Thursday, 16 November 2017

The Election Day Event

Today the LIGO and Virgo collaborations made their sixth announcement of a confirmed gravitational-wave detection. Once again the signal was from two black holes colliding — this time the black holes were each close to 10 times the mass of the sun, with the smaller object possibly as low as 5 solar masses, making this likely the lowest-mass binary-black-hole yet observed.

The signal was detected on June 8th, 2017, a full two months before the other two most recently announced detections. That was the day of the UK general election. I wanted to dub it the Election Day Event, but the name never stuck. That is not surprising; the UK election was hardly an inspirational historical moment. There were a few days of jubilation, when Theresa May’s clownish government failed to win the boost in support they had dreamed of (maintaining fanciful dreams being one of that government’s most striking features), and instead had to go into alliance with a gang of buffoons even more inexperienced, incompetent, politically marginal, morally repugnant and intellectually stunted than they were. For a while everyone was hooting that May’s days were numbered. The papers called her “Dead Woman Walking”. And yet, here we are, five months later, and she lives on, like rodents after a nuclear holocaust.

But I digress. Let’s just say that it was probably for the best that my “Election Day Event” suggestion was quietly ignored.

One of the most notable things about this detection, besides the notable name it does not have, is the accompanying explosion of publicity that it also does not have. Gravitational-wave detections have become normal science. The paper was written, without anywhere near the level of drama and stress of those following the first black hole and neutron star detections, then put on the preprint arXiv. There was no public announcement. No-one asked me for a quote for a university press release. No-one in our group has been assigned to go out and buy a couple of cases of champagne. Besides the preparation of some nice infographics and fact sheets, there was no fanfare. The revolution is over.

It may seem odd that this detection was published so much later than the detections made in August. In fact, the five months between detection and publication is similar to the lag-time of the first three observations. The rapid turnover of the binary neutron star result GW170817 was obviously influenced by the timescales of the wider astronomy community, and it was good to have the joint LIGO-Virgo observation of the binary black hole GW160814 out before that, as a clear example of a signal in Virgo. The latest result, GW170608, could wait a little.

The other notable thing about this detection is that the Hanford detector was undergoing maintenance at the time the signal arrived. This could have scuppered a detection claim. The Hanford data would be vetoed as unusable, and a signal in only one detector is not sufficient to claim a detection. However, the maintenance was not very intrusive and only affected the detector at low frequencies. Above 30Hz the data were deemed acceptable, and this left plenty of data from which to recover a clear signal.

What is interesting (at least to me), is that this is the kind of data inspection, characterisation and cleaning that would have caused much wailing and gnashing of teeth had it been necessary for the first detection. For forty years, gravitational-wave scientists were terrified of making a false detection claim. There was a good chance that the first detection would be weak, and we would have to provide a water-tight case that it really was a signal, and not a random blip in the data. The very suggestion that we might have to explain away some intentional mechanical interference would have given everyone the heebie-jeebies.

Having said that, if we had reached June 2017, six months into the second observing run, and there had not yet been any observations, I suspect that we would have been reluctant to throw back a nice clear binary-black-hole signal. But the hand-wringing over that “angular coupling minimisation” would likely have been tortuous. We can all be very pleased that the details are able to sit comfortably in an Appendix.

More Gravitational-Wave Stories

February, 2016:
The Discovery
How it Felt
How We Squeezed Out the Juicy Science

March, 2016:
Trying to Explain Gravitational Waves (Part I) (Part II)

June, 2016:
Book Review: Black Hole Blues
Detection Number 2 -- Black Holes Rule!
Rumours, Secrets and Other Sounds of Gravitational Waves

February, 2017:
One Year Anniversary (of being world famous)

June, 2017:
Detection Number 3 -- Nothing to see here: they are black holes
A hint of controversy

September 2017:
Detection Number 4 -- Virgo nails it

October 2017:
Did I just win the Nobel prize?

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