The twin Paradox

Karl W4KRL W4KRL at
Sun Jul 8 11:20:24 CDT 2012

At Tacos Terry M raised a question about relativistic time contraction: if a
space ship travels near the speed of light with respect to the earth the
space ship clock appears to run slow as seen by an observer on earth. When
the spaceship returns to earth the crewmen will be younger than they would
have been if they had stayed on earth.  Why aren't the earthmen younger than
the crewmen because from the spaceship point of reference the earth was
moving away at high speed so the earth clock appeared to run slowly?


The question assumes that there is a symmetry in the experiment: each
observer sees the other as moving so each sees the other's clock as running
slower. In fact, the experiment is asymmetrical. The space ship accelerates
to near light speed, decelerates to zero speed, accelerates and decelerates
in the reverse direction to get back to earth. Meanwhile, the earth does not
do any acceleration with respect to the spaceship. Since the inertial frames
of reference are not symmetrical, there is an asymmetrical dilation of time.
The crewmen do not age as much as the earthmen during the experiment.


My favorite story about time dilation is "Time for the Stars" by Robert A
Heinlein. It is found that certain twins can communicate telepathically and
instantaneously (not limited by the speed of light). One twin stays on
earth, the other travels in space to provide a communication link. They are
challenged by the effects of time dilation when the ship is near light speed
and also by the cumulative differential change in ages during the mission.
It's a good story.


73 Karl W4KRL

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