In replying to a recent question I stated:
Gravitational waves have not been yet experimentally observed so as to have their velocity measured.
Which after the fact prompted me to try and verify it. I found a "speed of gravity" claimed measurement, which has been in a controversial discussion. In the last paragraph of this wiki article I found:
In September 2002, Sergei Kopeikin and Edward Fomalont announced that they had made an indirect measurement of the speed of gravity, using their data from VLBI measurement of the retarded position of Jupiter on its orbit during Jupiter's transit across the line-of-sight of the bright radio source quasar QSO J0842+1835. Kopeikin and Fomalont concluded that the speed of gravity is between 0.8 and 1.2 times the speed of light, which would be fully consistent with the theoretical prediction of general relativity that the speed of gravity is exactly the same as the speed of light.[18]
There has been criticism, described in the paragraph.
Has this matter been resolved ? is the measurement valid?
The speed of gravity has also been calculated from observations of the orbital decay rate of binary pulsars and found to be consistent to the speed of light within 1% ( same link).
This is the speed that gravitational fields transfer information, and though the number itself depends on the theoretical framework solving for the system parameters it is true that just the existence of gravitational damping in the binary system would imply that the speed cannot be infinite.
Along these lines I was wondering whether studying the second star in a binary system where the first has gone nova would not give a cleaner and model independent measurement of the speed of transfer of information gravitationally. In this day and age where everything is digital one should be able to have data on this from now on.
LIGO has officially (Feb 11,2016) announced the observation of gravitational waves , at the same time consistent with the speed of light being c, and thus measuring it. See the webcast for a summary.
Best Answer
It is difficult to design empirical tests that specifically check propagation at c, independently of the other features of general relativity. The trouble is that although there are other theories of gravity (e.g., Brans-Dicke gravity) that are consistent with all the currently available experimental data, none of them predict that gravitational disturbances propagate at any other speed than c. Without a test theory that predicts a different speed, it becomes essentially impossible to interpret observations so as to extract the speed.
Kopeikin was wrong. See:
This is not really correct. GR predicts that low-amplitude gravitational waves propagate at c. The binary pulsar observations are in excellent agreement with GR's predictions of energy loss to gravitational waves. That makes it unlikely that GR's description of gravitational waves is wrong. However, it's not a direct measurement of the speed of gravitational waves. There is no way to get such a measurement without a viable theory that predicts some other speed.
The fact that a star goes nova doesn't produce any abrupt change in its gravitational field. If mass is expelled, there will be a gradual change.