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From: palmer@uw-june.UUCP
Newsgroups: net.physics
Subject: Re: Speed of gravity experiment
Message-ID: <1332@uw-june>
Date: Fri, 27-Apr-84 12:23:52 EST
Article-I.D.: uw-june.1332
Posted: Fri Apr 27 12:23:52 1984
Date-Received: Sat, 28-Apr-84 10:33:30 EST
References: <663@vax135.UUCP>
Organization: U. Washington, Computer Sci
Lines: 34

fnord

    The experiment described by Charles Poirier (two very-high-energy
lasers fire photons of gravitationally significant mass so that they
pass near to each other and are deflected) is very interesting, and
rather hard to solve.

    In the first place, the gravitational field due to the photon guns
will change when their mass-energy is put into the photon.

    A second, more major complication, is the effect of
gravitomagnetics.  Just as a moving charged particle produces a
magnetic field, a moving, massive particle produces a gravito-magnetic
field.  (Electric and magnetic fields are each the other as seen in a
different frame of reference.  Ditto for gravity and gravito-magnetism.)

    In the case of electromagnetics, for charged particles moving at a few
percent of the speed of light, the effect of magnetic fields is to make
the force on particle B (stationary in its frame of reference) due to
particle A (moving in B's frame) the same as if particle A were
stationary at the point it WOULD be if it continued on its straight-
line path while the electromagnetic signal was travelling from A to B.

    I do not know whether this result holds for the gravity of an
object moving at lightspeed, but the qualitative effect is the same.

    An additional wierdness may be due to the fact that the photon is
in its own gravitational well.  This may mean that it travels slower
than the speed of (low-energy) light due to time dilation.  Are there
any general relativists on the net who know anything at all about that
about which I know next to nothing? (whew, all that to avoid havingf a
preposition at the end :-))

                        David Palmer