Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site oddjob.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxn!ihnp4!oddjob!matt 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