As I am sure many are aware, we get a fair amount of questions about special relativity that are essentially variances of

Alice sees this event happen at this time over here, and Bob sees this event happen at this time over there, so then how can Alice see this if Bob sees this?

Of course questions like these don't necessarily need to involve an "Alice" and a "Bob", but the questions essentially boil down to how to reconcile time dilation, length contraction, relativity of simultaneity, etc. of various events as seen by various observers. Some (or many?) of these questions get closed/ignored due to quality, clarity, or "homework-ness", but there are also ones that survive and receive answers.

Some recent examples of questions like these, or questions recent posts have been duplicates of:

High Fives on the Relativistic Train of Death

Does Special Relativity Imply Multiple Realities?

Question about Lorentz transformations with time

Special Relativity - Regarding the Simultaneity of Events During the Train Paradox

2 Different Results for Length Contraction

I am sure there are better examples out there, but hopefully the type of question I am am referring to is evident. Questions like these can easily become convoluted in trying to specify when/where events occur in the various frames, what defines the events, etc. But, at least to me, it seems like many of these questions could "easily" be resolved with the appropriate application of Lorentz transformations, rather than some of the introductory equations of time dilation and length contraction.

Additionally, I know there are already great community wiki posts for other SR topics, and these serve as great posts to link to as a duplicate for other posts asking about these topics:

How can time dilation be symmetric?

What is the proper way to explain the twin paradox?

I am wondering if we can make a post like the two above that have a similar purpose.

Therefore, would it be useful to create a community wiki post that covers how to handle "event comparison" questions described above, or are the questions of this type actually not as frequent as I think or already adequately handled so that a community wiki post would not be useful? If one such post would be useful, what would this post look like, both in terms of the "question" that sets up the post and the type of answer(s) that would be ideal for such a post? Additionally, who would be responsible for creating the post/answer? I am by far an expert in SR, so I don't think I could handle creating an answer (even if it is open for the public to edit and make better).

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    $\begingroup$ I think there's some sort of rule that all SR & GR "FAQ" type questions just be written (and answered) by John Rennie. $\endgroup$
    – Kyle Kanos
    Dec 27, 2019 at 17:27
  • $\begingroup$ @KyleKanos I mean who else would do it? $\endgroup$ Dec 27, 2019 at 17:34
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    $\begingroup$ Could whoever gave the downvote explain why they think this isn't something that is needed? $\endgroup$ Dec 28, 2019 at 2:59
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    $\begingroup$ Good idea! I'm very much in favor of this $\endgroup$ Dec 28, 2019 at 9:14
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    $\begingroup$ The core problem is with new users not searching properly, if at all. So while the idea is not without merit it will not solve the issues of duplication, homeworkness etc. If only people cared to check even the suggested links before submitting a question... but one cannot force a donkey to drink, even it you bring it water. $\endgroup$ Dec 31, 2019 at 3:06
  • $\begingroup$ @ZeroTheHero I wasn't proposing this to fix those issues. Just a post that more experienced users can link to as a duplicate, just like how many twin paradox posts can be marked as a duplicate of the twin paradox community wiki. $\endgroup$ Dec 31, 2019 at 3:44
  • $\begingroup$ Right but if you can identify a problem that this would solve then I’d happily support. Don’t get me wrong: there’s a seed of idea here but I’m not sure - short of writing something that new users would not read - how the site is ahead. Maybe this needs to be fleshed out a little more? $\endgroup$ Dec 31, 2019 at 5:17

2 Answers 2


This doesn't seem like a good idea to me. The question would have to be extremely broad and vague. The answer would also have to be extremely broad and vague, something like "Use spacetime diagrams and the Lorentz transformation. Don't try to figure out all of special relativity using length contraction and time dilation. It doesn't work that way."

Possibly it would be useful to create a Q&A where the question was something like, "Can special relativity be reduced to length contraction and time dilation?," and the answer was an explanation of why not, with an explanation of spacetime diagrams and the Lorentz transformation. Is this different from what you're proposing?

  • $\begingroup$ Thanks for the thoughts. I'm not proposing anything specific. I just was thinking about the possibility of having something to direct users to who ask questions like the ones I discuss in the question. $\endgroup$ Dec 29, 2019 at 14:46

While I really like the idea at a naive level, I feel that it's implementation would be difficult at best and perhaps practically impossible.

The proposal is, essentially, to write the definitive introduction to relativistic mechanics - the one that somehow answers everyone's questions in the order in which they might ask them. I think that there are a variety of sources for learning relativistic mechanics and if the people asking questions were that way inclined, they could learn directly from these.

What Physics Stack Exchange does is allow people who do not feel confident with such resources to ask a question in their own way looking for an answer that respects the way in which the question was asked. Even when a question is a duplicate, it is often asked in a slightly different way, and the fact of it being a duplicate is part of the answer provided by the contributors of answers on stack exchange.

Telling people to go read the Wiki, while it sounds like what they should be doing, is likely to be off-putting to someone who is feeling confused about the concept behind the question. It is the one-on-one sense of support that makes stack exchange different from, say, the Wikipedia - which has many excellent articles on introductory mathematics and physics.

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    $\begingroup$ I'm not proposing a post that covers all of introductory relativistic mechanics. Just a post that takes posts that are trying to compare events using time dilation/length contraction and show how Lorentz transformations (1D would be sufficient) are a better, more general way to approach the comparison of various events as viewed in different frames. $\endgroup$ Dec 31, 2019 at 1:05
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    $\begingroup$ I am with you in intent. But, how do you get people to notice that this generic answer exits? Or is it that we should have the definitive self-answered question on the topic which people direct the original poster to? I suppose if we could get community support for a group of such definitive questions, that might prevent some otherwise wasted effort? $\endgroup$ Jan 1, 2020 at 23:33
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    $\begingroup$ The point isn't for people to notice it exists, although that would certainly be ideal. It would exist to serve as a question to link to as a duplicate, just like the other community wikis I have linked to in my question. If users fail to see the post, then others could still direct them to it and close their question as a duplicate. $\endgroup$ Jan 1, 2020 at 23:36
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    $\begingroup$ Okay, I see the idea. But, I remain unconvinced that it is really any different from everyone linking to a popular answer to the duplicated question. I think that the personal service is part of what attracts people to a forum rather than the Wikipedia - which I would not expect us to improve on as far as being a wiki is concerned. $\endgroup$ Jan 3, 2020 at 0:16
  • $\begingroup$ So you think the community wikis that are already established should not have been made? $\endgroup$ Jan 3, 2020 at 1:37
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    $\begingroup$ Well, this question has caused me to think a lot on the issue. For something for which there is not another commonly known wiki, I can see a point, but don't feel it's core to what is important about Stack Exchange. For something for which well written wiki's are available, I feel it is not Stack Exchange's role to compete. What Stack Exchange does is give personal service (so to speak). You get to talk to a person, not directed to a FAQ. $\endgroup$ Jan 6, 2020 at 2:58
  • $\begingroup$ I'm not proposing to do something that reflects the "core of Stack Exchange", nor am I asking for a competition with other sites. It would essentially be just like a normal post you would see on SE, but more free for others to edit as they see fit. $\endgroup$ Jan 6, 2020 at 3:02
  • $\begingroup$ Could you not do this unilaterally - with encouragment to those with enough reputation points to edit - and then link to this in your answers? If that became popular you would get it by default. And if not, then the question of whether has been answered in a practical sense. $\endgroup$ Jan 7, 2020 at 4:49
  • $\begingroup$ A friend of mine suggested that perhaps in the answer archetype a list of duplications should be kept. $\endgroup$ Jan 15, 2020 at 0:17

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