# time dilation

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I understand that according to the special theory of relativity, time passes more slowly as speed increases. So, a spaceship travelling from the earth to some distant place would experience a time-slowing effect, as viewed from the earth. But also according to the special theory, there is no preferable inertial frame. So it would be just as correct to say that the earth is moving away from the spaceship. Therefore it is us who are experiencing the time dilation effect, in the spacehip's frame. Logic tells me something is wrong here. It seems to me the time dilations would effectively cancel, leaving no time dilation at all. Apparently, this is not the case. Can somebody explain to me where I'm going wrong?

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O.K., I'm interpreting a section of my physics textbook (this has to do with the "twin paradox")...

The theory of relativity only applies to inertial reference frames. Now, if we assume the space ship started at Earth, accelerated to near the speed of light, decelerated when it reached whatever destination it had, accelerated when going back to Earth, and decelerated when it reached Earth, then the space ship is no longer an inertial reference frame. On the other hand, any acceleration Earth has is negligible, so it can be treated as an inertial reference frame. Therefore, it's only the time on the space ship that increments more slowly relative to the time incrementation of the Earth.

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 Original post by ZaxxO.K., I'm interpreting a section of my physics textbook (this has to do with the "twin paradox")...The theory of relativity only applies to inertial reference frames. Now, if we assume the space ship started at Earth, accelerated to near the speed of light, decelerated when it reached whatever destination it had, accelerated when going back to Earth, and decelerated when it reached Earth, then the space ship is no longer an inertial reference frame. On the other hand, any acceleration Earth has is negligible, so it can be treated as an inertial reference frame. Therefore, it's only the time on the space ship that increments more slowly relative to the time incrementation of the Earth.

Hmm. Interesting. I knew the definition of inertial frame, I guess I just never thought of the consequences of the 'inertial' part. Thanks!

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 Original post by ZaxxNow, if we assume the space ship ... accelerated ... decelerated ... accelerated ... decelerated, then the space ship is no longer an inertial reference frame. ...

Yes and no. The amount of acceleration in irrelevant since the paradox still holds if the period of acceleration is instantaneous -- but it does indicate which twin will be younger.

The solution to the paradox is realizing that each twin's experience is not the same. One twin experiences two separate inertial frames while the other experiences only one.

[Edited by - JohnBolton on May 23, 2005 5:33:00 PM]

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If the ship travels at 0.9c away from Earth, then each will thing time is travelling slower for the other. But they can only check this by exchanging information which travels at a finite speed.

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