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But the cosmic microwave background looks the same - thus the same „age“ - from all directions. If it was instantaneous in one direction, wouldn’t the CMB be different in different direction ?

Martin Embeh

this video has consumed half my day so far.... The whole concept of “right now” could be a great video (or two, or three…). For the first rabbit hole, you could then discuss why GPS is dependent on extremely accurate measurements of time delay, and how the GPS system estimates the changes in the speed of light caused by the Ionosphere, as well as the signals travelling from (near) vacuum to full atmospheric pressure, which changes the speed of light is changing during the entire trip. The next rabbit hole is how relativity causes GPS satellites need to periodically re-synchronize their clocks instead of just synchronizing them once. The Earth causes time dilation, so the clocks of the satellites travel through time at a different rate when compared to clocks on the ground. Then take a moment to discuss geodetic precession vs. frame dragging effect. Then raise that crazy to the power of crazy when discussing the trip of a signal from Earth to Mars. The time dilation caused by the mass of the Sun, the rotation of the sun, different orbital distances of both planets, the elliptical orbits of the planets, the mass of each planet, and the rotation of each planet, all affect the observed passage of time. The final brain-bender (for me, at least) is how useless the concept of right-now is. Using the space-travel experiment, where two people start on the ground, one travels in a spaceship some great distance and back at high speed. Both people start at the same “now”, and they both end up at the same “now”, but each has experienced a different amount of time between those two “now”. This really shows how “time” is actually another dimension. Just as two people can walk a different path between their house and the store, two people can travel different paths between two “now” locations in time. Actually, that’s not the final brain-bender. The final one is how the speed of light doesn’t change based on how fast the observer is moving relative to the light source, but the frequency of the light does change. After all that, sit back and enjoy the fact that you have not only destroyed the concept of “now”, but also completely decimated what it means to “observe”, and thrown the concept of “travel” into complete disarray.