The last research aim to find gravitons which are boson, so if I correctly understand they are virtual particles.
Out of theorical assumptions,
Is there exist an experimental proof that the propagation of the gravity isn't instantly ?
I try my best to write correct english, I hope you understand my question.
There is this evidence for the speed of gravity, i.e. whether it is infinite as Newtonian physics postulates or has a specific velocity. Since the force of gravity is very weak laboratory experiments cannot be made. In this article the question is explored :
While current observations do not yet provide a direct model-independent measurement of the speed of gravity, a test within the framework of general relativity can be made by observing the binary pulsar PSR 1913+16. The orbit of this binary system is gradually decaying, and this behavior is attributed to the loss of energy due to escaping gravitational radiation. But in any field theory, radiation is intimately related to the finite velocity of field propagation, and the orbital changes due to gravitational radiation can equivalently be viewed as damping caused by the finite propagation speed. (In the discussion above, this damping represents a failure of the "retardation" and "noncentral, velocity-dependent" effects to completely cancel.)
The rate of this damping can be computed, and one finds that it depends sensitively on the speed of gravity. The fact that gravitational damping is measured at all is a strong indication that the propagation speed of gravity is not infinite. If the calculational framework of general relativity is accepted, the damping can be used to calculate the speed, and the actual measurement confirms that the speed of gravity is equal to the speed of light to within 1%. (Measurements of at least one other binary pulsar system, PSR B1534+12, confirm this result, although so far with less precision.)
Bold mine.
To the best of my knowledge, there is currently no experimental evidence of the velocity at which gravity is propagated. Due to the tiny size of the gravitational constant, direct evidence of the propagation velocity of gravity would be extremely difficult. Direct measurement would have to involve large masses moving at high velocity with respect to one another, using equipment to map the changes of their motion due to their mutual gravitational attraction. By determining the amount and direction of force needed to make the course changes observed, the velocity of gravity could be estimated reasonably well.
However, the installations which have been built to detect the existence of "gravity waves" would give an indication of the velocity, especially if they can detect the origin of such waves. Unfortunately, they have yet to get a positive detection signal.
