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I was wondering if light can travel faster when it is within a region of moving spacetime. I know in general, that when you move a medium such as water, like in a river, water waves travel slower when against the current and faster when along with the current.

Is this true with light? It seems to me that when light is travelling in the ergosphere of a black hole (a region where spacetime moves at the speed of light), it would travel 2 times faster. Fact check me about the ergosphere.

Qmechanic
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Tachyon
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    Does this answer your question? Can light travel faster? – StephenG - Help Ukraine Jul 04 '20 at 14:22
  • @StephenG No, it doesn't answer my question. – Tachyon Jul 04 '20 at 14:25
  • It sounds like you are asking about the Luminiferous Aether. https://en.wikipedia.org/wiki/Luminiferous_aether – D. Halsey Jul 04 '20 at 22:31
  • @D.Halsey Well, I am asking about light speed being affected within regions of moving spacetime, not the ether. I don't think you should confuse the ether with spacetime, that would be bad. – Tachyon Jul 05 '20 at 11:25
  • “according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time” - Albert Einstein, 1920. – safesphere Jul 05 '20 at 23:36
  • @safesphere Well, I was taking a lecture on SR, and the professor said that Einstein said that the Ether was unnecessary. Einstein disproved the ether according to my professor from Stanford. The course link here: https://www.coursera.org/learn/einstein-relativity . – Tachyon Jul 06 '20 at 13:55
  • @safesphere Also, the ether would break the postulate of relativity of Einstein because it provides an absolute reference frame to see whether or not you are moving, which is a contradiction of the postulate of relativity. In addition, the ether has never been proven experimentally. – Tachyon Jul 06 '20 at 14:01
  • Did you notice the above quote is by Einstein? See this: “Because the same mathematical formalism occurs in both, it is not possible to distinguish between LET [Lorentz Ether Theory] and SR [Special Relativity] by experiment.” - https://en.m.wikipedia.org/wiki/Lorentz_ether_theory – safesphere Jul 06 '20 at 15:31

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The speed of light relative to a local observer is always c, but relative to a far away observer it can indeed receed faster than c; for example, a photon receding at the hubble radius has a speed of 2c relative to the observer in the center of the frame, and a photon travelling in your direction at that distance has a speed of 0c relative to you (the proper distance relative to you stays fixed), but still 1c relative to a local observer who is also located at the hubble radius, see Susskind, Cosmology 2 at timestamp 1:30:30. The same goes for the ergosphere or the horizon of a black hole, relative to a local observer it still has exactly 1c.

Yukterez
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