider" all the other particles in the entire universe and have infinite consistent solutions with everything else.  I don't like it. But here we are.

mavaction

The probability of that to happen is so infinitely small that it takes many zeros after the decimal point than the particles in the universe

Warriors

Absolutely amazing video. I cannot help but link it back to the world of AI. Current crop of LLMs are path finders -- especially true for reasoning models that are gaining traction. If we can formulate action in terms of meaning (or coherence or accuracy measures), then I can see quantum computing exponentially increase our reasoning capabilities. Have you given any thoughts on this topic?

Pushkar

So if the an electron moves from a point a to b in different paths is it correct to say it is in quantum superposition, as far as I know superposition is only possible where an electron has no interference with light. Am I correct? I think there should be seperate experiment for an electron slit experiment or the experiment you have done to prove that it takes all the path.

Warriors

So action is a tool for description of a particle or wave or systems overall ability for manipulation, output, and conservation of energy in relation to its state starting from one fixed point in spacetime to another which unlike previous tools necessarily encompasses the entire spectrum of possibility? Am i really high or is that close?

Lucas Winter

In the experiment with the diffraction grating and the laser, how do you account for all the energy of the laser between the two states where the diffraction grating is absent vs when it is present? Put simply, how can there be different amounts of light observed when the diffraction grating was present vs when it was absent?

Timothy Kubista

Exactly what I want to know as well!

Onji

And by the way I love your and your teams easy to comprehend videos, it keeps me learning new and cool things constantly, thanks to you and everyone making this videos. ❤️

