# The detection of gravitational waves and the assumption that gravity travels at the speed of light

I read from this wikipedia article that we have obtained some empirical evidence that gives us an upper bound on the difference between the speed of light and the speed of gravitational waves. What I am confused about is: it seems that the inference from those data to that conclusion (i.e. the speed of gravity is close to the speed of light) involves the detection of gravitational wave signal, and yet, to my knowledge, in order to infer from experimental data that we have detected gravitational wave signals, one must assume that gravity travels at the speed of light. But of course this would be circular. So I must be missing something here. Any pointer/clarification would be greatly appreciated.

• By "we", do you mean all if humanity? Jan 15, 2021 at 18:02
• Your assertion that "order to infer from experimental data that we have detected gravitational wave signals, one must assume that gravity travels at the speed of light" is the source of the confusion. Such an assumption is not needed. Jan 15, 2021 at 18:04
• I see. Thanks! (And by "we" I don't have anything specific in mind -- that a placeholder pronoun.) Do you happen to have any particular reference about this fact (i.e. that the assumption is not needed)? I learnt about gravitational waves mostly from reading works of sociologist Harry Collins, who at some places assert that such assumption is needed. Jan 15, 2021 at 19:40

Here is the starting paper,

Abstract

On 2017 August 17, the gravitational-wave event GW170817 was observed by the Advanced LIGO and Virgo detectors, and the gamma-ray burst (GRB) GRB 170817A was observed independently by the Fermi Gamma-ray Burst Monitor, and the Anti-Coincidence Shield for the Spectrometer for the International Gamma-Ray Astrophysics Laboratory. The probability of the near-simultaneous temporal and spatial observation of GRB 170817A and GW170817 occurring by chance is $$5.0\times {10}^{-8}$$. We therefore confirm binary neutron star mergers as a progenitor of short GRBs. The association of GW170817 and GRB 170817A provides new insight into fundamental physics and the origin of short GRBs. We use the observed time delay of $$(+1.74\pm 0.05)\,{\rm{s}}$$ between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity and the speed of light to be between $$-3\times {10}^{-15}$$ and $$+7\times {10}^{-16}$$ times the speed of light,

The logic is simple. First they establish that there is a coincidence by deriving the probability of two random events, a gravitational event(GW) and a gamma ray burst(GRB) to happen at the same time. The two events were detected at the same time by four completely different experimental setups (I am speking of only LIGO and Fermi for simplicity).

Assuming the signals were both the result of the same binary neutron stars merger , LIGO measuring the gravitational waves and the Fermi gamma ray burst detector independently the time the gamma burst arrived , if the speed of gravitational waves and gamma rays is the same, they should arrive at the same time ( after correcting for the different laboratory locations, in addition to the model of the merger ). So they investigated whether they did come simultaneously and give the very strict constraints to the difference between light and gravitational wave velocity.

There are corrections etc that play a role but that is the rough picture. There is no circular logic. The main assumption lies in assuming it is not a random coincidence that the gravitational wave and the gamma ray burst came at about the same time, and attributed it to the same neutron binary mass merger.

In this article a method independent of electromagnetic signals arriving concurrent to the gravitational waves is explored, but has very much smaller accuracy.

LIGO detects gravitational waves when the gravitational waves hit the detector,so there is no assumption that "one must assume that gravity travels at the speed of light".