Monday, 15 February 2016

How to decode gravitational waves from black holes

The gravitational-wave signal that we observed on September 14th last year, and announced to the world last Thursday, came from two black holes spiralling into each other. Those are the first two black holes we have ever observed colliding. The result of the collision was an even bigger black hole, and the last moments of the signal were the closest we have ever come to measuring a signal from a black hole. These are now three famous objects in the history of science. What do we know about them, and how can we be sure?

Friday, 12 February 2016

What it feels like to detect gravitational waves

I wrote yesterday's blog post before we announced our gravitational-wave detection. At the time I wrote, I had no idea what the announcement would feel like.

I was at a press conference in London. Well, almost. Before the press conference started I was whisked away to a BBC radio studio, so that I could talk about the discovery immediately after it was announced.

I got into the studio at 3.20pm, ten minutes before the official press conference was due to start.

“I can’t say anything before 3.30pm,” I explained. I’m sure it didn’t matter anymore, and I’m sure the strict news embargo was falling apart all over the world, but a five-month habit was hard to break. I wasn’t even sure if I would dare to say anything after 3.30.

I sat in a chair and put on headphones. The programme presenters were actually in Manchester. (Brian Cox must have been unavailable. Perhaps washing his hair.) The current programme was something completely different. In the studio with them they had spoken-word poet Hollie McNish. She was talking about her life. I could see them all talking on a video monitor. This was cool! My wife went to a Hollie McNish show last year, and we have a poster on the wall at home, and now I got to listen to her live! With ten minutes to go, I settled down to listen to something that, for the first time in five months, had nothing to do with gravitational waves. Then the interviewer said, “I’m sorry, but we have to stop now to go live to a major scientific announcement in the United States.”

Hey! I was enjoying that! What’s going on?

Oh yeah. Gravitational waves.

For a few minutes it was a bit painful, watching the British broadcasters and the British poet sitting in a studio listening to a stirring speech about the greatness of American science. I was worried about how long the fluffy background would last. How long were radio listeners going to be subjected to pre-announcement puff-talk? Especially the bits that involved playing a movie, which no-one listening to the radio could see.

Fortunately it didn’t last long, and then good old Dave Reitze came on. He was looking cool. Hi, Dave!

He didn’t beat about the bush. He just got up and said, “We have detected gravitational waves. We did it!”

And all of my flippant cynicism evaporated. The reality of it hit me.

We had been working for five months to get these results out. We wrote one big paper on the detection, and 12 other “companion” papers on the science we squeezed from the observations, and the backup evidence and explanations for how we got the results and why we were sure they were correct. Many people worked insanely hard. Sometimes it was horribly frustrating. There were interminable teleconferences. There were hours of discussions devoted to obscure technical points that probably didn’t matter — but everyone wanted everything covered. Everything had to be checked and re-checked, then doubted, debated, and checked again.

And the emails! Oh, the emails! My email filtering skills have now reached combat level.

By the end of it, lots of people were exhausted, frustrated, and ready for it to be over. Nerves were strained and tempers easily flared. The calm facade of scientific discussions was beginning to crack. And just when everyone was at their most frazzled — then we had to plan press conferences.

Amidst all of that, it was easy to forget the significance of what we observed.

Then Dave made the announcement, and everyone applauded, and suddenly it was hard to breathe properly.

In a few minutes they were going to expect me to speak. Was I allowed to just blubber on live radio?

There was some time to compose myself. The radio show stuck with Dave for a little longer, then went back to the studio in Manchester. Then there was a pre-recorded explanation from Stephen Hawking. Hey, who told him? Weren’t we supposed to keep this a secret? It turned out that he served a useful purpose. Getting grumpy at whoever blabbed to The Hawkster was really helping me get my tear ducts under control.

By the time I got on, I almost had it together. I can’t remember what I said, although they did ask me who was going to win the Nobel Prize. All I could think was, “If I say a name on a recorded broadcast, I’m a dead man!”

The whole thing is still sinking in. It might take years. That’s fine — it will also take years for the implications to sink in to the world of science. The day will come when hearing the clomps and thumps of the universe will seem as natural as looking into space with a telescope. That’s when we will really understand what all this has meant.

Previously: We detected gravitational waves!

Next: 1. How to decode gravitational waves from black holes.
          2. Why bother trying to explain gravitational waves? 
          3. Is spacetime really curved?

Thursday, 11 February 2016


Today we announced it to the world: we did it! We detected gravitational waves!

This has taken some time to prepare. Since the 0.2-second-long blip appeared in the data in mid-September it has taken five months to verify and write up the results. Before that, it took two decades to build the LIGO detectors. But that’s nothing. The event we observed — two black holes whirling around each other before merging into one — occurred over a billion years ago.

In the same spirit of advance preparation, I wrote this in January, so that today I could concentrate on more urgent matters. Like partying. I wrote quite a lot on what we have found, and what it means, and also what it has been like to be part of a major scientific discovery. (The word “major” is meant as an understatement, in an effort to pretend that we’re cool about all this. We’re not.) I'll post more over the next few days. But for now...

There are in fact three incredible observations here.