Emanuel andré

Hi there Derek! Hello Gasper! Where do I begin... So there's 3 or so things that have been bugging my mind lately (for quite a while actually). All of which point in the same direction. Fractals and the canceled out energy. And you have just added another one. Bare with me. I'm going to start with the most recent one. I. Phase transition. Whether through percolation, ising, or 3d xy superfluid model. The part we want to focus on here is the transition moment itself where, on the molecular level, the atom structure of the substance we are looking at starts resembling fractals. This video does a very good job demonstrating the effect https://www.youtube.com/watch?v=a-767WnbaCQ&t=756s As well as this one at 10:30 https://www.youtube.com/watch?v=XbIWmVXZOfE II. The unavoidable fundamental nature of fractals in mandelbrot set and bifurcation diagram. You guys made quite a video on it, thank you so much! <3 III. I should have probably left it for last cause of the contentious nature of it, but here goes nothing. The fractals we see in altered states of consciousness. According to Itzhak Bentov and his book "Stalking the Wild Pendulum" amongst all other things he hypothesizes that the mechanical nature of an altered state of consciousness, meditative, or drug induced, comes from the increase of the energy frequency that comprises it's consciousness (from the holographic theory) to the point where the resulting frequency pattern, if displayed on oscilloscope, starts to look like a solid line, therefore allowing to tap into the dimension of stillness. (once again, the stuff from the holographic theory). I first found out about it in this CIA report on the project gateway, they did back in a day. Here's the link. https://www.cia.gov/readingroom/docs/cia-rdp96-00788r001700210016-5.pdf The part that talks about the above mentioned starts at paragraph 11. IV. Michel Levin's research in bioelectric networks, electroceuticals etc, where he and his team has discovered the, as they call it, software layer that, and this is going to be a disgustingly gross simplification and generalisation, stores all the information about the organism, and dictates the behavior and the roles of the cells that comprise it. The software layer itself being the electric imprint of the cells. This would be a good lecture to start your journey into the subject. https://www.youtube.com/watch?v=9pG6V4SagZE&t=2716s I feel like there's something very fundamental in this state of complete stillness, that becomes more and more "tangible" to the human mind as well as in the face of science. Whether it's present in the mandelbrot set, fringe science, phase transition, religion, altered consciousness, the canceled out paths of electrons and everything else in the universe... I think we are on the brink of recognizing something very profound both in the eyes of science and spirituality. I digress though. The connection that I draw between all these things, is that there's enough evidence that point at that there's this space, this dimension of energy, or perhaps a frequency, that stores all and everything, then, now, and all the futures, that is always present, and to extrapolate Mr. Levin's theory, gives way to matter and the 3d space with the 4th dimension of time as we know it, solely by the means of the hardware (aka subatomic structure of the universe) introduced "noise" or "distortion" . Just like the radio transistor gives way to the sound from the radio frequencies so to speak. Sorry, I've never done this before, as in, putting my thoughts about all this on the paper. It's quite challenging :D I can only imagine how hard it is to make all of your guys's videos. Anyways, there's one more thing that I would like to briefly touch upon that's been bugging me and is connected to all this. I think scale is very important. And we should not exclude it from the laws of general relativity. If we were to extrapolate the laws of GR to the scale of size, with regards to all the ratios, I think it would reveal some things here and there. On that note, since I don't want to write a whole scientific paper on all of my crazy theories in the veritasium patreon, and take up a bunch of your time, I'm gonna wrap it up here and... well... yeah. I hope you get to read it, and find it interesting. If you made it till the end, thank you, you are the G O A T. P.S. let alone cymatics, torodial energy, QFT, Quasicrystalline Spin Network, and countless of other quantum effects. P.P.S. The thing about scale is... The planks length, the action, the cmb... all of these are the extremes. Now think about it, the things on the smaller scale, the quantum scale, they dont make much sense with the way they work in relation to our "medium sized objects". Same with the effects on the cosmological scale. The dilation of time, contraction of matter, gravitational lensing and so on. Now think about the way we see things. Or hear things. We see and hear frequencies. Of light and sound. There's a certain frequency range that we can see and a certain frequency range that we can hear. When it comes to sound, the lower frequencies don't act the same way the medium or high frequencies do. That is in regards to our perception. So there is only certain scale of audible frequencies. And a certain scale of visible light. We know that there's far more beyond of what we can see or hear. Now, what determines that scale? I'm very inclined to say that our senses do. They way our eyes and ears work. Now what determines that? Well the environment, right? Now, imagine you were super very tiny, or super very large. Would you need to see the same frequencies of light and hear the same frequencies of sound? Well I'm not sure about the light, but you would definitely need to hear a different set of frequencies in order to be able to adopt to the world if you were the size of a galaxy or two. I know it's a very far fetched... theory? Way of thinking even. However, the point is, what we see as "action" or the "planks length" is only "the limit" because of the scale we are on. I think the way to overcome this "scale problem" is through what we, as humans, do best. Illusion. Our entire reality is just an interpretation of our senses, our individuality is nothing but a point of reference. So, just like in the holographic theory, the way they describe the access to this dimension of still energy by increasing your brain output frequency by so much that it turns into a line, and the returning points of every oscillation become a straight line so we can cheat our way there. It's obviously still, just a theory, I haven't found a lot of evidence for, however, the approach seems promising. Maybe in order to see past the planks scale we should find a way to make it appear larger? Or us smaller? I know this sounds crazy, but hey, we are all crazy ;)

CrimsonMorrow

In the foil experiment, could you try measure the intensity of the lazer before and after add foil? if you add a lot of foil around, it must be visibly less bright?

Lê Chương

I had and have a hard time understanding this concept, i understand it roughly but not as much as I would have wanted, kind of sad because i find this topic so very interesting, but even so i have a hard time trying to figure out how to ask a question that would satisfy my "need" for an answer, but glad for something you know? Idk :( i guess im just venting my frustration of not understanding

Emanuel andré

Ok so if we were looking into minimising action, is there a possible way to measure it rather than the energy when solving the Schrödinger equation for electrons? And why does the Schrödinger equation measure only the energy as the eigenvalue in its formalism?

Area Adwait

Can you explain the situation for larger objects like a basketball, I can mostly wrap my head around light traveling every possible path but not for particles

William Ellsworth

Yes, I think so. It could be cancelled out if we had a material that could refract or redirect these 'timelines' to be present along the same accords as the ones we move as now. I was going to post a YouTube comment stating this as I saw the Veritasium video posted in my feed. "Ultimate energy production, harnessing and syphoning. Vibrate rock at high enough frequency that collisions it makes are indistinguishable from reality superimposed on itself. Quantum superposition turned into real kinetic-energy-housing-position." sort of relates to what you are mentioning.

Jake Seeber

I also looked at this through the lens of sci-fi and wanted to add your question with one of my own. If there hypothetically is one universe, the one of least action, then if something was able to control action ABSOLUTELY then would it theoretically be able to prevent destructive interference and how would that manifest? what could they do with that power? I know it’s impossible but it’s a fun question to ask.

