# Question about speed of light

**URL:** <https://boards.straightdope.com/t/question-about-speed-of-light/960784>\
**Category:** Factual Questions\
**Created:** [March 10, 2022, 12:59am UTC](https://boards.straightdope.com/t/question-about-speed-of-light/960784 "2022-03-10T00:59:04Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\
**Post date:** [March 10, 2022, 12:59am UTC](https://boards.straightdope.com/t/question-about-speed-of-light/960784/1 "2022-03-10T00:59:04Z")

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Shine a flashlight on a reflector. The light travels to the reflector, and then bounces back to the flashlight.

[This video by Veritasium](https://www.youtube.com/watch?v=pTn6Ewhb27k) _seems_ to be saying the speed of the light from the flashlight to the reflector is probably the same as the speed of the reflected light (from the reflector back to the flashlight), but _we don’t know that for sure_.

So is possible the speed of the light from the flashlight to the reflector is _ **not** _ the same as its speed from the reflector to the flashlight? Or am I misinterpreting the video?

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [March 10, 2022, 1:16am UTC](https://boards.straightdope.com/t/question-about-speed-of-light/960784/2 "2022-03-10T01:16:42Z")

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Only if you define “reality” in a really weird way.

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**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\
**Post date:** [March 10, 2022, 2:00am UTC](https://boards.straightdope.com/t/question-about-speed-of-light/960784/3 "2022-03-10T02:00:33Z")

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> [@Crafter\_Man](#):
>
> So is possible the speed of the light from the flashlight to the reflector is _ **not** _ the same as its speed from the reflector to the flashlight? Or am I misinterpreting the video?

That is what he is saying, and it is technically true that you cannot just measure the speed that a photon travels just by looking at its passage, albeit for reasons not clearly explained in the video. Of course, the Michelson-Morely experiments demonstrated the invariance of orientation on the speed of light (thus disproving the existence of luminiferous aether, albeit quite unintentionally) and later Ives–Stilwell experiments demonstrated the effect of relativistic time dilation on such observations consistent with special relativity.

So, while we can only practically measure the so-called “two way speed of light” (and this is a fundamental problem because of the lack of simultaneity between two sufficiently distant points) there is no plausible physical mechanism by which light could be somehow faster in one direction than in the opposite direction when corrected for any difference in inertial states (and in the cast of cosmological observations, the expansion of underlying spacetime). This measurement issue is purely a philosophical point, not some kind of paradox. Note this is only true within Einstein relativity; in quantum field theory, an individual photon has a probability of taking effectively an infinite number of paths, including a not-entirely-zero chance of a path that would require it to go faster than _c_, although in the mean it will move exactly at _c_, which of course is consistent with observation.

Stranger