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I’m sure everybody is aware of the question that people love to ask, i.e. some variation of “what’s the experience of a photon (or someone traveling at $c$, etc)?” This question is always roundly downvoted. Why?

I understand that the question is not strictly about physics because...why exactly? Aren’t there clear ways to interpret this question that are consistent with reality? I feel that we are missing a pedagogical opportunity for a question that is obviously quite popular.

For example, we could talk about limits. What is the experience of an observer in the limit $v \to c$? We could talk about line elements or spacetime diagrams. What’s the length of a light-like line? What does that mean?

I have never answered one of these myself because they’re widely derided and closed. But I’m worried that we’re interpreting this question too rigidly and literally, to the detriment of the curious public.

So why don’t we answer these?

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    $\begingroup$ Links to these questions? $\endgroup$
    – More Anonymous
    Sep 11, 2021 at 4:37
  • $\begingroup$ @MoreAnonymous One example I can think of right away is this. The full comment thread can be found here. $\endgroup$
    – Vincent Thacker
    Sep 11, 2021 at 8:49
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    $\begingroup$ aren’t you just saying this kind of questions already have multiple duplicates? $\endgroup$
    – ZeroTheHero
    Sep 11, 2021 at 13:13
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    $\begingroup$ @ZeroTheHero sure, they have duplicates. But most duplicate questions don’t get downvoted so mercilessly. It’s like we’re indignant that someone would ask this question at all. $\endgroup$
    – Gilbert
    Sep 11, 2021 at 15:19
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    $\begingroup$ well... if an OP doesn't due proper diligence to look for duplicates, that would be a reason for downvotes. I'm pretty sure I'm guilty of downvoting questions for which there are multiple near duplicates, although not sure about this specific case. see also the last part of the answer by @WillO $\endgroup$
    – ZeroTheHero
    Sep 11, 2021 at 15:34
  • $\begingroup$ It could make an interesting question of why, physically, it appears to be impossible to have a observer (capable of processing information internally) that is massless and thus travels at speed $c$. But that's not usually what these questions seem to be looking for. $\endgroup$
    – Buzz Mod
    Sep 11, 2021 at 19:35
  • $\begingroup$ A comment from How does a photon experience space and time?: I would also add a "neuroscience" comment. Collections of photons propagating in some direction, because they move by $v=c$ exactly, can't have brains that would send signals back and forth. $\endgroup$
    – PM 2Ring
    Sep 11, 2021 at 20:20
  • $\begingroup$ (cont) If an electric signal were sent back, against the direction of motion of the photon, it could never get back because to do so, these signals would have to travel faster than the original photons - faster than light - and that's not allowed. So nothing moving at the speed of light can actively think, at least not a nonzero number of operations per second. ;-) – Luboš Motl $\endgroup$
    – PM 2Ring
    Sep 11, 2021 at 20:20
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    $\begingroup$ "What is the experience of an observer in the limit v→c?" - but you're traveling arbitrarily close to $c$ relative to an infinity of inertial observers and your experience is... that you're at rest relative yourself. There isn't a limit here. No matter how close one gets to $c$ according to some inertial observer, one has one's rest frame. How does looking at $v \rightarrow c$ help here? An entity with speed $c$ according to any inertial observer is not at rest in any inertial frame, correct? $\endgroup$
    – Alfred Centauri
    Sep 12, 2021 at 0:05
  • $\begingroup$ @AlfredCentauri I think most would agree that when this question is asked, there is an implicit assumption that the velocity is relative to some “stationary” objects in the universe. If you’re moving near $c$, relative to the universe of things, how do you experience the universe? How long does it take to get from stationary planet A to stationary planet B? What do things look like? I think answering this version of the question would be satisfying for most. $\endgroup$
    – Gilbert
    Sep 12, 2021 at 0:16
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    $\begingroup$ @Gilbert, count me in the camp of those that disagree. $\endgroup$
    – Alfred Centauri
    Sep 12, 2021 at 3:04
  • $\begingroup$ @AlfredCentauri fair enough. $\endgroup$
    – Gilbert
    Sep 12, 2021 at 11:43
  • $\begingroup$ I used to be open-minded about these questions, but it's the same thing every time. There's precisely one useful thing you can say about this picture (you can take a limit of inertial frames as $v \to c$) and every other conclusion one draws is misleading. For example, a common misconception is that because a photon's "life" and "death" happen at the same time for it, every photon must be absorbed before the universe ends. Therefore, things like the CMB cannot exist. The more you try to work within this picture, the more confused you get. $\endgroup$
    – knzhou
    Sep 13, 2021 at 1:10
  • $\begingroup$ @knzhou thank you, and I’m sympathetic to your point of view. Could it be a helpful answer to simply introduce the fact that a light-like spacetime interval is zero, and argue that therefore a photon couldn’t experience anything (even for a fictionalized, anthropomorphised photon)? This might be satisfying for folks. $\endgroup$
    – Gilbert
    Sep 14, 2021 at 16:00

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I don't think I have ever downvoted this kind of questions, or at least the ones I think you are referring to. I do think I had voted to close a couple in the review queues.