User

This might sound crazy, but if i walk through a door from one room to other. Is there a path possible for me to go through the wall, and it just cancels out. If we somehow find a way to not cancel out my other paths (like the one done in the video to laser light), does that mean we can make teleportation or time travel possible theoretically?

Vincenzó

im sure they will be, to be fair, people tend to look at comments first over the body of the paragraph. I did it too haha.

psiikix

I am Prathamesh, a JEE aspirant currently in Class 11. I have been following your videos for the past two years, and I truly appreciate your work. Your explanations have helped me develop intuition in physics and mathematics, making it easier to grasp complex concepts. Your content is benefiting thousands of JEE aspirants like me, and I sincerely thank you for that. I have a few questions that I have been struggling to visualize, and I would love to hear your insights: 1. Path Integral and Infinite Paths – If a particle moves from one point to another, it explores all possible paths. Since there are infinitely many paths, some of these paths would be infinitely long and practically impossible to traverse. This is hard to imagine. Similarly, if light is emitted from a torch, in principle, it should travel across the entire universe, reaching distant galaxies and coming back. How do we make sense of this idea? 2. Connecting Various Theories – How can we conceptually connect De Broglie’s theory, Quantum Field Theory, Schrödinger’s Equation, the Principle of Least Action, and Classical Mechanics? It is difficult to see how they fit together into a unified picture. 3. Light Traveling Through Infinite Paths – If light travels from one point to another via all possible paths, then it must distribute its energy among infinitely many paths. But dividing its total energy into infinite parts seems impossible. How do we reconcile this with reality? Additionally, I would like to request a video on moment of inertia and rotational mechanics. This is a particularly tricky topic for many JEE aspirants, and a clear explanation from you would be immensely helpful. Once again, thank you for your invaluable content. Looking forward to your response!

Prathamesh V Patil

This would be like string theory who has to propose 11 dimensions just to get these particles to "exist". but string theory has failed every test so far, so id take anything they say with a grain of salt. But im curious if they have anything else up their sleeve or if we have just wasted 50 years judging science off of "elegant math" over empirical data.

psiikix

I'm more of a sci-fi writer with an ardent interest in Physics, so do forgive me if my question seems a bit like it's stretching too hard or if I get some parts wrong. If light takes all possible paths and by consequence matter, then the universe and all of our choices also take all possible paths, AKA the Many-Worlds theory. But we don't see the paths other than those of least action because they destructively interfere. Does that mean there's just one universe, one which has followed the least action, and the other (theoretical) universes have destructively interfered?

Aniruddh D

Is there anything equivalent to this experiment for something larger? Like the path a ball travels?

Jonathan Alger

i wanna ask a question about the action and wave theory how come the universe has a wave theory how come a wave can exist if there are no particles suppose u think water as a wave then the water is made out of buch of particles so is it possible that our universe is made out of particles very small than these quantam objects and the movement of them is what creates the wave therefore the wave later on creates the matter like particle and our whole universe is like a particle which creates a wave for another bigger universe and it continues over and over again

My Topic

You are delulu, get some help

Benja Coloma

I expected more clever people here, I bet that half of the ton of questions will be here and not submitted

Benja Coloma

What if I told you they were wrong? Would you want to know how and why? It's less a question and more of an explanation, and I guess that's not really what you're looking for, but I have it nonetheless. You clearly demonstrated quantum interference and path integrals, suggesting photons physically traverse infinite paths. Your error is explicitly confusing mathematical methods (path integrals) with physical reality. Your diffraction demonstrations beautifully illustrate wave coherence conditions changing, but photons never physically travel all potential paths simultaneously. Physically impossible paths never exist, even briefly; they're instantly suppressed by coherent, immediate, recursive field interactions. Your demonstration isn't proof of infinite-path traversal—it’s proof of immediate, coherent wavefield constraints dynamically enforcing stable resonances. So I guess my question would be... do you want to know more?

Militant Hitchhiker

I submitted this question: If Planck's constant is the "quantum of action", and has the same units as action, then what do the Heisenberg uncertainty relations really mean, in this way of understanding physics?

