From: utzoo!decvax!cca!JGA@MIT-MC@sri-unix
Newsgroups: net.physics
Title: paradoxical twins
Article-I.D.: sri-unix.3302
Posted: Thu Sep 16 08:24:26 1982
Received: Fri Sep 17 03:18:44 1982

From: John G. Aspinall 
    Date: 15 Sep 82 19:28:39-PDT (Wed)
    From: harpo!ihps3!houxi!deimos!ariel!houti!trc at Ucb-C70
    Article-I.D.: houti.154
    Via:  Usenet; 16 Sep 82 4:08-PDT

    Now that simultaneity has been settled (I hope) how about the good old
    twins paradox?

    My question here is, if both frames of reference are equally good,
    then why should one twin age differently than the other?  In fact,
    it would seem to me to be reasonable to argue that, there is in fact
    a prefered (stationary) frame of reference, but that the only way to
    detect the difference is to check the relative time dilation effects
    between two frames.

    I have heard it argued that the difference is that one undergoes
    acceleration and the other doesnt, and that this somehow explains
    the effect.  However, the difference in ages doesnt occur during
    the acceleration, for the most part, but rather during the long
    period of free fall.

Sigh.  You have just invoked simultaneity again.  That is, you have compared
their ages ("at the same time") when they're in different inertial frames.

The "stay-at-home" twin stays in one inertial frame, the
"rocket-traveller" twin switches from one inertial frame to another.

I hate to end this here, but a good explanation would take more time
and effort than I have available.  I highly recommend French's explanation
in "Special Relativity" (Norton, MIT Introductory Physics Series, 1968)
on pages 154-159.  He makes the asymmetry between the twins clear, and also
shows that special relativity is adequate for handling the problem.

John Aspinall.