But I reckon that the vast majority of these questions, especially when asked by (new) low reputation users, can always be answered by some variation of "there is no rest frame for the photon". Which is counter-intuitive, sure, but also a popular answer present in a myriad other posts both on this site and on others.

I think that if the poster were to ask about more specific and interesting nuances, such as the limits $v\rightarrow c$ that you are mentioning, that would be an indication that they have already done some research on the subject or given some thought about the question. Which would then make it more stimulating for people to engage in writing an answer and participating in the ensuing discussion. Instead of, for example, closing it as a duplicate or for lack of focus/needs more details.

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  • $\begingroup$ I still think if someone doesn't show much research it's beneficial to ask the poster what research he/she has done before voting to close a question, as I think if someone knows little about physics it can be harder for that person to know what to research, and so if the person does research he/she may look up something different from what's really relevant to the topic, making it look like to someone who understands more about the topic like the poster has done less research than he/she actually has. $\endgroup$
    – Anders Gustafson
    Sep 11, 2021 at 10:34
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    $\begingroup$ I would have said that asking about limits $v\rightarrow c$ is one of the best possible indicators that the poster has not given some thought to the question. $\endgroup$
    – WillO
    Sep 11, 2021 at 14:13
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    $\begingroup$ @WillO I would argue some thought has gone into it. Maybe not "enough thought", but it at least recognizes the problem with $v=c$. If everyone had perfect thought then there would be no questions. $\endgroup$
    – BioPhysicist
    Sep 11, 2021 at 15:12
  • $\begingroup$ @WillO: It could mean "I don't actually know what $\gamma$ is, please explain it to me." Which is a perfectly on-topic question, but probably a duplicate of something. $\endgroup$
    – Kevin
    Sep 14, 2021 at 5:22
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One could, of course, find some version of such a question that has some nuance that makes it novel. For the vast majority that you mention, however, you've essentially answered your own question: The large number of variations of the question with answer here and on other easy-to-find resources suggests strongly that the asker of such question now didn't do sufficient research before posting. So that's cause for a downvote.

Beyond that, such question is almost certainly a duplicate. That means that you should be flagging / voting to close as duplicate rather than answering. You should not be answering questions that you know are duplicate.

(Obviously it sometimes happens that you answer a question not realizing it's duplicate. There might also be some reasonable disagreement about how exactly matched questions have to be to be duplicate for closure. But by premise of your question, that doesn't apply in these cases.)

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    $\begingroup$ I’ll admit that I did not link to examples of this question (I’m doing this on my phone, so it’s not easy to find links). That said, if you look at any of them, you’ll find that they are not closed as duplicates, with a link at the top pointing to a friendly, insightful answer. Rather, they are downvoted with seemingly unwarranted snark piled on. $\endgroup$
    – Gilbert
    Sep 14, 2021 at 0:44
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    $\begingroup$ I just did a search trying to find a specific answer related to the comments on a different answer. There are a lot of duplicates marked as such. The generic question along these lines at this point should be both closed (as duplicate) and downvoted (for lack of research). That some slip through without the duplicate sticking is not surprising since it takes someone time to find the duplicate and then 4 more timely close votes to complete the action, whereas the downvotes always stick. Downvoting in this situation is a step toward cleanup via roomba in the even the duplicate doesn't stick. $\endgroup$
    – Brick
    Sep 14, 2021 at 14:50
  • $\begingroup$ The "snark" as you put it, is not appropriate, wherever it arises. But addressing that in the abstract is not specific to the type of question that you mentioned. $\endgroup$
    – Brick
    Sep 14, 2021 at 14:51
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"Aren’t there clear ways to interpret this question that are consistent with reality? " None that I can think of. What's an example of one of these "clear ways"?

The nonsense about limiting behavior is clearly not an example, since every single change in velocity is an increase in velocity in some frames and a decrease in others.

I understand what "talking about spacetime diagrams" means, but I have no idea how that talk could yield a clear way of interpreting this question.

More here.

As for this: "This question is always roundly downvoted. Why?". I downvote them to keep them off the front page, where I believe they convey a misleading message that this is not a site for thoughtful people.

