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: Mon, 28 Dec 92 05:28:05 GMT

Given the positive responses from Tom Van Flandern, Daniel Gottesman,
and Daryl McCullough - and based on my experience working through the
proposed counterexample that Matt McIrvin sketched - I'm convinced that
the technical details of my original argument were correct.  However
there remains much confusion over what the technical result says about
the conflict between SR and FTLC, and I think that many people - including
professionals - misunderstand this conflict.

Disclaimers:  I'm not a professional physicist.  Also this article would
be ten times as long if I qualified every point.  Instead of trying to
anticipate and address every conceivable minor nit-pick, I've just stuck
to the main issues.  So onwards ...

In order to see what is going on you need to separate out two very
different things that are lumped together in SR:
  
  1)  The Principle of (Special) Relativity.
  
  This principle says that it is only relative motion which is relevant
  to the laws of physics.  In other words, nothing about the laws of
  physics can allow you to detect absolute motion with respect to empty
  space.  However, the laws of physics *are* allowed to refer to
  acceleration with respect to empty space - this is why it's called
  Special (restricted) Relativity as opposed to General Relativity.
  
  2)  The Lorentz Transformation.
  
  Given that you know the relative state of motion between yourself and
  another observer, the Lorentz transformation is a formula which allows
  you to calculate the spacetime coordinates which this observer assigns
  to an event in terms of the coordinates which you assign.  That's why
  it's called a "transformation": it transforms the coordinates assigned
  in one reference frame into the coordinates assigned in another frame.

These two things are independent.  Old-fashioned Newtonian mechanics uses
the Galilean transformation, not the Lorentz transformation (2), and yet
Newtonian mechanics satisfies the principle of relativity (1).  Many
people are aware of this.

What's not as well understood is that the Lorentz transformation (2) does
not imply the principle of relativity (1).  Given the very basic notion
of assigning spacetime coordinates to events, it becomes immediately
meaningful to ask how coordinates transform between frames in the real
world.  One can determine by experiment that coordinates in fact transform
according to the Lorentz equations and not the Galilean equations.  This
is an experimental fact that stands on its own merit and is quite
independent from the principle of relativity (1).

To put this succinctly, there is a very big difference between the validity
of the Lorentz *transformation* and the validity of Lorentz *invariance*.
They are very different things!

Getting back to FTLC, people understand that the validity of the Lorentz
transformation (2) is fundamental to the very framework of current
physics, and that you cannot change (2) without invalidating a massive
body of existing theory that has an equally great body of experimental
confirmation.  Then the mistaken belief that FTLC violates (2) comes into
play, and leads to a common misconception that the mere existence of FTLC
would invalidate current physics.  This simply is not true, and this
serious misconception creates an unfairly strong bias against FTLC.

I believe that, except for gravity which is addressed by General
Relativity, all currently accepted theory is formulated within the
framework of Quantum Field Theory (QFT).  Would the existence of FTLC
invalidate the current body of QFT and the great amount of experimental
confirmation of QFT results?  No!  All that you can say is that FTLC
would involve some currently unknown phenomenon, and that a theory which
described the laws governing this phenomenon could not be given within
the framework of QFT.

I personally see no reason to believe that some weird new class of
phenomena which violate the principle of relativity (1) will ever be
uncovered.   For that reason I think it's very unlikely that FTLC
is possible.  But that is simply saying that FTLC is unlikely because
it goes against the general *trend* of currently successful theories
and experimental results concerning *known* phenomena.  This is a very
far cry from saying that FTLC would *contradict* the existing body of
theory and experiment: that simply is not the case.  That's my point.

So, is it true that misconceptions about FTLC and SR really are common?
They seem to be; what do other people think?  How many people were aware
that FTLC is consistent with the Lorentz transformation (2)?  How many
have seen "proofs" that FTLC violates causality, proofs which make use
of nothing but the Lorentz equations and therefore can't be valid?  Does
anyone besides me agree that it's worth understanding that neither (1)
nor (2) alone contradict FTLC, but only both taken together?

Mark Corscadden
markc@smsc.sony.com
work: (408)944-4086
Newsgroups: sci.physics,rec.arts.startrek.tech