"Special relativity is based on the observation that the speed of light is always the same, independently of who measures it, or how fast the source of the light is moving with respect to the observer. Einstein demonstrated that as an immediate consequence, space and time can no longer be independent, but should rather be considered a new joint entity called "spacetime."
https://www.bowdoin.edu/news/2015/04/physics-professor-baumgarte-describes-100-years-of-gravity.html
So spacetime and LIGO's gravitational waves (ripples in spacetime) do not exist if the speed of light is not "always the same". It is obviously not the same for the stationary observer and the moving observer here:
View: https://youtube.com/watch?v=bg7O4rtlwEE
The speed of the light pulses relative to the stationary observer is
c = df
where d is the distance between subsequent pulses and f is the frequency at the stationary observer. The speed of the pulses relative to the moving observer is
c'= df' > c
That is, the speed of light relative to the observer VARIES with the speed of the observer.
The speed of light VARIES with the speed of the emitter, as posited by Newton's theory
and unequivocally proved by the Michelson-Morley experiment:
"Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887...The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)."
https://en.wikipedia.org/wiki/Emission_theory
Banesh Hoffmann, Einstein's co-author, admits that, originally ("without recourse to contracting lengths, local time, or Lorentz transformations"), the Michelson-Morley experiment was compatible with Newton's variable speed of light, c'=c±v, and incompatible with the constant speed of light, c'=c:
"Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether." Banesh Hoffmann, Relativity and Its Roots, p.92
https://www.amazon.com/Relativity-Its-Roots-Banesh-Hoffmann/dp/0486406768