1

I just don't get how if two people are moving relative to each other, either clock as seen from the other's perspective is slower.

Consider this: A and B are stationary relative to each other and begin with t=0s on both their clocks. Then A and B start moving relative to each other at 0.80c. The Lorentz factor is therefore 5/3.

From A’s reference frame, B is moving. When A’s clock reads 5s, B’s clock should read 3s.

But from B’s reference frame, A is moving. When B’s clock reads 3s, A’s clock should read 1.8s.

So what is the reading on A’s clock when B’s clock reads 3s? And worse yet, couldn't we keep going? When A's clock from A's perspective reads 1.8s, B's clock should read 1.08s, and so on...

This seems similar to the twin paradox but not exactly since there is no acceleration here.

John Smith
  • 308
  • Does this answer your question? How can time dilation be symmetric? – John Rennie Nov 03 '23 at 16:15
  • Why the downvote? It seems an excellent question and the fact it's been asked before doesn't warrant downvotes. – John Rennie Nov 03 '23 at 16:16
  • "So what is the reading on A’s clock when B’s clock reads 3s?" --' in who's frame? You've answered both versions of this question in your two preceding paragraphs. – WillO Nov 03 '23 at 16:17
  • These times are not "real" (and you do not see any clocks). They are a mathematical consequence of "relativity of simultaneity" (IMO a failed teaching method). 1) to tell if a clock is slower (rather than just behind yours) you need to observe its readings over a finite time. It is traveling at (up to) 1,000,000 times the speed of a bullet, so good luck with that. 2) All these scenarios are taking place in 1D space - you cannot see anything but a dot in front of and behind you! 3) If you insist on taking this approach you will come up against the "andromeda paradox". No downvote from me! – m4r35n357 Nov 03 '23 at 16:20
  • It does seem confusing if you interpret the explanation literally, but that would be a mistake. Time dilation does not mean that clock A runs more slowly than clock B! What it means is that time in one frame is out of synch with time in another. Suppose you pass two clocks, one of which is running a second ahead of the other. If you pass the first when it and your watch read 10:00:00, then when you arrive at the next it will read 10:00:02 while your watch reads 10:00:01. That is not because your watch runs more slowly than the clocks! – Marco Ocram Nov 03 '23 at 19:44

0 Answers0