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https://m.youtube.com/watch?v=ljoeOLuX6Z4 in this video they measure the same event at different times does that tell you the speed of light in that direction? Since the event doesn’t appear to happen at the same time instantaneously does that mean that the speed of light isn’t infinite in that direction? Can you measure the speed of light by looking at similar events and in different directions?

Jesse
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1 Answers1

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Let me try to explain what these astronomers did. In a very distant galaxy, a supernova happened a long time ago, emitting light in all directions. All this light traveled in "straight lines" and at the same, constant speed (which is not infinite), until some of it reached us. However, the "straight lines" followed by light are not your usual intuitive straight lines, because space is not Euclidian. In General Relativity, space-time is a manifold which is curved in the presence of mass and energy, and in which "straight lines" are called geodesics. (Globes are more tangible examples of curved manifolds, in which geodesics are circle segments.) Roughly speaking, the fact that space-time is curved in the presence of mass means that distances are longer near massive objects. So all the light that traveled from this supernova did not cover the same distance before reaching us, this is why some of it arrived later and allowed the astronomers to observe this multiple times.

Now, I don't think this kind of observation can help us measure the speed of light, because we don't know very precisely the length of the different paths that light took. In fact, the astronomers mentioned in the video did the opposite : they estimated these different lengths using other methods to successfully predict the next time that some light would arrive. In this process, they had to input the speed of light, which is well known thanks to other measurements (see here).

Anthony Guillen
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