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From: matt@oddjob.UUCP (Matt Crawford)
Newsgroups: net.physics
Subject: Re: FTL and time-travel
Message-ID: <851@oddjob.UUCP>
Date: Sat, 13-Jul-85 18:51:31 EDT
Article-I.D.: oddjob.851
Posted: Sat Jul 13 18:51:31 1985
Date-Received: Sun, 14-Jul-85 09:04:19 EDT
References: <375@sri-arpa.ARPA>
Reply-To: matt@oddjob.UUCP (Matt Crawford)
Organization: U. Chicago, Astronomy & Astrophysics
Lines: 25

In article <375@sri-arpa.ARPA> Purtill@MIT-MULTICS.ARPA writes:
>>[from: mit-eddie!nessus@topaz.arpa]
>>According to Special Relativity, faster-than-light travel is
>>exactly equivalent to traveling backwards in time: there is no
>>difference.
>
>Is that really true?  If so, could someone please explain how?

I wouldn't say "exactly equivalent", since a single trip at a speed
faster than light will have you arriving at a time which to *some*
observers is earlier than your departure but which to other observers
is later.  However ...

If I suppose the existence of boxes which can send out signals at
a fixed faster-than-light speed (in the reference frame of the
sending box) and receive these signals from other boxes, then I can
place two of these boxes in motion at speeds slower than light in
such a way that the first box can send a signal to the second and
receive the reply before it sends the initial signal.  Thus any SF
writer who wants to invoke FTL signalling will have to either deal
with these violations of causality or invoke some preferred frame of
reference such as the rest frame of the microwave background.
_____________________________________________________
Matt		University	crawford@anl-mcs.arpa
Crawford	of Chicago	ihnp4!oddjob!matt