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    $\begingroup$ I think we can be thoughtful and serious while also being open to the possibility that people don’t understand exactly what they’re asking and don’t understand why their question, if interpreted literally, is nonsensical. The people who ask this question are non-professionals, curious about relativity. Perhaps we could forward them to a friendly, pedagogical answer rather than slapping their wrists (as we do for, say, the homework questions)? $\endgroup$
    – Gilbert
    Sep 11, 2021 at 15:42
  • $\begingroup$ As for what’s nonsense and what’s not, I, for one, am not against saying that a light-like (or at least $v \to c$) line element has length zero, and so the proper time and distance one would calculate over such a world line are zero (despite the fact that no such reference frame could actually exist). Is this really such a sin? $\endgroup$
    – Gilbert
    Sep 11, 2021 at 15:58
  • $\begingroup$ One last thing regarding your link, I’m not saying that there is a meaningful frame-independent notion of traveling at speed near $c$. I think the questioners implicitly assume a stationary background, e.g. traveling from one planet to another or something. Then the question becomes “how would one moving near $c$, relative to the rest of the stuff in the universe, experience the universe?” $\endgroup$
    – Gilbert
    Sep 11, 2021 at 16:08
  • $\begingroup$ Gilbert: "i think we can be thoughtful and serious while also being open to the possibility that people don’t understand exactly what they’re asking and don’t understand why their question, if interpreted literally, is nonsensical. " I agree. But the fact that we can be open to this does not change the impression left on first-time visitors who are making quick judgments about whether this site is worth their while. $\endgroup$
    – WillO
    Sep 11, 2021 at 18:09
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    $\begingroup$ @WillO I highly doubt a high quantity of new users are seeing questions like these and thinking the site isn't worth their while because of it Just because you think the way to think about it is obvious doesn't make them poor questions. $\endgroup$
    – BioPhysicist
    Sep 11, 2021 at 18:25
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    $\begingroup$ "I for one, am not against saying that a light-like (or at least $v \rightarrow c$) line element has length zero, and so the proper time and distance one would calculate over such a world line are zero." This is objectively wrong both mathematically and physically. <irony>There are several good answers on Physics.SE that explain this.</irony> It's more than just that the "rest frame" doesn't exist. @Gilbert $\endgroup$
    – Brick
    Sep 14, 2021 at 0:04
  • $\begingroup$ @Brick okay, convince me. How is it wrong to say that the integral of zero is zero? $\endgroup$
    – Gilbert
    Sep 14, 2021 at 0:39
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    $\begingroup$ @Gilbert: I suspect that before Brick can convince you, you're going to have to tell us what a "$v\rightarrow c$ line element" is. $\endgroup$
    – WillO
    Sep 14, 2021 at 2:19
  • $\begingroup$ @WillO I’m not trying to reveal any deep insight with my above comment (which was obviously ill-advised since it’s merely proving a distraction). What I was trying to say is that, for the “photon experience” question, it’s relevant and maybe useful to point out that a “light-like” interval has length zero (do we agree? Do you prefer this language?). Then, what I am not opposed to doing (although this may be controversial) is to interpret this (i.e. ascribe to this some physical meaning). I understand that this could be misleading given that no light-like reference frame exists. $\endgroup$
    – Gilbert
    Sep 14, 2021 at 2:47
  • $\begingroup$ @WillO that said, perhaps noting that the light-like interval is zero is a better way (from a pedagogical point of view) to argue that a photon can’t experience anything because the spacetime distance between its emission and absorption is zero. This brings be back to two legitimate (as I see it) ways to answer this question: (a) “a photon doesn’t experience anything,” (this being an actual answer, not a dismissive comment) and (b) “an observer traveling arbitrarily close to $c$ relative to a stationary universe will observe —insert insightful analysis here—.“ $\endgroup$
    – Gilbert
    Sep 14, 2021 at 2:58
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    $\begingroup$ @Gilbert Meta is not the place for a discussion about the physics, but it is covered in multiple main-site Q&A. Regarding your last comment, there is no (b) because it depends on a non-existent, preferred reference frame. There is no consistent way to "ascribe to this some physical meaning", and you've touched on several fallacies in your last few comments. I tried to find my favorite answer, but it's taking to long to sort through all of the duplicates and bad answers - which goes to the point of your meta. The site is littered with this topic to the point that good material is buried. $\endgroup$
    – Brick
    Sep 14, 2021 at 14:46
  • $\begingroup$ @Brick I think a good (albeit unlikely) outcome of this meta would be the discovery or establishment of a good, canonical answer (or set of answers) to this question. Regarding (b), I don’t see what the problem is because we’re just talking about two reference frames (one with an observer and one with stars/planets/etc) with a relative velocity that the observer observes to be arbitrarily close to $c$. This this not well-defined? $\endgroup$
    – Gilbert
    Sep 14, 2021 at 16:11
  • $\begingroup$ A canonical answer would be great, although it will be difficult to untangle the mess of Q&A already on this topic. Your pursuit of what was labeled (b) earlier follows a common fallacy. It is not well-defined, and that is covered in some of the main-site Q&A. If I find the one that I think is best, I'll link it. $\endgroup$
    – Brick
    Sep 14, 2021 at 16:14

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