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From: matt@oddjob.UUCP (Matt Crawford)
Newsgroups: net.physics
Subject: Re: FTL and time-travel -- exercise for the reader
Message-ID: <860@oddjob.UUCP>
Date: Wed, 17-Jul-85 17:17:06 EDT
Article-I.D.: oddjob.860
Posted: Wed Jul 17 17:17:06 1985
Date-Received: Thu, 18-Jul-85 20:33:44 EDT
References: <375@sri-arpa.ARPA> <851@oddjob.UUCP>
Reply-To: matt@oddjob.UUCP (Matt Crawford)
Organization: U. Chicago, Astronomy & Astrophysics
Lines: 23

In article <851@oddjob.UUCP> I give an example of how faster
than light signalling can lead to causality violations.  As
an exercise for the readers of this newsgroup who think they
understand special relativity, I pose the following question.
I provide the answer at the end of this article.

Suppose that at time t=0 person A emits a signal with velocity
u > c in A's own frame of reference.  This signal is received
by B who at that instant is at a distance d from A (as measured
by A) and is moving away from A at speed v, with c^2/u < v < c.
B immediately replies by sending back a signal at speed u in
B's own reference frame.

At what time does the reply signal from B reach A?

Answer:  In A's frame, the reply arrives at time

	t = d/u - d*(uv/c^2 - 1)/(u-v).

Under the assumption c^2/u < v < c, this is negative.
_____________________________________________________
Matt		University	crawford@anl-mcs.arpa
Crawford	of Chicago	ihnp4!oddjob!matt