Wednesday 15 June 2016

Black holes rule!

Finally, the news is out. LIGO conclusively observed a second pair of black holes spiralling into each other. For the record, that's a total of six black holes observed with LIGO in its first four months of operation: for every binary merger, two black holes go in, and one comes out. Not bad for a machine that many grouchy astronomers claimed would never see anything [1].

The black holes in the latest binary were less massive than those in the first detection, and the signal was weaker. This time the signal was not clearly visible in the data — the fancy search algorithms and waveform models that we have spent so many years working on were absolutely essential to the observation [2].

So: what does all this mean? And do we have any more secrets up our sleeves?

Let's first deal with what it does not mean.

Since the first detection announcement in February, there has been a lot of gibberish spouted in the science press about probing the big bang, proving Einstein right, testing quantum gravity, verifying string theory and, inevitably in the on-rush of any stinking stream of science hype, time travel. I can understand the drooling from the journalists -- opportunities to send their story to the top of the front page don’t come along often. The BBC tagline when I turned up for interviews on February 11 was, "Scientists uncover the secret of the Universe." I don’t even know what that means. Fortunately they refrained from asking me to explain it, and I refrained from asking who thought up such nonsense.

But it's not just the journalists -- the scientists have done it as well! It is clear that someone's one shot in life at a quote in some online rag can really go to their head. I could roll out the excuse that a lot of this has not been from actual gravitational-wave researchers, but from the crowds of "related" scientists furiously flaunting their PhD certificates and an ability to read wikipedia. Sadly, though, a fair amount has come from the gravitational-wave crowd themselves.

There are of course many deep questions that we do dream of answering with gravitational waves, but most of these will come (if at all) from much more sensitive future detectors. Last week the LISA pathfinder team demonstrated that the technology is ready for a space-based gravitational-wave detector. This is the kind of machine that will do cosmology. But a space-based detector is 15 to 20 years in the future. Right now we have ground-based detectors. What are they going to find?

The answer, which today’s announcement should make abundantly clear to anyone who is not drivelling about the big bang, is: black holes! We are going to find a lot of black holes. By 2020 we are likely to have observed hundreds of black holes. We may be observing a black-hole merger every day.

Credit: LIGO / Caltech Press Office
Compare that with our other favourite compact astrophysical object, the neutron star: we currently have mass measurements for about 60 neutron stars. By 2020 we are likely to know the properties of more black holes in the universe than neutron stars, and although radio astronomers have been finding binary-neutron-star systems since 1974, in five years of GW observations we may have found more binary black holes in the universe than binary neutron stars.

The point is: up until now astrophysicists have been trying to understand the make-up of our universe, and how it got that way, and where it is going, by studying data from observations of numerous exotic objects. They hungrily devour each batch of new data to update models and make new discoveries. More observations of the same old objects -- more pulsars, more gamma-ray bursts, more supernovas -- are all desperately needed to increase our understanding of the universe.

LIGO has just identified an entirely new entity -- binary black holes -- and is about to deliver to astrophysicists' doors a massive sack of data just as large as the sacks they have for each of their other favourite objects. That is the big message of the latest announcement.

If you have been trying to follow all the gravitational-wave burble, you might ask: wait, isn't LIGO going to find binary neutron star systems as well? And isn't a joint astronomical observation of an electromagnetic counterpart going to be the most exciting observation yet  [3]?

Sure, we hope there will be binary neutron stars, and sure, an electromagnetic counterpart would also be very nice. (Especially for the conventional astronomers desperate to be players in this new game.) But we may have to find 100 binary neutron stars before we also find an electromagnetic counterpart, and for every binary neutron star we find, it is quite possible we will find ten binary black holes. Regardless of how you rate the science that can be done with each observation, the fact remains: there is going to be an avalanche of black-hole data to play with.

That is the big message of the second detection: the first one was not a one-in-twenty-years fluke, gravitational-wave astronomy has truly begun, and the big business is going to be black holes.

Black holes rule!

As for my second question -- do we have any more secrets up our sleeves? -- I'll leave that for next time...

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".


1. There are another three we are fairly confident we found, but the signal was not strong enough for us to claim the magical "five sigma" statistical significance, so this binary continues to officially languish as a mere "candidate". We can only hope that history promotes it to bona fide "observation" status.

2. If you want some idea of how tricky this is, try playing the Black Hole Hunter game.

3. Translation, in case you’re not quite up to speed on all the techno-babble: conventional telescopes observe electromagnetic (EM) signals — light, radio waves, x-rays, gamma rays. One especially exciting source for these telescopes is a burst of gamma rays, which is creatively known as a “gamma-ray burst”. One of the possible causes of a gamma-ray burst is the collision of two neutron stars. If a gravitational-wave detector found a signal from two neutron stars colliding, and a telescope saw a gamma-ray burst at the same time and place, then that would be very strong evidence that gamma-ray bursts are indeed produced when neutron stars collide. Having “joint observations” from entirely different kinds of observatory — a conventional EM telescope, and a gravitational-wave detector — would yield rich information greater than the sum of two individual observations. That said… well, return to the main story for caveats.

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