From: markc@smsc.sony.com (Mark Corscadden)
Subject: Re: FTL communication in SR does not violate causality
Keywords: FTL SR causality Special Relativity
Organization: Sony Microsystems Corp, San Jose, CA
Date: Fri, 18 Dec 92 01:57:30 GMT

I think I see the point where a flaw creeps into Matt McIrvin's argument,
but the fault is at least partially mine.  In my original article I said:

MC> ... The existence of a preferred frame violates
MC> the principles of Special Relativity (I believe) but it does not contradict
MC> the mathematics of SR, and it's the math that has been confirmed over and
MC> over by experimentation.

Instead of being timid I should have said that my use of a preferred
reference frame violates the principles of Special Relativity, period!
Also, the "mathematics of SR" I refer to are only the mathematics of the
Lorentz coordinate transform, which I *do* assume to be universally valid.
I could have been more clear about these points.

In article <mcirvin.724046110@husc8> mcirvin@husc8.harvard.edu (Matt McIrvin) writes:
>If you just examine E1, E2, and E, there's no trouble.  But consider this:
>
>Suppose that there is apparatus at rest in O2's frame that is just like
>O1's, and vice versa.  O2 can set up events F1 and F2 which are both at
>the origin of O2's spatial coordinates; F1 follows E and F2 follows F1,
>according to O2's time coordinate.  F1 causes event F which is at the
>origin of O1's spatial coordinates, which in turn causes F2, all of this
>reckoned using O2's time coordinate.  This must be possible if SR
>applies and the apparatus works everywhere. 

Okay so far, except that the phrase "reckoned using O2's time
coordinate" seems not wrong but misleading.  F1 causes F causes F2
is not reckoned in relation to any time coordinate; causality is an
objective relation that holds in all reference frames regardless of
time coordinates.  It is a bit odd that the time coordinate of F1 can
follow (i.e. be greater than) the time coordinate of F (in O2's frame)
even though F1 caused F, which was why I was careful to say just what
FTLC entails.

>Now if O2's frame is moving at a sufficiently high speed relative 
>to O1, it's easy to see that F can *precede* E1 according to O1's time
>coordinate.

This is where the argument goes wrong.  It's true that F can precede *F1*
in O2's frame, or more exactly the time coordinate assigned to F in O2's
frame can be less than the time coordinate assigned to F1 in O2's frame,
but I don't see how you can show that F must precede *E1*.  Without a
specific example with all space-time coordinates spelled out explicitly
I can't be sure that I'm addressing the point you have in mind, though.

The rest of your argument is correct, in that having F precede *E1* would
lead to a violation of causality, but I don't think you can show that F
can precede E1.  I don't want to give the mistaken impression that I'm
promoting the bizarre "information forgetting" communication you describe
or some equally unfair reworking of the words "communication" or
"causality".  You can use "communication" and "causality" according
to their normal meanings, it shouldn't effect my point.

>Matt McIrvin

Mark Corscadden
markc@smsc.sony.com
work: (408)944-4086