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From: dgary@ecsvax.UUCP
Newsgroups: net.physics
Subject: Re:  Relativity questions
Message-ID: <2416@ecsvax.UUCP>
Date: Tue, 1-May-84 13:38:51 EDT
Article-I.D.: ecsvax.2416
Posted: Tue May  1 13:38:51 1984
Date-Received: Wed, 2-May-84 05:48:12 EDT
References: sri-arpa.107
Lines: 41


RE the twin "paradox"  ...

The classic pu\zzlement runs thisaway:  "If A goes to Alpha Centauri at .995c
and comes back younger than B, the twin who stays home, OK.  But howcome
(if 'all motion is relative') we can't assume B is 'at rest' and A (along
with the majority of the Cosmos) goes gallavanting away and back at .995c?
Then B would be younger than A!!"

Many textbooks discuss this problem and quite a few get it wrong.  Many claim
that since accelerations are involved, General Relativity is necessary.  But
this is not strictly so.  By increasing the length of the trip but keeping the
accelerations constant, we can vary the relative contribution of General
Relativity effects as much as we like.

A simple way of explaining the problem is to note that special relativity
deals with inertial reference frames, and if you feel an acceleration, you're
not in a single inertial frame but moving between them.  So B is (more or less)
staying in a single frame on Earth while A jumps into one frame headed out and
another coming back.  (By being 'in' a frame I mean at rest in it.)
This makes it clear that A and B are NOT symmetrical and cannot easily be
exchanged.

I find this a little too simple, though.  For detailed analysis, see if you
can dig up the APS book of reprints named "Special Relativity - Selected
Reprints" or something like that.  There's a partial rerun of the famous
Dingle exchange in Nature (Dingle was a physicist who believed the Twin
Paradox proved Einstein wrong).  You might also dig up Science News from
about 1978 which had a long exchange of letters on the subject, almost all
of which were nonsense but worth reading for amusement.  The editors finally
called a halt to it by yelling, "Shut up on this relativity crap!!" or
something.

Finally, Taylor and Wheeler's Spacetime Physics is absolutely tremendous, and
may be the best physics text ever written.  I recommend it for an analysis of
many well-known relativity "paradoxes."

Best,
D Gary Grady  Duke University Computation Center  Durham,  NC  27706
              (919) 286-4296    (919) 684-4146
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