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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.space
Subject: Re: speed
Message-ID: <165@utastro.UUCP>
Date: Fri, 31-May-85 12:12:17 EDT
Article-I.D.: utastro.165
Posted: Fri May 31 12:12:17 1985
Date-Received: Sat, 1-Jun-85 13:44:12 EDT
References: <359@osiris.UUCP>
Distribution: net
Organization: U. Texas, Astronomy, Austin, TX
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> I've heard the twin paradox, and until recently I thought I had it
> straight.  I thought of something recently, though.  When one twin
> takes off, leaving the other here, why does the one in space age
> more slowly?  Why can't you use a reference frame travelling with
> him and say that the earth is travelling at a great velocity?  Am
> I missing something?  (Obviously I am.)
> 
> 				Rob St. Amant
 
You can use any reference frame you want.  You have a choice of three
obvious ones:  Earth, astronaut on his way out, and astronaut on his
way back.  The only requirement of the theory is that when the astronaut
returns and compares his age the answer should be consistent with description
in any reference frame.  We have

Earth:  Joe consistently aged more slowly than his hidebound brother John who
        stayed home.

Outward bound frame :  At first John, sitting on a rapidly moving Earth, was
        aging more slowly than Joe.  However, Joe decided to catch up to
        the Earth and during this time aged hardly at all.  The net effect is
        that more time passed for John than for Joe.

Inward bound frame: At first John and Joe are first moving at high speeds.  
        However Joe is moving much more rapidly than John.  After a while
        he stops moving and John (and the Earth) catches up to him.  
        However, Joe spent very little time standing still and the net effect
        is still that less time passed for him (because of his earlier high
        speed motion).
-- 

"Don't argue with a fool.      Ethan Vishniac
 Borrow his money."            {charm,ut-sally,ut-ngp,noao}!utastro!ethan
                               Department of Astronomy
                               University of Texas