As I understand, we (currently) have no way of verifying whether light travels at the same speed in different directions. However, if it did, would we not observe "skewed" black holes? If the speed of light were smaller on one side of the black hole, the photons passing around that side would have a bigger minimal radius of rotation around the black hole than the photons on the other side. I.e., photons could get closer to the black hole without getting sucked in on one side than the other. Thus the black hole would appear bigger on one side than the other with respect to the center of mass. I am imagining something like this

Something like this

Moreover, by one theory light moves instantaneously in one direction and at speed of c/2 in the other, would we not actually physically see the black hole in this case? Am I misunderstanding something?

  • 1
    $\begingroup$ Its not like we've looked at black holes from a lot of different directions, mind you... $\endgroup$
    – Jon Custer
    Apr 4, 2021 at 21:56

1 Answer 1


If the speed of light were different in different directions then this would be apparent without needing to look at black holes. This anisotropy would have shown up in Michelson-Morley experiment, which compared the speed of light in many different directions. Even before that, it would have shown up in astronomical observations within our own solar system. For example, when observing the motions of the moons of Jupiter, the correction made for the distance between Jupiter and the Earth would have had a dependence on the absolute position of Jupiter in its orbit, rather than just on the relative positions of Jupiter and the Earth.

  • $\begingroup$ I suspect that the OP is referring to the one way speed of light $\endgroup$
    – PM 2Ring
    Apr 5, 2021 at 10:16
  • $\begingroup$ This answer is incorrect, see sciencedirect.com/science/article/pii/S0370157397000513 $\endgroup$
    – Eletie
    Apr 5, 2021 at 10:18
  • $\begingroup$ @PM2Ring If they are then they seem to be misinterpreting it. We can still rule out a general anisotropy in the speed of light, even if measuring the two-way speed does not rule out correlations between the speed of light in opposite directions, which would be a very specific (and suspiciously geocentric) anisotropy pattern. $\endgroup$
    – gandalf61
    Apr 5, 2021 at 10:35
  • $\begingroup$ For clarification, I was indeed referring to the one-way speed of light. I came up with the question after stumbling upon Vertasium's video. $\endgroup$ Apr 5, 2021 at 11:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.