We have numerous questions on the site about the speed of light, and we have the tag for them. However, since the end of October 2020, when the video Why The Speed Of Light* Can't Be Measured was posted by Veratasium on YouTube, we have had a constant stream of questions about the one-way speed of light. Currently, that video has 4,956,740 views so I guess it's not surprising that we're getting so many questions about it.

There were already a few questions here on this topic, but now it seems that a large percentage of new questions about the speed of light are specifically about the one-way speed of light. There are currently 2,251 questions with the tag, and 769 results in the https://physics.stackexchange.com/search?q=%5Bspeed-of-light%5D+one-way list.

I think it would be helpful if we had a tag specifically for this sub-topic. There have been some great questions and some excellent answers posted, with various useful links, but IMHO there really isn't much more that can be said on the topic that hasn't already been said here. A specific tag would make it easier for potential askers to find those existing questions, and for curators to quickly find suitable duplicate targets.

Yay! We now have a tag: . So now we need to create suitable tag info & tag wiki entries...

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    $\begingroup$ FWIW, this search targets the one-way speed of light questions. $\endgroup$
    – Qmechanic Mod
    Mar 9, 2021 at 23:53
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    $\begingroup$ Almost all the questions are just the OP misunderstanding the issue and asking "why can't <simple setup> solve the problem?" I think a tag is alright, but an even better option would be to make a canonical Q/A and close all future ones as duplicates. $\endgroup$
    – knzhou
    Mar 10, 2021 at 3:16
  • $\begingroup$ @knzhou A good canonical Q/A would certainly be useful. As I said above, we already have some great existing answers, but they aren't particularly easy for new OPs to find, and even regulars familiar with the site and the topic have to spend time wading through all the stuff that's accumulated in recent months to find the best dupe targets, or related posts that may suit the OP. A canonical would simplify that, and allow us to concentrate all the goodies in one place. And a new OP is more likely to actually read stuff on one page than stuff spread over a bunch of pages. $\endgroup$
    – PM 2Ring
    Mar 10, 2021 at 3:46
  • $\begingroup$ But I think we need a tag as well, because there are countless minor variations of <simple setup> that OPs have posted, and will continue to post. So it'd be good to be able to point them to a question that's closely related to their simple setup, as well as to the canonical. And a tag would make it easier to manage that "cat herding" exercise. ;) $\endgroup$
    – PM 2Ring
    Mar 10, 2021 at 3:52
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    $\begingroup$ Shouldn't tags be based on a significantly separate existence of a subtopic -- either in academia or in the structure of the subject matter of physics? One-way speed of light doesn't make much sense as a separate subject in this context IMHO. $\endgroup$
    – user87745
    Mar 10, 2021 at 22:12
  • $\begingroup$ @DvijD.C. That's a reasonable objection. The one-way speed of light is a sub-topic, not a separate topic in its own right, but I think it would be useful to have a tag for it, to make it easier to separate such questions from general speed of light questions. IMHO, although it's important, I don't think that beginner students of SR should be worrying about it. OTOH, I don't want them to get the impression that it's some kind of flaw in SR theory. I could have just created the tag myself, but I thought it'd be a Good Idea to discuss it here first. $\endgroup$
    – PM 2Ring
    Mar 11, 2021 at 11:42
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    $\begingroup$ This question made me finally go and watch that video. It was so bad. It threw so much good physics right out the window. At one point, it talked about symmetries as it was violating rotational invariance symmetry. He mentions seeing things in space as they are but never stopped to think of what that would mean for the CMB (ie, there would not be one if light moved instantaneously). How have physicists not stood up and said "ummm, no. You're wrong" to this video yet? $\endgroup$
    – Jim
    Mar 11, 2021 at 15:33
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    $\begingroup$ @Jim: You may want to read through some of the answers to this question, because most of the highly voted ones actually seem to say "Yeah, the video is basically right, if a bit sensational." $\endgroup$
    – Kevin
    Mar 19, 2021 at 6:33
  • $\begingroup$ @Kevin No need. I know my physics. I know how much of it was right, but it leaves out a good deal of physics and makes assumptions that make it look more correct than it is. I mean, I can think of a couple ways to do this measurement and one way to determine whether or not there is a difference without using any clocks or synchronization. But my biggest issue was that they completely skipped over the evidence we have that what they said can't be true. $\endgroup$
    – Jim
    Mar 19, 2021 at 12:22
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    $\begingroup$ @Jim If you can measure the one-way speed of light you should get on that and publish it! That would be a huge deal! $\endgroup$ Mar 19, 2021 at 12:32
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    $\begingroup$ @Jim What do you mean? As Dale's answer explains in the question linked by Kevin, there is no observation or experiment which can determine the one-way speed of light. So, using Reichenbach's notation, any choice of $\epsilon$ is a convention, and $\epsilon = \frac12$ is merely the simplest convention, and the most convenient for calculations. $\endgroup$
    – PM 2Ring
    Mar 19, 2021 at 12:37
  • $\begingroup$ @BioPhysicist I would if I had a handy way of artificially generating muons. And a large facility on the equator. And a lot of money $\endgroup$
    – Jim
    Mar 19, 2021 at 13:27
  • $\begingroup$ @PM2Ring the theoretical framework for special relativity is fairly well established and confirmed. All one has to do is ensure the "clock" is travelling with known kinetic energy across a known distance and then compare the proper time the clock measures one direction with the return trip. If there is a discrepancy in the one-directional speed of light, the direction with a higher speed of light will be the direction for which the travelling clock experiences more proper time $\endgroup$
    – Jim
    Mar 19, 2021 at 13:34
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    $\begingroup$ Sorry, @Jim, but those ideas cannot establish a one-way speed of light. However, a comment thread on meta isn't a good place for an extended physics discussion. If the existing answers on this topic don't convince you, perhaps you should ask a new question. Or come to The h bar and chat about it. Also see the site of noted relativity historian & philosopher, John D. Norton. $\endgroup$
    – PM 2Ring
    Mar 19, 2021 at 15:02
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    $\begingroup$ @Jim Even a method that is theoretically correct would be ground breaking. You should definitely publish if you have it. $\endgroup$ Mar 19, 2021 at 15:36

1 Answer 1


I think a tag is warranted.

This search for "one way" within speed-of-light-tagged questions returns 59 threads, most of which fit the proposed tag and many of which form a steady trickle up to very recent dates, as well as several quite-old threads (as opposed to a single glut right after the publication of the Veritasium video).

I think this is more than enough to show that this specific sub-topic is popular enough on this site that a dedicated tag is a useful organizational tool. And that is in itself sufficient to set it up.

  • $\begingroup$ I created a one-way-speed-of-light tag for now. (If it doesn't work out this action can easily be reversed.) $\endgroup$
    – Qmechanic Mod
    Dec 1, 2021 at 12:04

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