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We have lots and lots of questions on the symmetry of time dilation. That is when two observers are heading towards each other how can each think their clock is running faster? These questions typically come from people who have just started studying special relativity.

I think I have come up with a good analogy to explain what is going on, so I propose to post a canonical Q/A. A draft of my proposed Q/A can be viewed here.

But I'm aware that these canonical Q/As aren't viewed with enthusiasm by everyone so I won't proceed unless the consensus is that it would be a useful contribution to the site. To this end I'd like two things:

Firstly please upvote or downvote this question to indicate your approval or otherwise of the idea.

Secondly please answer if you think there are better ways to approach this. For example there may already be a question that is in effect a canonical question on the issue (I did look but while there are dozens of related questions I haven't found one that I thought a good candidate for a canonical question).


Thanks to everyone who voted or commented. I've now posted the Q/A (reluctantly making the names more sensible :-).

How can time dilation be symmetric?

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  • $\begingroup$ Looks good to me, though I am not a huge fan of the names. $\endgroup$ – Kyle Kanos Jan 29 '18 at 12:14
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    $\begingroup$ @KyleKanos humph. I went to considerable trouble to invent names that I thought would amuse. $\endgroup$ – John Rennie Jan 29 '18 at 12:16
  • $\begingroup$ sorry :(, perhaps if it were a more common reference? But it is your post, so your choice $\endgroup$ – Kyle Kanos Jan 29 '18 at 12:18
  • $\begingroup$ FWIW, 1. The classical twin paradox questions so far are this & this Phys.SE questions. 2. If moreover space is periodic, it is this Phys.SE question. $\endgroup$ – Qmechanic Jan 29 '18 at 13:20
  • $\begingroup$ Sure, go ahead. I don't know why anyone would oppose canonical Q/A posts. Ideally speaking, shouldn't all posts be "canonical"? (to avoid repetition, etc.) $\endgroup$ – AccidentalFourierTransform Jan 29 '18 at 16:07
  • $\begingroup$ I personally think it'd be much clearer with just A, B, and C, or Alice, Bob, and Charlie. I agree that this is not the same as the twin paradox (it's something much more fundamental, i.e. why can both clocks 'be slower than the other', without the acceleration/meeting up again part) and is an extremely common separate confusion that deserves its own thing. $\endgroup$ – knzhou Jan 29 '18 at 17:39
  • $\begingroup$ Sorry, but I also found the names distracting. At the same time I'm tired of Alice and Bob. What about Brownie and Cheesecake? ;-) $\endgroup$ – stafusa Jan 29 '18 at 23:35
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    $\begingroup$ It managed to get a VTC for the "homework" reason. I'm pretty impressed by that. $\endgroup$ – JMac Jan 30 '18 at 22:15
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    $\begingroup$ @JMac yes, I was a tad surprised by that. I was expecting votes to close as a duplicate, but as homework? :-) $\endgroup$ – John Rennie Jan 31 '18 at 5:20
  • $\begingroup$ @JMac hmm now I want to know who voted to close as homework... how about we all vote to close it just to find out who was that first close vote? :-P $\endgroup$ – AccidentalFourierTransform Feb 2 '18 at 3:30

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