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I can't get something strait... Lets use the usual example: A man is flying away from earth on a very fast spaceship. So, His time is now not moving in the same "speed" like earth time but slower. But why do we say that the man is moving faster than earth? why is it not that earth is moving faster than the man? I mean, it's not like there is a direction that if you move that way then you are faster and the other way means you moves slower.

So, how can we assume that's time is moving slower for the man and not on earth? Maybe the man is flying the "other" way and now he is slower than earth... If the man is not accelerating than it could appear to him that earth is moving fast and he is stationary... isn't it?

So, I can sum it as: In relation to what do we say that something is faster than some other thing?

Thanks!

roiberg
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  • there are mountains of questions here regarding time dilation. For the "moving away/towards" business, don't confuse time dilation with the doppler effect (see e.g. here: http://physics.stackexchange.com/questions/99175/does-time-dilation-correct-for-the-doppler-effect). For the refernce question, also see e.g. here: http://physics.stackexchange.com/questions/6147/time-dilation-how-does-it-know-which-frame-of-reference-to-age-slower – Martin Feb 26 '15 at 10:45

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So, I can sum it as: In relation to what do we say that something is faster than some other thing?

We just arbitrarily decide that something is motionless. Very often we pretend that earth is the motionless thing, although earth revolves around the sun.

We know for sure that earth is not motionless, so we can conclude that earth for sure experiences time dilation.

If an astronaut revolves around the sun faster than the earth revolves around the sun, then that astronaut experiences more time dilation than the earth.

When motions are revolving motions, or back and forth motions, we know which motion is faster.

A little addition:

Earthlings looking at the sun must see:

1: time dilation as they see the sun moving

2: time ticking fast on the sun as they are revolving

Admittedly that is slightly odd. For an observer on a revolving platform things are slightly odd.

stuffu
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Special Relativity only applies to non-accelerating inertial reference frames, thus special relativity cannot be used to model your example.

But why do we say that the man is moving faster than earth? why is it not that earth is moving faster than the man?

That's a subtle misunderstanding of relativity - the critical factor is the frame of reference of the observer. If you are an observer on Earth (assuming Earth is not accelerating), you would observe the man moving with respect to you. If you were the man, not accelerating, you would observe the Earth moving with respect to you.

Nick
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  • Lets say they are not accelerating... or what happens when the spaceship stops to accelerate. who is faster than? – roiberg Feb 26 '15 at 10:28
  • Acceleration and deceleration are covered by General Relativity. Special Relativity is the special case of non-accelerating inertial reference frames. – Nick Feb 26 '15 at 10:31
  • See my edit to my comment about observers. There is no "who is absolutely faster" only, "who is moving with respect to me". – Nick Feb 26 '15 at 10:34
  • @Nick: you say "Special Relativity only applies to non-accelerating inertial reference frames" but this isn't true. See for example Can a ultracentrifuge be used to test general relativity?. It is true that the Lorentz transformations don't apply to accelerated motion, but that just means the Lorentz transformations are a special case not that SR in general can't handle accelerated motion. – John Rennie Feb 26 '15 at 10:39
  • @Nick - So why is it "known" that if an astronaut is flying from earth for a few years he will get back "younger" than his friends on earth? Or is it wrong? If it's right than it means that the man was faster from both perspectives... – roiberg Feb 26 '15 at 10:41
  • @JohnRennie very interesting. I do know it's more complex, but as a rule-of-thumb for people just discovering the principles of relativity I think it's a reasonable assumption. – Nick Feb 26 '15 at 11:01
  • @Nick: if this was the Discovery channel I'd agree, but this is the Physics Stack Exchange. We're all serious about physics here. – John Rennie Feb 26 '15 at 11:17
  • @JohnRennie I'd say it's more important to tailor your response to the level of the questioner, so they can learn something, rather than over-confusing them with something that doesn't actually improve their understanding. – Nick Feb 26 '15 at 13:56
  • Nic, the first sentence of your answer is simply wrong; see, as a counterexample, Rindler observers. Also, the final sentence in the 2nd comment is simply wrong; what is 'special' about SR is that the spacetime metric is constant and flat. Accelerated reference frames are not a problem in SR. – Alfred Centauri Feb 26 '15 at 15:53
  • @Nick: this is my problem too. It's not a question of necessary simplification. You have made a statement that is simply incorrect. – John Rennie Feb 26 '15 at 16:03