From: hora@ecf.toronto.edu (HO ROBERT ANDREW)
Subject: Re: FTL travel/communication
Organization: University of Toronto, Engineering Computing Facility
Date: Fri, 14 Aug 1992 02:37:29 GMT

In article <1992Aug13.181330.6422@tfs.com> paigen@tfs.com (David Paigen) writes:
>hinson@maxwell.physics.purdue.edu (Jason W. Hinson) writes:
>
>>I wrote the following message to someone who asked me to explain why FTL
>>travel/communication impies a causality problem, and I thought it might
>>be of some interest to readers of r.a.s.tech:
>
>Ok.
>
>>senario:
>>Person A is on earth, and person B speeds away from earth at a velocity v.
>>To make things easy, lets say that v is such that for an observer on
>>earth, person B's clock runs slow by a factor of 2. 
>
>Right, person B has a slow clock.
>
>[deleted]
>
>>NOW we must switch to B's point of view.  From his point of view, A has
>>been speeding away from him at a velocity v.  So, to B, it is A's clock
>>that has been running slow.
>
>*BZZT* wrong, but that you for playing :-)
>
>If A thinks B has a slow clock, then B thinks A has a fast clock.  From
>this point onwards your scenario is meaningless.  I have seen arguments
>about this causality violation before, but I can't remember how it works.
>Please try again, only with a bit more care.

This would be true if there was such a thing as an "absolute" time which was
the same for all frames of reference.  From Special Relativity, we know that
this is not true, that the observance of the passage of time in another
frame of reference depends on the velocity of that frame of reference with
respect to your own "rest" frame.  Hence the (so-called) twin paradox,
where twin A stays on earth and watches twin B speed off to some distant
star and back.  From twin A's perspective, B is always moving with some
velocity v(t) with respect to A's rest frame.  Thus, according to SR which
calls for time dilation in the moving frame, B will age slower than A and
return to earth much younger than A.  From B's perspective, it is A who is
always moving with the (reversed) velocity v(t) with respect to B's rest
frame, and therefore A should be younger when B returns due to time dilation.

Unlike the twin paradox, the scenario proposed by the original poster involves
two inertial frames of reference moving with some constant speed with respect
to one another.  Therefore, it is equally valid to say that A moves with
velocity v away from B in B's point of view, or that B moves with velocity
v away from A in A's point of view.  Which means that from SR, A observes
B's clock to run slow while at the same time, B observes A's clock to run
slow.  

>
>I have been unable to construct a scenario that violates causality using
>telephone (instantaneous) communication and horseback (c) travel.  Is it
>required that one party be traveling near the speed of light in order to
>violate causality?
>
>----------------------->"Lay on, McDuff,    14th c. ideals in a 21st c. world
>David Paigen            and damn'd be he        N: PLO Bush IRA CIA KKK
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-- 
=============================================================
Robert Ho               | "Have you got a plan m'lord?"
Univ. of Toronto        | "Yes, and it`s so cunning you could
hora@bullet.ecf.toronto.|     brush your teeth with it."
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