From: metares@well.sf.ca.us (Tom Van Flandern)
Subject: Re: The Hole Story
Organization: Whole Earth 'Lectronic Link
Date: Wed, 16 Dec 1992 16:45:11 GMT


Earlier, I wrote:

>> 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.

and max@west.darkside.com (Erik Max Francis) replied:

> I'm confused.  I thought that general relativity made a very clear
> distinction that no information can travel faster than light -- not even
> gravitational information.  So if you move a mass very quickly out of the
> way, somebody far away feeling the effects of that mass won't notice it
> until that information (carried by virtual gravitons) reaches them.

     The confusion is not yours alone.  Actually, it is SR, not GR, which
says that no information can travel faster than light.  Then when GR is
taught, the point about gravity acting without detectable delay is glossed
over, leaving students in a state of disbelief when they learn about it later
in their careers.

     First, here's what is true.  Light propagates through the solar system
at lightspeed, taking minutes to hours to reach the planets.  When light from
the Sun applies a force, for example, the Poynting-Robertson effect, that
force seems to come from the Sun's retarded position, as one logically
expects.  As a direct consequence, dust in the inner solar system slowly
spirals into the Sun, since delayed forces are non-central forces and always
change the angular momentum of an orbit.

     Gravity, by contrast, always appears to act from the true, instantaneous
position of the bodies involved.  It never appears to associate with retarded
positions.  And the Sun's gravity does not change the angular momentum of the
planets.  By direct observation, the directions of the Sun's arriving photons
at the Earth and the direction of its gravitational force on the Earth are
not parallel, since one comes from the retarded position and the other from
the true position.

     This can be verified by pure computation in a computer.  If we insert
lighttime delay into either Newtonian or GR equations of motion of the
planets, they quickly spiral out of their known orbits.  Gravity acts without
detectable delay.

     Now, the interpretation.  In GR, the static gravitational fields of
bodies out to infinity are supposed to accompany them in some sort of rigid
fashion.  When the Earth arrives at a point in its orbit, space-time there is
already curved without need for anything to propagate out of the Sun.
Unfortunately for ease of interpretation, this must also be true even when
the source of this curved space-time is accelerating, as in the case for
contact binary stars or binary pulsars.  If it were not, the orbits would fly
apart in a matter of relatively few revolutions.  The angular momentum
transfer for non-central forces is a large effect if propagation speeds are
at all comparable to lightspeed.

     So why can't you use this feature to send ftl signals?  In truth, you
probably can -- see the thread in sci.physics with the subject "FTL
communication in SR does not violate causality," where (surprisingly) there
seems to be agreement about this possibility.  But the usual explanation is
that all masses simply follow geodesics, so their motions are predictable
into the past or future, so no new information is communicated ftl.

     You are sure to object that you could send a signal ftl by interfering
with the normal, geodesic motion of a large mass.  In such a case, we have no
experimental evidence to guide us about what would happen.  There are two
possibilities, and each causes a bit of a paradox under some conditions.

     My own unique answer to this dilemma will appear in my forthcoming book
as part of a complete new cosmology, the "Meta Model," which has already been
much discussed in sci.astro.  The book is now about 10 weeks from its
publication.  -|Tom|-

-- 
Tom Van Flandern / Washington, DC / metares@well.sf.ca.us
Meta Research was founded to foster research into ideas not otherwise
supported because they conflict with mainstream theories in Astronomy.