🚀 go-pugleaf

There recently was a discussion of gravity waves and I asserted that 
the speed of such waves must necessarily be Cem, the speed of electromagnetic
waves, by virtue of the masslessness of the graviton.  I have since learned
that my answer was not complete.  Here is as better explanation:

The speed of gravitational radiation (Cgw) depends upon the specific model of
Gravitation that you use.  There are quite a few competing models, including of
course Einstein's but also Brans-Dicke and several families of others.  
All metric models can support gravity waves.  But not all predict radiation 
travelling at Cgw = Cem.

There is a class of theories with "prior geometry", in which, as I understand
it, there is an additional metric which does not depend only on the local
matter density.  In such theories, Cgw != Cem in general.

However, there is good evidence that Cgw is in fact at least almost Cem.
We observe high energy cosmic rays in the 10^20-10^21 eV region.  Such
particles are travelling at up to (1-10^-18)*Cem.  If Cgw < Cem, then
particles with Cgw < v < Cem will radiate Cherenkov gravitational radiation
into the vacuum, and deccelerate from the back reaction.  So evidence of these
very fast cosmic rays good evidence that Cgw >= (1-10^-18)*Cem, very close
indeed to Cem.  

Summary:  In a purely Einsteinian universe, Cgw = Cem.  However, a class
of models not yet ruled out experimentally does make other predictions.  

A definitive test would be produced by LIGO in coincidence with optical 
measurements of some catastrophic event which generates enough gravitational
radiation to be detected.  Then the "time of flight" of both gravitons and
photons from the source to the Earth could be measured, and strict direct
limits could be set on Cgw. 

-Scott