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From: ethan@utastro.UUCP (Ethan Vishniac)
Newsgroups: net.space
Subject: Re: Re: twin paradox
Message-ID: <200@utastro.UUCP>
Date: Thu, 6-Jun-85 18:00:56 EDT
Article-I.D.: utastro.200
Posted: Thu Jun  6 18:00:56 1985
Date-Received: Sun, 9-Jun-85 01:45:08 EDT
References: <2094@mordor.UUCP>
Organization: U. Texas, Astronomy, Austin, TX
Lines: 29

> From: Charles.Fineman@CMU-CS-SPICE
> 
> > The key to the twin paradox is that the travelling twin goes on a ROUND
> > TRIP, so his frame of reference is an ACCELERATED FRAME (you cannot
> > return to Earth without changing direction, and you cannot change
> > direction without acceleration) whereas the stationary twin has an
> > INERTIAL FRAME. This is what makes their frames of reference
> > non-equivalent, thereby they will experience time differently. 
> 
> What if we assume that the universe is closed? Then it would be possible to 
> return to earth without changing your accelration. What happens then?

Propagation in curved space requires the application of General Relativity.
In this case the answer is that it isn't possible to circumnavigate
the universe travelling at less than the speed of light between the
the Big Bang and the Big Crunch (chomp).  However travelling *at*
the speed of light it is just possible.  In which case no time at
all passes for the traveler and the entire history of the universe
goes by for the stay at home.  Why?  Well basically the equivalence
of all uniformly moving frames does not apply in a curved space.
All that you are guaranteed is that *locally* the laws of physics
will be the same for the two.  However, the shape and evolution
of the universe are perceived differently by the two observers.
-- 

"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