In my mind, gravity must self interact due to moving at exactly the same speed as light (and light is affected by gravity). To put this in quantitative terms, the paper "Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A" (https://arxiv.org/abs/1710.05834) constrains the "difference between the speed of gravity and the speed of light to be between −3×10^−15 and +7×10^−16 times the speed of light."
Yep I feel like ligo and Virgo are the Michelson Morley experiment of this century.
They have shown something that is not well accounted for in current theories. Just as MM showed light is always observed travelling at the same velocity Ligo has shown gravity is self interacting. Now we need to change our models and build mathematical frameworks to account for this. It is not acceptable to have a theory that doesn't account for this post Ligo.
I'm not convinced of that interpretation of MM. MM found a signal below the margin of error. I think if they spun the table faster the signal would grow in strength. But subsequent experiments stopped rotating the assembly at all and just relied on the rotation of earth, and thus had even slower rotation and even smaller signals.
But at this point nobody is going to revisit MM, attempting to do so would immediately ruin your scientific reputation.
> The expected deviation of the interference fringes from the zero should have been 0.40 of a fringe – the maximum displacement was 0.02 and the average much less than 0.01 – and then not in the right place.
The data doesn't look anything like a signal, and wiki also says:
> Roberts (2006) has pointed out that the primitive data reduction techniques used by Miller and other early experimenters, including Michelson and Morley, were capable of creating apparent periodic signals even when none existed in the actual data.
> But subsequent experiments stopped rotating the assembly at all and just relied on the rotation of earth
> Our measurement compares the resonance frequencies of two orthogonal optical resonators that are implemented in a single block of fused silica and are rotated continuously on a precision air bearing turntable
Thank you for providing that research, I wasn't aware of that paper.
But when I read it, I found something interesting.
"The final precision could be reached by integrating over more
than 130 000 rotations relying on a careful suppression of
systematic effects caused by the turntable rotation"
I wondered what effects they were suppressing, and while they discuss that to some extent they also say:
"While even faster rotation would have allowed to acquire more
data and thus improve statistics, it resulted in increased
residual systematic effects presumably due to modulated
centrifugal forces and was thus not implemented"
And they state:
"active rotation potentially
causes a systematic modulation of the beat frequency
and might thus mimic an anisotropy signal. For exam-
ple, gravitational or centrifugal forces that act on the
resonators may get modulated with the turntable rota-
tion and therefore modulate the length of the resonators."
So I would ask, how do we distinguish between new physics and systematic errors that look like new physics and need to be excluded from the experiment?
I would love to revisit MM myself, but I don't have the expertise to actually setup the experiment. I might be able to resolve that gap, but I definitely don't have the funding either, and that is a gap I don't know how to solve.
I like this gedankenexperiment: Imagine you're in the vicinity of a supergiant black hole orbited by a close binary pair of neutron stars about to merge with each other.
The neutron stars are close enough to the black hole that you see multiple images of them, and the images are highly distorted. E.g.: You can see them even when they're "behind" the black hole due to the highly bent path of the light.
You have hundreds of probes scattered over a large volume of space, each equipped with an optical telescope and a gravity wave sensor similar to LIGO.
What would you expect to observe?
Would the frequency of the final "ringdown" chirp before the merger be observed identically in the optical and gravity sensors? Would they be observed as "coming from" the same spots in the sky? Would you expect to feel a higher contribution to gravity by the neutron stars due to their multiple images? Etc...
It's interesting that our intuition about the consistency of the rules related to the speed of causality imply that the answers ought to be: Yes, identical, yes, yes, etc...
But that means that gravity is definitely self-interacting, and it also means that gravity can be focused, just like light can be focused by gravitational lensing.
This implies that stars at the edge of a galactic disk "see" more gravity from the opposite side of their galaxy due to the lensing effect of the central bulge!
My bet is that difference between gravitational waves and gamma rays due to the effect known as "surfing". A gravitational wave pushes light back from peak, where light is slower, to bottom, where the speed of light is normal again.
The difference between gravitational wave and gamma rays is more dependent on initial magnitude of gravitational wave, than on distance traveled.
