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Time Dilation Confusion

  1. Mar 22, 2015 #1
    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. jcsd
  3. Mar 22, 2015 #2


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    You need to find out about the so called 'twin paradox'. http://en.wikipedia.org/wiki/Twin_paradox.[/PLAIN] [Broken]

    Relativity is not about time-dilation and length contraction. These are just linked coordinate effects of the Lorentz transformation that connects any two inertial frames' coordinates. The important things are those that are *not* dependent on which observer measures them.

    The most important ( for some) is the fact that every clock measures its own time, which is a mathematical function of the spacetime length of its four dimensional path through spacetime.
    Last edited by a moderator: May 7, 2017
  4. Mar 22, 2015 #3
    Thanks for the pointer Mentz114! No need to beat a dead horse here, close this thread/delete it. You have given me plenty to chew on!
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