🚀 go-pugleaf

In article <1992Sep30.160233.17640@CSD-NewsHost.Stanford.EDU> pratt@Sunburn.Stanford.EDU (Vaughan R. Pratt) writes:

> Hmm, this is peculiar.  My feeble Quayle's-eye view of gravity led me
> to the thought that finite propagation of gravity was only detectable
> in accelerating situations (such as two stars rotating about each
> other).  I am now hearing an equivalent statement, but expressed in
> terms of a distinction between gravitation and gravitational
> radiation. 

In both electromatnetism and gravitation, the only time you can see a
finite speed of propagation is when something is propagating---that
is, when there is radiation.

There is, however, a real difference between electromagnetism and
gravitation, which probably shouldn't be glossed over too quickly:
radiation is much more important in electromagnetism than in General
Relativity.  The (naive) way I look at it is that since there are two
different signs of electric charge, you can get electromagnetic
radiation from a time-varying dipole moment.  In the case of
gravitation, however, the lowest-order term that can produce radiation
is a time-varying quadrupole moment, which thus results in a large
suppression factor.

What this means, then, is that even in situations where something is
accelerating, the gravitational radiation that is produced may well be
either absent or very tiny.  (In the case of the Solar System, for
example, I don't think there's any hope of detecting it.)  And, again:
if radiation is completely negligible, then you're not going to see
any propagation delays.
--
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