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In a recent popular video by Veritasium it is claimed that no one has devised an experiment that measures the one-way speed of light, and that in theory a directional difference in the speed of light is possible.

I can understand that it's difficult to design experiments to test this on earth, but does cosmology not provide us with pretty strong bounds on the devation of the one-way speed of light versus the two-way speed of light?

I would imagine that if light traveling in one direction is significantly slower than the other way around we would see cosmological effects that we do not:

  1. A significant gradient in the temperature of the cosmic microwave background radiation. In the extreme event that in one direction the one-way speed of light is instant, we would not see a CMBR in one direction at all.

  2. A significant difference in the development stage of galaxies in one direction v.s. the other. E.g. more young stars that pulsate less than older stars.

Is my intuition correct? Or would the night's sky in a universe with a one-way speed of light that significantly differs from our conventional two-way speed of light look identical to one that matches it?

orlp
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  • You don’t need to go outside our own solar system. If the speed of light was different in different directions, the movements of the moons of Jupiter would appear to be irregular. – Mike Scott Nov 05 '20 at 08:09
  • What if the speed of light is instant in one direction, but the Big Bang is also moving slowly in that direction, instead of happening everywhere instantly? What if the CMB in that direction is new universe that just got created? – user253751 Nov 05 '20 at 10:05
  • https://en.wikipedia.org/wiki/One-way_speed_of_light says that the problem is related to synchronizing clocks between the emission point and the receiving point. It does seem to be a problem with respect to cosmology which is limited to approximately managing the problem somewhat with redshift and brightness and such other indirect techniques. – Buzz Nov 22 '20 at 21:32
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    Does this answer your question? Is it possible to prove or disprove the Einstein Synchronization Convention with astronomy? – nick012000 Nov 25 '21 at 07:18

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