Re: Speed of Light/Gravity

George Murphy (
Tue, 28 Apr 1998 07:35:45 -0400

Bill Payne wrote:
> 27 Apr 1998 20:40:26 -0500, Glenn R. Morton wrote:
> > Nothing
> > travels faster than light and to travel 1000 light years in one year would
> > violate the speed of light.
> I'm only going to respond to this one point in Glenn's excellent post.
> I understand that gravity travels much faster that light, almost
> instantaneously. If it did not, then the planets would be unstable in
> their orbits.
> I think it works like this. When sitting still in a car while it is
> raining, you can see the rain outside falling vertically down. When you
> begin to move, the rainfall appears to slant back. Likewise, because
> the earth is orbiting the sun, the light rays from the sun slant in the
> direction of the earth's travel relative to the sun, and the sun's disk,
> as observed from the earth, is the sun's apparent position, not it's
> true position. During a total solar eclipse, the moment of totality
> comes either before or after (I can't remember which) the gravational
> maximum exerted on the earth by the sun and moon combined.
> Hopefully someone else can shed more light on this, the speed of
> gravity.

In general relativity, currently the best gravitational theory
we have, gravitation is propagated at the speed of light. We have
indirect evidence for the existence of gravitational waves, though not
yet any direct measurement of their speed.
Note 8 of the 2d ed of Eddington's _The Mathematical Theory of
Relativity_ has a comment which is germane. In discussing the emission
of energy from a spinning rod via gravitational radiation Eddington
"If gravitation is not propagated instantaneously the lag may
cause tangential components of the force to occur, so that there will be
a couple presumably opposing the rotation. Laplace anticipated that if
gravitation were propagated with the speed of light this disturbing
couple would be large enough to be appreciable in astronomical systems,
and deduced from its absence that gravitation must have a much greater
speed. We know now that the first order effect which Laplace expected
is compensated; but the loss of energy (1) [the formula from general
relativity] is actually the residual Laplace effect of the third order
of small quantities, as determined by modern theory."


George L. Murphy