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From: Purtill@MIT-MULTICS.ARPA
Newsgroups: net.physics
Subject: Re: FTL and time-travel -- exercise for the reader
Message-ID: <445@sri-arpa.ARPA>
Date: Sun, 28-Jul-85 19:28:00 EDT
Article-I.D.: sri-arpa.445
Posted: Sun Jul 28 19:28:00 1985
Date-Received: Wed, 31-Jul-85 08:15:12 EDT
Lines: 21

From:  Mark Purtill <Purtill@MIT-MULTICS.ARPA>

<Fnord>
>          t = d/u - d*(uv/c^2 - 1)/(u-v).
>
>Under the assumption c^2/u < v < c, this is negative.
I think you're wrong.  Try u = 3c, v = .5c.  (I assume d > 0).  You get:

t = (d/c)*( 1/3 - ( ((3/2)-1)/(3-.5) ) ) = (d/c)*(1/3 - 1/5) = 2d/15c > 0.

The correct condition is:

u > v > 2u/(1+(u/c)^2) > 0.

This implies u > c, but note that v can be >= c. Now, when u >> c, we
can ignore the 1, and get v > 2u/(u/c)^2, i.e., v > 2 c^2/u, which is
sort of like what you had.

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