Gravitational Waves are Compression and Expansion Waves on the Fabric of Spacetime...
so says LIGO. Specifically, they make the audacious claims:
"Gravitational waves [GW] are 'ripples' in the fabric of space time caused by some of the most violent and energetic processes in the Universe. 'Waves' of distorted space would radiate from the source (like the movement of waves away from a stone thrown into a pound). Furthermore, these ripples would trave at the speed of light"
Where does the authority come from making such outlandish statement? Let's start with the most obvious refutations.
1 If GW waves are the result of such cataclysmic processes, then why resort to a linearization without first deriving the full set of equations? Answer: There are no periodic solutions to the nonlinear Einstein equations.
2. Even in the linearized version, spherical waves are not supported, only plane waves. The pebble analogy falls apart because it would be tantamount to a localization of energy--which is impossible in the Einstein theory.
3. The expansion and compression of the fabric of spacetime describes compressional waves, like sound waves, which are longitudinal. The error committed by claiming that GWs are transverse waves, and hence, polarizable is wrong because the TT waves uses the time components of the polarization of the 4-vecotr. Polarization is a spatial phenomenon, and time does not enter. This is just one example where grinding out the formalism in 4D, without regard to the physical phenomena being described fails miserably.,
The gross error is in claiming that GW waves travel at the speed of light. There are two types of waves: longitudinal, non-polarizable, and transverse, waves, which are. These are known as p- and s-waves in seismic theory. The p-waves, in general, travel faster than s-waves which are shear-waves. The p-waves are dilatational waves in which the volume changes but not the shape, whereas for s-waves, the shape changes but not the volume.
Since light is a s-wave, it would appear that the speed of light is not the ultimate speed. However, s-waves cannot propagate through liquids, and gases, whereas light can. This is because the fundamental oscillations are between a couple of equivalent forces acting at right angles to themselves and to the direction of propagation. It is absurd to imagine that the so-called fabric of space is made up of this couple of forces. Gravity has no magnetic vector from which it can propagate in such a fashion as light.
If expansion and compression waves are what are causing the mirrors to separate in the arm of the Michelson-type interferometer, it is a whole new ball game. And to exclude such a possibility could not be based on the comparison of the spectra with that obtained from numerical relativity, since the latter has absolutely no physical basis.
As far as light waves are concerned, the Maxwell stress tensor contains both a shear modulus involving the electric and magnetic vectors, and a bulk modulus involving their energy densities which is related to a radiation pressure. The same does not apply to GWs. The type of motion referred to by LIGO involves the motion of particles in one direction only. There is nothing in the fabric of spacetime that would mimic the oscillation of two force fields in orthogonal directions. That would be too much to ask of the new "ether."