Monday, 20 June 2016

Rumours, secrets, and other sounds of gravitational waves

Last Wednesday LIGO announced another detection of a binary-black-hole merger. Four months passed between the first announcement (in February, of a detection the previous September) and the second (a detection from late December). Which begs the question: are more on the way?

No. We're done. The data from the first Advanced LIGO observation run have been fully analysed for compact-binary signals, and the results our out: two (or three) binary-black-hole mergers, and that's it. No neutron-star binaries, no mixed neutron-star/black-hole binaries. Just black holes [1].

Clearly, we knew all about the second detection when we announced the first. That made the announcement in February a little odd. As far as the world was concerned, we had made one spectacular, but also very surprising, discovery. Had we been incredibly lucky? Was this a one-in-twenty-years event?

What if the detectors had not been working at that moment? There were plenty of tests still being done. In the early hours of September 14th, people at the Livingston site were considering testing how well the detector would measure "signals" from a car loudly starting and braking outside the building. The detector would be out of "observation mode" while the tests were carried out. At 4am they decided against it and went home. Less than an hour later, the gravitational-wave signal passed through [2].

Oh my God -- what if they had been doing their test when The Event came!!?? Would the start of gravitational-wave astronomy have been delayed for years?

By February we knew that the answer was No.

It would have been a terrible shame -- it could be a long while before we have a signal as strong as that first detection. From no more than the (quite reasonable) assumption that sources are uniformly distributed across the universe, we will most often observe signals from further away (there are more of them) and they will be weak. Only one in every twenty to thirty events are likely to be as strong as that first detection. To have missed it would have been heartbreaking -- except, of course, that we would not have known.

But we know that there are plenty of other signals out there. There was a weak signal waiting only a few weeks later (poor old LVT151012), and a couple of months later, the binary from the latest announcement. The detectors were switched off on January 19th for further upgrades, and it is likely that in the intervening five months there have been several more juicy signals that have passed unnoticed through them. We are cool about missing those.

LIGO Magazine cartoon --
from March.
We knew all of that when we announced the first detection, but could not say anything. The “Boxing Day event" was still being analysed, and LIGO stuck with its "we'll tell you when we're done!" policy. Since then, anyone who has asked, "What about those other three months of data?" has been told to wait, although often with nudge-nudge-wink-wink comments like, "We saw one signal and one strong candidate in the first month of data. There have been three more months. Work it out for yourself." (There was also a hint in the LIGO magazine "detection issue", from March.)

Now that's over. All the secrets are out.

All of this secrecy has skewed the story a bit. When people said, "Wow, you were so lucky to have seen something!", we could not respond, "No, there were more on the way.” And we could not clarify, "What was surprising and rare about the first detection was that it was so loud! There won't be many like that one."

Indeed, the second confirmed detection was about half as loud as the first. The black holes were also less massive, and the detectors were sensitive to more orbits of the late inspiral of the two black holes before they merged, which improved a little our ability to measure the black holes -- the signal was half as loud, but we knew the black holes' properties with roughly the same accuracy.

Of course, the irritations of the secrecy around the second announcement were nothing compared to the first. In December I had an old collaborator and friend visit, who is a veritable hero of gravitational physics, and I could not utter a word to him. He arrived during a LIGO teleconference. There were several students and postdocs gathered in my office. When he knocked on my office door I jumped up, rushed out, and cried, "Thank God! You've just saved me from the most boring telecon! Let's leave those guys to suffer and go get a coffee!" and whisked him quickly away.

I was at a conference later in the month that was full of non-LIGO people giving talks about what amazing science we could do, if only we knew whether binary black holes existed in Nature. It felt perverse to be unable to cry out, "They do! They do!" Worse were the people giving talks on what the consequences would be if stellar-mass black holes do not form. These people were wasting their time -- they were investigating a question that was now irrelevant. And they would continue to investigate it for two more months. Wasn't it criminal to be unable to appraise them of reality?

A scientist's strict dedication to Truth and Honesty makes it hard to keep such a secret for so long. Worse: we were missing invaluable opportunities to show off and inflate our egos. Thankfully the suffering was entirely compensated by a delicious feeling of smugness and intellectual superiority.

What were harder to accept were the losers spreading rumours. Fortunately there were not many of them. The only one of note was the esteemed Professor Laurence Krauss. I do not know him personally, and I do not work in his field, and he very clearly does not work in ours', but from a purely behaviourist interpretation of character, and based on this one small event, we are forced to entertain the possibility that he may be a dipshit. Of course, one should not draw strong conclusions from limited data, and it may well be that he is in fact the Mother Teresa of theoretical physics. (Although he has an old friend who would hardly consider that a complement.)

Fortunately none of that happened for the second detection. I did not hear any of leaks or rumours. LIGO scientists did not receive any "loose talk costs lives" emails from the Spokesperson. It has all been much quieter.

Now we can settle down and get ready for the next batch of LIGO data. The detectors switch back on in just a few months, the data will flow again, and there are sure to be plenty of black holes lurking in it. And all the fun will begin again.

More on Gravitational Waves

We detected gravitational waves!

What it feels like to detect gravitational waves.

How to decode gravitational waves from black holes.

Why bother trying to explain gravitational waves?

Is spacetime really curved?

Book review of "Black hole blues".

Black holes rule!


1. There are a few searches for other sources that take much longer and are still underway, but I have no idea what their status is. I only care about black holes.

2. See pages 9-10 of the March 2016 LIGO magazine.

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