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Say I have the following setup.

Person A is stationary. Person B is to the very far right of person A and is travelling towards person A in a plane at very high speed but sufficiently far away so that the collision time is some big number. Also say both people are waving at each other frantically (say once per second in local frame time).

From person A's frame, person B's clock became very slow. So, to take as an example, by the time (in A frame) that B passes A, only one tick of person B's clock has gone by. That means we can only see person B waving once.

But from B's POV, the reverse is true, it is person A's clock that has only ticked once. From B's frame, he has waved many many more times.

So what happened to those extra waves that person A cannot see?

Is it because in B's frame, although he waves many many times, only one tick of person A has occured, so person A can only see "one tick worth" of waving?

Shuheng Zheng
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  • It is not an exact duplicate of the other question. I asked about where the extra hand waves go. But I think I have an explanation. – Shuheng Zheng Jun 05 '19 at 07:01
  • There is no contradiction here. Like you said, each person measures their own clock as being faster than the one they percieve as moving. So they would in fact "see" less hand waves. But you have to remember they are in different reference frames. What happened to the hand waves? They haven't happened yet in that frame. Each persons point of view is true. The only time when they can objectively say one persons clock is slower are when they are brought together. This is the gist of the twin paradox. I recommend you look into it, it confused me a bit at first too. – Thatpotatoisaspy Jun 05 '19 at 08:15
  • But basically until they can compare their clocks locally the times they measure are truly relative. – Thatpotatoisaspy Jun 05 '19 at 08:18

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