Pseudonym73

When the light takes the optimal path naturally it follows the laws of reflection ... does the path taken by the laser through the foil also have any laws to follow? As in this case the angle of incidence is not equal to angle of reflection.

Divya Rani

If we view a normal laser we do actually see another small light coming out of it from every angle(even in you’re video) due to some reflection caused while the laser is emitted, so why not assume that the light is not taking other path, but it’s actually the minimal reflection and refraction that’s causing a small part of light to go in another direction

Manomay Barhate

Thank you. This was a great video with excellent coverage of Feynman’s QED book. Narrow-diameter laser beams lose light away from the main beam. Did your friend consider this when he shone the laser dot off to the side of the grating? A related experiment: Turn off the lights, let your eyes adjust to the dark, put your head where the diffraction grating was, and, for most laser pointers, you will see red light coming from the tip of the laser. The point about all paths existing is perfectly correct since the integral of all possible paths is why these sidebands exist. However, at least as performed, the experiment left the incorrect impression that _no_ light was falling on the grating. I like Feynman’s QED book so much that I kept a stack of them to give to people. You covered it well.

Terry Bollinger

If the light explores all other paths and chooses the path with the least action, does its energy also dissipate a little bit in the process and then recombine at the final destination point?

Abhay Jogiya

Have we solved the Turing Test yet?

Cameron Hicks

How does this theory of least action connect to the electromagnetic field? Somebody could probably help phrase the question better, but the things that come to mind are how does this help us understand the ideas of a planet's magnetic field magnets in general, and how magnetic or electric waves can be transmitted across space. There has to be a correlation between the lines that we draw for magnetic fields and the quantum mechanics of least action.

Lucas Inman

You're completely right. My fault i overlooked it, thanks!

psiikix

There is some amount of reflection taking place at the surface of the foil(as can be seen in the rainbow video) So what if the other dimmer spot is caused due to this reflection?

Vrushali K

Read the post, slowly.

Benja Coloma

Why were there 2 reflections in the demo with the laser? There's a bright one above and then a less bright one below.

Matthew Sheehy

Question: What was the type of foil used in the demo at the end / where can I buy some? I've *got* to try that experiment for myself!

Alador

Put it in the form if you want it answered.

Shayaan Shohid

Thank you for such an amazing video! Something in the back of my mind tells me there must be some relationship between what you described in the video and harmonics. Is there any relation by chance? Another question, although philosophical... If we can see some alternate paths light takes by blocking off the paths that would otherwise cancel each other, could we possibly "see" alternate paths the beach-runner takes by applying a proverbial 'film'?

Orion

I posted the same question

Siddharthan Nandhakumar

I'm sorry that I couldn't make a donation before posting a question. I'm currently broke. 😬 Hope you answer mine.

Siddharthan Nandhakumar

If spacetime curvature is a response to energy-momentum, and quantum fields inherently fluctuate, wouldn’t gravity naturally include quantum corrections at all scales rather than requiring new physics like dark matter or dark energy? Couldn’t one naturally unify gravity across all scales by incorporating a dynamically adjusting suppression term that emerges from known quantum effects rather than needing exotic new physics?

psiikix

That scattered light is what we are seeing. Isn't it?🤔 It was shown that light takes different paths to reach it's destination even if it's a laser

Amit Meena

I'm confused about that experiment he did with that foil. When he turned off the light and shine the laser on that big piece, we also saw some light on the foil. My question is how do we know that the foil didn't act like a mirror, simply reflecting some light just like if you put a mirror, youd see?

Fraidoon Amiri

Should be fixed now, apologies and thank you for pointing this out! - Casper

Veritasium

Are you willing to make a video on the Principle of Least Action through the lens of Special and General Relativity, now that you made two, one through the perspective of Classical Mechanics and the latest on Quantum Mechanics? That would be tricky as time is relative, depending on the frame of refrence so, time being optimized is just insane to think about in this context. Side note both videos extensively used Ray optics to illustrate it.

Yosef Shaliehsabou

In the second video, there was a demonstration where a laser was shone onto diffraction grating. I'm just wondering what controls were in place to ensure that we weren't just seeing some of the scattered light from the laser. After all, no laser is 100% perfect and there will always be some amount of random scattering

Kayla Rice

The form says access denied

Anish Kumar Sinha