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From: demillo@uwmacc.UUCP (Rob DeMillo)
Newsgroups: net.sf-lovers
Subject: Re: Time travel, take 2
Message-ID: <1656@uwmacc.UUCP>
Date: Tue, 5-Nov-85 13:10:50 EST
Article-I.D.: uwmacc.1656
Posted: Tue Nov  5 13:10:50 1985
Date-Received: Fri, 8-Nov-85 05:49:51 EST
References: <291@caip.RUTGERS.EDU>
Organization: UWisconsin-Madison Academic Comp Center
Lines: 41

> From: Alan Wexelblat 
> 
> Thanks to those who replied to my earlier posting on time-travel.
> One thing still puzzles me though:  Is it the case that the center of
> mass of the universe doesn't move?  Is is (theoretically) possible to
> calculate our position/velocity w.r.t. this non-moving point?  People
> seem to talk about our universe as expanding, but exapanding away from
> what?
> --Alan Wexelblat
> WEX@MCC.ARPA

If we are to believe Einsteinian physics...there is no center of the 
Universe. When astrophysicists speak of the Universe expanding, they
are not talking about galaxies fleeing away from a common center.
What is being talked about is the fabric of spacetime (one word, please)
being expanded, and everything on it is going along for the
ride.

The common analogy that is used is: consider the Universe as 
the dough for raisin bread. The spacetime is the dough, and the
raisins are everything else. (i.e. galaxies, stars, people,
small marine mammals, whatever) As you let the dough rise, or
expand, it appears (regardless of what raisin you happen to
be) that all the other raisins are rushing away from you.
The result: everyone is rushing away from everyone else, but there
is no common center.

A subtle, but important, difference..


-- 
                           --- Rob DeMillo 
                               Madison Academic Computer Center
                               ...seismo!uwvax!uwmacc!demillo


     "...I suppose you think the concept of a 
         robot with an artificial leg is amusing?"

                    -- Marvin, the Paranoid Android