From: [m--e--e] at [infonode.ingr.com] (Maurice Beyke)
Newsgroups: rec.games.frp.advocacy
Subject: Re: Realistic FTL?
Date: 20 Oct 93 19:05:34 GMT


 Okay, I'll try to keep this simple (mainly for my own benefit; being a
 dopey engineer and not a physicist, it took me a while to clearly see
 the causality violation in FTL).  I apologize for any inaccuracies this
 may have.

 Say the lost Orion party craft flies past the earth at 0.866c, headed
 toward Proxima Centari (which, since I don't have my references handy,
 I'll fudge and say is 4 lightyears away).  As they zoom past, they send
 us the schematics for an ansible, some sort of FTL communications device.

 Now, *in our reference frame* they will arrive at PC in 4.62 years.  Say
 it only takes the brilliant earth scientists a year to decode their language
 and build the ansible, but the UN can't decide what to say to them until
 they reach PC.  When send them a congratulatory message as they zoom past
 PCentari, only 2.31 years would have passed for them, *in both reference
 frames*, but this is enough time for them to get to PC in their RF due to
 length contraction.

 Now, since they see us as moving past them at 0.866c, *in their reference
 frame* they see us as experiencing time dialtion as well, and in that RF
 *only 1.15 years has passed on Earth*.  Now, according to SR, their
 reference frame is just as valid as our own.  So, when they use their
 ansible to send a message back, it is received on Earth, *in both reference
 frames*, 1.15 years after they left the Solar system, which is 3.47 years
 *before* we sent them the message.  Causality is violated.

 Now note that nowhere in this discussion is the nature of the FTL
 communication stated.  The ansible could be a hyperspace wave radio, a
 warp drive message torpedo, a instantaneous matter transmitter, or
 any other device.  The only assumptions required are that Special
 Relativity (which has been rigorously tested) is accurate, and that the
 FTL communication device can cross from one reference frame to another.
 If you disallow the latter, then FTL and causality may both be retained,
 according to my limited understanding.  This is the case for both the
 "Alderson Jump Drive" of _Mote_in_God's_Eye_ and the FTL teleport in
 _The_Nimrod_Hunt_.  In the former, a reference frame is defined by the
 two corresponding Alderson Points, and the ship "jumps" point to point
 in that RF.  In the other, a universal RF is defined by the builders of
 the teleport stations.

 I hope this helps.
 ----
 Boris Mikey                |"Here the ways of men part: if you wish to
 aka Maurice Beyke          | strive for peace of soul and pleasure, then
 [m--e--e] at [batman.b11.ingr.com]| believe; if you wish to be a devotee of
 These views are mine,      | truth, then inquire."
 Ingr. doesn't want them    |                                  Nietzsche