Wednesday, 27 September 2017

The Power of Three

Another day, another announcement of LIGO detecting gravitational waves from two black holes colliding. What's the big deal?


The big deal is that this time it was not only the LIGO detectors that found the signal. This time the Advanced Virgo detector, near Pisa, Italy, spotted it as well. The Virgo detector joined in LIGO’s second observing run for its last month, from August 1st to August 25th. On August 14th all three detectors recorded the sound of two black holes whirling together.  If you're keeping track, the black holes were about 30 and 25 times the mass of our sun, so not very different from those of the first detection in 2015, and the binary was between one and two billion light years away. All the funky facts are here.

This was obviously very exciting for the Virgo team, who have been itching for their detector to join the fun ever since the first LIGO detection in September 2015. You might wonder, though, why it should matter to anyone else. As we know very well by now, two detectors are enough to find these signals and to make decent measurements of their properties. If you look at the paper, you will see that Virgo is not yet as sensitive as the LIGO detectors, and the Virgo measurement was much weaker. If the two LIGO detectors had the same sensitivity as Virgo did, then the measurement would have been too weak to count as a confirmed detection. So what does Virgo add?

The answer: location, location, location. Gravitational-wave detectors are microphones, and microphones are bad at pinpointing the direction a sound came from. With two microphones, though, you can note the time when a sound arrived at each one, and narrow down the direction to a circle in space. For example, if the sound arrived at both microphones at the same time, then the source must be the same distance from both of them, and if the sound came from outer space, all possible directions mark out a circle on the sky. With three microphones, you can think of them as three different pairs (microphones 1 & 2, 2 & 3, and 3 & 1). Using the time the sound is recorded in each microphone, you can mark out three circles, and where all three circles cross — that’s the direction the sound came from!

Credit: LIGO/Caltech/MIT/Leo Singer
(Milky Way image: Axel Mellinger)
Notice that in that example, we do not care how loud the sound is. We just need all three microphones to hear it. It is the same with gravitational-wave detectors. So long as Virgo heard the signal, we can narrow down the part of the sky the signal came from. In the figure you can see the sky localisation for the latest LIGO+Virgo detection, compared to all the previous LIGO-only detections.

This is very important, because it allows conventional astronomers all over the world to point their telescopes at just the right part of the sky, and verify that they see nothing at all. They see nothing at all, of course, because the objects are black holes. But it is always fun to watch them try.

But seriously, this is useful, because if we do observe something that also gives off electromagnetic waves (colliding neutron stars are the source everyone dreams of), then the astronomers want to be able to find the “EM counterpart” to the gravitational-wave signal. They have been itching to join the fun as well! They care a lot about improving the sky localisation by ten times, which is what Virgo allowed. Put another way, if we did observe a source with an EM counterpart, it would be a real shame if Virgo was not operating.

Six gravitational-wave polarisations (stretching patterns) that we could
imagine. Only (a) and (b), i.e.,  + and x, are possible in Einstein's theory.
(From Cliff Will's review article)
The other nice science that Virgo made possible was an extra check on Einstein’s theory. In general relativity, gravitational waves can oscillate spacetime in a “plus” shape (the up-down direction stretches while the left-right direction shrinks, and vice versa), or a “cross” shape (the same thing, but turned by forty-five degrees). One detector can be aligned to detect the “plus” part, but would miss the “cross”, because a “cross” signal would stretch and shrink both detector arms by the same amount; it is the difference that they are sensitive to. To measure the “cross” as well, you need a second detector. A third detector can make sure there are no other options. If there were others, then Einstein’s theory would be in trouble. In case you are getting nervous for the old guy, fear not: even with three detectors, we found only two polarisations.


The previous gravitational-wave announcement was for a detection in January. This new detection was in August. You are probably wondering if there was anything else in the data. You may even have heard rumours. Do not waste your time with that nonsense. In science, the results are all that matter. Rumours are for dilettantes. Jeez, people, have some patience! We are working on it.


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

October 2017:
Did I just win the Nobel prize?

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