Re: [Phys-l] twin paradox question

[John Mallinckrodt <ajm@csupomona.edu>]

The twin paradox is usually and perfectly well explained in terms of the shift of reference frames that the traveling twin (and ONLY the traveling twin) undergoes at the turnaround point.  Well drawn spacetime diagrams make that explanation quite transparent.

Nevertheless, I think it is even easier and certainly more concrete to think about it in terms of Doppler shift.  Both twins SEE (literally SEE) the other twin aging more slowly as they move away from each other because of the Doppler shift and both SEE the other aging more quickly as they move towards each other.  The traveling twin SEES equal periods of slower and faster aging because the Doppler shift changes for her immediately upon turning around.  In contrast, the stay at home twin sees a much longer period of slower aging, because he doesn't see the Doppler shift change until the light from the turnaround event gets to him.  Thus, the traveling twin is younger when she returns.

See http://www.csupomona.edu/~ajm/materials/twinparadox.html for a more complete explanation and spacetime diagrams.

John Mallinckrodt
Cal Poly Pomona

On Dec 18, 2010, at 5:36 PM, Michael Barr wrote:

> I have always wondered something about the twin paradox.  If one twin is
> moving away from the Earth at high speed time moves slower for him compared
> to his brother.  If the brother on Earth were able to see his brother on the
> fast moving ship everything would appear in slow motion.  Here is my
> question.  If the on the fast moving space ship were to look at the Earth is
> it moving away from him at a very high speed too.  So doesn't everything on
> Earth appear to be moving in slow motion compared to time on the space ship?
> So, why upon return is the traveling twin young and the twin that stayed on
> Earth old?  I don't get it.
> Thanks,
> Mike Barr
>
> -- 
> Please consider the environment before printing this e-mail
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l