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 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