# Time Dilation Confusion

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1. Mar 22, 2015

### thrush

Hi everybody,

I'm learning Special Relativity, and probably ok with four vectors, the metric equation, the Lorentz Transform, and the Doppler shift, etc., but enough about me.

I'm still a little confused about time dilation. In several hypothetical examples of SR that I have seen, two spaceships, call them A, and B, are traveling apart. From A's frame, B's velocity is 0.9C, and B's spaceship has contracted in length and increased in mass. From B's frame, A's velocity is 0.9C, and A's spaceship has contracted in length and increased in mass. When both slow down so that the relative velocity becomes zero, all is back to 'normal' and they then occupy the same reference frame (with standard orientation, as some say).

But what about time? From A's frame, time is slowed on B's spacecraft. But from B's frame, time is slowed on A's spacecraft. Would it not then be the case that their clocks would read identically even until their relative velocities became zero again? Which astronaut aged more slowly? Do not these examples about an astronaut aging more slowly on a high speed round-trip imply that she is moving with respect to some more fundamental frame upon which we sit back here on Earth? And does not SR imply that there is no "fundamental frame?"

I know this must be a thread many times here, apologies, but clearly I am missing something. Thank you.

2. Mar 22, 2015