From: palmer@sfu.ca (Leigh Palmer)
Newsgroups: sci.astro
Subject: Re: The Hole Story
Date: 18 Dec 92 22:06:52 GMT
Organization: Simon Fraser University

I'm posting this for a temporarily communicationally challenged colleague. I
offered him a pair of very massive semaphore flags, but he prefers this medium.
Disclaimers apply.

From Benjamin Weiner:

metares@well.sf.ca.us (Tom Van Flandern) writes:
>     First, gravitational radiation == gravity waves; the terms are used
>interchangeably.  So I disagree with your statement, "You need _radiation_ to
>communicate."  One should be able to communicate by moving masses around
>without generating any gravity waves.

Tom, you are obscuring the issue.  I am perfectly willing to accept your
point that the static field has no propagation delay.  I would phrase
this as: given the distribution of masses and their velocities, one can
then calculate their future positions (Cauchy problem is solved) - and your
point is that the "forces" which one sees (really curvature of the geodesics)
point to instantaneous, not retarded, positions, et cetera.  Fine.

It's my contention that, to communicate, you have be able to send a message
and that this "message" should not be predetermined by the Cauchy data.
Suppose I decide to sacrifice myself for science, and leap into a
10^6 M_sun black hole, carrying two giant iron spheres.  The tidal force
at the event horizon of such a big BH is small and so I cross it
without being smushed.  I decide to send a message (say "I'm
through OK") to Tom, who is smart enough to stay outside, with my iron
spheres.  How do I do it?

Tom knows what my velocity and that of the spheres were as he waved
a forlorn goodbye to me.  So he could predict (in theory; the problem
is intractable except maybe numerically) the positions of the spheres,
via the geodesic equation, without knowing what the hell I was trying
to say.  No communication.  To communicate I have to wiggle the
spheres *and it has to be done by non-gravitational forces* otherwise
Tom could predict it, and we assume Tom could not predict that I was going
to say "I'm through OK" rather than "Knicks by six in OT."

So fine, I wiggle the spheres back and forth with some kind of big piece
of construction equipment, which is held together by electromagnetic
forces.  Exerting EM forces on the spheres, they no longer move along
geodesics, and produce some kind of disturbance (postulating the
existence of gravitational radiation) - a perturbation in the metric.
Such gravitational radiation (assuming its existence) propagates along
null geodesics - lightlike lines.  It cannot escape the black hole.
My message will not reach Tom.

Essentially my argument, with or without black holes, is this: for
communication to mean anything, it must involve non-gravitational
forces - in a spacetime that has a Cauchy surface, anyway (which includes
all reasonable spacetimes).  If one communicates by "moving masses around" 
via rockets or whatever then one is bringing in non-geodesic motion.  You 
can't communicate via the static field because it is predictable, and if 
you have non-geodesic motion then the field is not the static field.

>     In GR, it is implicit that nothing needs to propagate to maintain the
>static field.  If it did, that would cause a problem for SR, since the
>experimental evidence is that any such propagation must be ftl.  In GR, it is
>required that the static field around any mass pre-exists out to infinity
>without detectable delay.

>     Masses accelerate in response to the static fields of other masses.
>Gravity waves are not the cause of this acceleration.  The acceleration
>could, in principle, take place without generating any gravity waves; for
>example, if the space-time medium were extremely resistant to wave
>disturbances.

>     Gravity waves are small ripples through space-time set off by the
>acceleration of masses.  They are not to be confused with the changes of
>curvature of space-time as masses move that happen without detectable delay.
>If these motions set off waves also, those propagate through the medium at
>the speed of light.
Newsgroups: sci.astro