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Imagine the classic Elevator thought experiement. Einstein said there's no physical experiement that shows the difference between gravity and upwards acceleration.

What if we casted a light beam from the roof of the elevator to the floor? In the upwards accelerating elevator the light will hit the floor faster than the speed of light. However in the gravity elevator, gravity can't make the light beam travel faster than the speed of light.

So is this a violation of the EP?

Karveiani
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    According to SR, no, the person in the elevator does not measure light to travel faster than c. It does not violate the equivalence principle. In both cases the person in the elevator measures the wavelength of light shift. – user1247 Feb 11 '20 at 17:32
  • SR only deals with inertial frames of reference, no? – Not_Einstein Feb 11 '20 at 17:42
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    @Not_Einstein no, SR is perfectly capable of dealing with accelerated frames as well. The only requirement for SR is that the spacetime is flat i.e. the Riemann tensor is zero. It is true that the Lorentz transformations can only be used in inertial frames, but they are just part of SR. – John Rennie Feb 11 '20 at 17:53
  • Yes, If you could show by experiment or prove from theory that a light beam would behave the way you claim it would behave in the "elevator" experiment, then that would cast serious doubt on the equivalence principle. But, I suspect that you may find it more difficult than you think to obtain that proof or experimental result. – Solomon Slow Feb 11 '20 at 18:22
  • @SolomonSlow I think that a proof from theory would be meaningless. I can prove the statement in the Galilean Relativity, it doesn't cast any doubt on the equivalence principle though. – AnOrAn Feb 11 '20 at 21:42

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