The second postulate of STR was made exclusively for light (electromagnetic waves)?
If gravitational waves also travel at velocity $c$, then are they obliged to fulfill this second postulate?
Any wave of field (anti matter, dark matter, dark energy), traveling at velocity $c$, must obey this same postulate?
Since gravitational waves are not contained in the second postulate, does the invariance of these waves need to be proved?
Is Ligo results a proof of the velocity $c$ of gravitational waves?
Can Ligo prove the invariance of gravitational waves?