# Time Dilation happens on moving frame - but which one?

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1. May 19, 2015

### controlfreak

As per theory of special relativity the time in moving frame R' is supposed to run slowly compared to the stationary frame R. But isnt the stationary frame R moving wrt frame R' in the opposite direction. For an observer in stationary frame R', R would be moving with the same velocity wrt to it and wouldnt he think time will dilate in R which is the moving frame according to that observer in frame R'? So if time dilates in both, wouldnt both measure the same time? This has been confusing me for a while though I understand the equations which Lorentz framed to satisfy the absolute velocity of flight.

2. May 19, 2015

### A.T.

Yes, each frame measures the same clock rate for the clock that moves in that frame.

3. May 19, 2015

### PeroK

If two frames are moving wrt each other, then they both observe time dilation in the other frame - in the sense that both observe clocks and any physical processes (hence time itself) running slower in the other frame. Time dilation, like motion itself, is relative, and not "absolute".

Also, "moving frame" is relative not absolute; a frame can only move relative to another and is not absolutely "moving".

Understanding how and why this symmetry of time dilation can be resolved is a central aspect of understanding SR. It can be done by the using the Lorentz Transform or by directly studying the behaviour of clocks in the two frames relative to each other.

4. May 19, 2015

### controlfreak

I realize I was talking about the twin paradox. The explanation as given in this link is that (http://en.wikipedia.org/wiki/Twin_paradox) asymmetry is introduced when the twin returns back to compare the times as to return back one needs a change in velocity, an acceleration.

5. May 19, 2015

### controlfreak

6. May 19, 2015

### controlfreak

This provides an analysis of Twin Paradox resolutions - http://www.iisc.ernet.in/~currsci/dec252005/2009.pdf - This whole thing doesnt seem to be fully and satisfactorily resolved. First of all why do we make the assertions that the space ship twin is younger? Has that been proved by experiments which indirectly attest to that? or does theory predict that?

7. May 19, 2015

### A.T.

Use the search function then. It has been explained to death here.

8. May 19, 2015

### Staff: Mentor

That's not correct; it is.

We haven't done it with spaceships, obviously, but we've done it with subatomic particles many times. As A.T. says, use the PF search function; you will find many, many threads where the twin paradox has been discussed to death. Another good discussion is the Usenet Physics FAQ article on the twin paradox (which you will also find linked to in many of those other threads):

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html

Pay particular attention to the "spacetime diagram analysis"; that is the most general method of analysis and generalizes to all possible scenarios in curved spacetime as well. It also, as the FAQ article says, allows you to put all the other methods of analysis in their proper perspective.

If you have further questions after reading through what's already been written and discussed, please start a new thread with a specific question. This thread is closed.

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