Re: Speed of Light/Gravity

George Murphy (
Tue, 28 Apr 1998 07:47:44 -0400

Vandergraaf, Chuck wrote:
> Bill,
> I've often wondered about the speed of gravity, but I have never heard any
> reasonable argument that information (e.g. gravity) can be transmitted
> faster than the speed of light.
> The hypothetical question I have posed to physicists is the following: if
> the sun all of a sudden disappeared, it would take ~8 minutes for us to be
> engulfed in darkness. Would it also take 8 minutes for the earth to fly off
> in space (and would we have 8 minutes to see the sun become progressively
> smaller, or would it happen sooner?

Note my parallel post to Bill. According to general relativity
(or any relativistic theory of gravitation) it would indeed take 8
minutes before we could in any way sense the sun's absence.
But this brings out one reason why gravitational radiation is
very weak: Because of the conservation laws, the sun's mass can't just
disappear & its center of mass must also remain the same.

> For that matter, is there any reason, physical or otherwise, why nothing can
> go faster than the speed of light? I understand that mass approaches
> infinity as the speed of light is approached, but does this apply to gravity
> as well? (whatever gravity is, anyway)

If things which could carry signals could move faster than light
then there would be violations of causality. (Signals could be received
before they were emitted.)
Relativity doesn't actually prohibit anything going faster than
c. There might be tachyons which _always_ move faster than c. But
nothing can be accelerated or decelerated _through_ the speed of light.
This is not in the most basic sense due to increase of mass with
velocity. It is a consequence of the fundamental space-time relations
of the Lorentz transformation & the resulting formula for addition of
velocities. If velocities u and w are added, the result is not the
common sense u + w but (u + w)/(1 + uw/c^2). If u & w have any values
less than c, the result will always be less than c too.

George L. Murphy