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From: Purtill.StudentNS@MIT-MULTICS.ARPA
Newsgroups: net.sf-lovers
Subject: Re(n): FTL Travel
Message-ID: <2819@topaz.ARPA>
Date: Fri, 19-Jul-85 19:08:39 EDT
Article-I.D.: topaz.2819
Posted: Fri Jul 19 19:08:39 1985
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From: Mark Purtill <Purtill@MIT-MULTICS.ARPA>

<Fnord> 
>or other, you have to "get out of the game"; warp drives, for,
>example, bop out of normal space into a different sort of 
>environment and bop back into normal space somewhere else, 
>by-passing the normal space in between.  Another approach is to 
>diminish the mass of your ship in some currently unknown way, to 
>compensate for the diminishing, return you're getting from the force 
>you apply.

Unfortunately, that doesn't help.  As long as your FTL drive obeys the
"Principle of Relativity" (which just states that physics is the same in
all inertial frames of reference) FTL travel => time travel.  I will try
to explain (and will probably fail).  For convienince, we'll assume an
instantanious drive, but a similar argument works for any velocity > c.
(This explaination assumes you've seen special relativity before.)

          t |    t' /            |         We have two frames of
            .A      .B           |      reference: the x,t frame and
            |      /             |      the x',t' frame.  The '(primed)
            |      |             |      frame is moving at about .5c
            |     /              |      relative to the unprimed frame.
            |     |              |        Given an instantanious drive,
            |    /               |  x'  we'll show that S can communicate
            |    |               |___/  with T, which is in it's past on
            |   /             ___.S     the world-line ST.  Bad news.
            |  |          ___/   |
            | /       ___/       |
            | |   ___/           |
            |/___/               |
           O./___________________.T___ x

Okay, so what we do is start off at S moving at .5c relative to the x,t
frame, ie, in the x',t' frame of reference.  We use our drive, and end
up at O (which is simultaneaous to S in the x',t' frame of reference.)
Waiting there is another ship, in the x,t frame of reference.  We send
(via radio or whatever) S's message to that ship, which uses it's drive
to get to T (simultaneous in *it's* frame of reference.) But T is just S
in the past!

Now, *if* one is willing to postualte a special frame of reference,
wherein one can travel faster than the speed of light, one is okay.  If
there are two such, you get time-travel.  Also, note that Tachyons avoid
the time-travel paradoxes by not being detectable from the STL side of
the universe.

[This is gotten from a physics book called _Special_Relativity_ or some
such, by an author whose name starts with a 'B,' but which I've
forgotten.  If my explaination is incomprehensible, you might try
looking in a book with the same name for a better explaination.]

       Mark
^.-.^  Purtill at MIT-MULTICS.ARPA    **Insert favorite disclaimer here**
(("))  2-032 MIT Cambrige MA 02139