First, we observed gravitational waves. That’s what we’ve been trying to do for decades. It had to happen eventually, so long as the detector was as sensitive as promised. (It wasn't -- it was better!) The only question was when, and when depended on just how many loud sources there are out there. There could have been so few that we would have had to run the detectors at their most sensitive for five years before we found anything — which could have been as late as 2025. Or there could be so many that we found something in the first three months. What we didn’t expect was that we would detect a signal before we even started. More about that later.

For gravitational-wave enthusiasts, the big question was not whether we would ever detect gravitational waves, but what the source would be. Producing a gravitational wave that we can detect requires a stupidly violent, intense astronomical event — we need massive objects moving extremely fast in tiny spaces. A likely option would be two neutron stars in close orbit around each other. Take the sun and squash it into a ball twenty kilometres across, and you’ve got something pretty much like a neutron star. That is a very small, dense star. Since neutron stars are so small, they can orbit each other very close without touching — so close that they can orbit hundreds of times every second. That is fast. We now have two massive objects moving very fast in a small space -- a perfect gravitational-wave source. And the great thing about neutron-star binaries is that we have seen several already in the universe. None in such tight orbits as I just described, but we assume that we have only seen a tiny fraction of the total number out there.

In principle we can do better than that. We can put two black holes in orbit. If the sun were a black hole, it would be only three kilometres across. Now that is dense! Also, neutron stars are limited to be no more than twice as massive as the sun. For black holes there is no limit. And the more massive they are, the stronger the signal. The only problem with binary black holes is that we have never seen any. Sure, that’s not so surprising, given that black holes are, well, black, but that’s no excuse when you’re asking for a billion dollars to build a detector. You either know the source is there, or you don’t. And when it came to binary black holes, we just did not know for certain if any existed [1].  

Until now. LIGO just heard two of them crash into each other. They were both about thirty times more massive than our sun. (To be precise, which is what we’ve spent the last five months struggling to be: one was 36 times the mass of the sun, and the other 29. Give or take a few suns.) To make one of these black holes you would have to take thirty of our suns, and squash them together, and squash them all into a ball about 200 kilometres across. Don’t forget, we didn’t just find those two black holes — we also found the black hole that resulted from the two of them eating each other. In fact, we witnessed the millisecond-long gulp. The final black hole is about 60 times the mass of our sun. During the last rapid orbits (we heard the two black holes circle each other about five times in 0.2 seconds) and merger, the binary gave off three suns worth of energy. What that means is, if you took three copies of our sun, and turned them all instantly into pure energy in an incredible nuclear reaction, that would be the amount of energy released when these two black holes collided. In terms of energy, this makes it among the brightest events humanity has ever observed. If this energy had been released as light, for an instant it would have been the brightest thing in the sky. Instead, the energy was locked in gravitational waves, which interact so weakly with the rest of the universe that they passed through the earth completely unnoticed -- except, of course, for the blip that peeked above the noise of the massive experiment built with the express purpose of noticing it.

The signal in both detectors.
Top: The data
Middle: General-relativity calculations of the signal from two black holes.
Bottom: The data minus the predicted binary-black-hole signal. 

Let’s recap. We’ve just directly observed gravitational waves for the first time. Also the first ever binary black hole. Also black holes tens of times as massive as our sun. Anything else?

Well, as a matter of fact, yes. After its monumental collision, the final black hole bled away its scars as a last dying dribble of gravitational waves. Immediately after merger the black hole was like a big distorted spinning spacetime jelly. It quickly settled down into the squashed-ball shape of a spinning black hole (not as round as a ball, but not as a flat as a pancake), and as it settled down it released more gravitational radiation, like a bell ringing as it settles down after being struck.

Why is that radiation so interesting? Encoded in it are the properties of the final black hole. More than that, this signal is the closest we can imagine to measuring a signal from a black hole. We have never seen a black hole, but we have heard one.

That is incredible observation number three: we have measured a signal from a black hole.

In the future, when we find much stronger signals, we will be able to use that part of the signal alone to probe in detail the physics of black holes. That used to be our dream, but no more. It won’t be long now.

This all comes back to the point I hinted at earlier: we saw this signal before we even started looking. The first run of the detector, where we would officially take “science data” to analyse, started on September 18th. Two weeks before there was a final “engineering run”, a dress rehearsal where data was collected, verified and analysed, to make sure that all of the equipment and processes were working correctly and in place. That was when the signal was observed — on September 14th, during the last engineering run.

Needless to say, the beginning of the official “observing run” was a little more hectic than originally planned. But data continued to be collected until January 12th. Much of that data is still being analysed. As you will have realized by now, if there were more signals in that data, we already know about them. But just as before, the details will have to wait until the analysis is finished.

Where do I fit into all this? I just happen to study binary black holes. I assumed I was going to have to wait a lot longer before my work would be of any use at all. We all knew that the first observations would be neutron-star binaries. Maybe a binary where one object was a neutron star and the other a black hole. But binary black holes? We didn't even know for sure if they existed.

So the first detection was a very pleasant surprise indeed. And it gives me a perfect opportunity to tell you all about binary black holes, and how we study them. Stay tuned!

More Gravitational Waves:

Next: What it feels like to detect gravitational waves.

More science: How to decode gravitational waves from black holes.

Explanations: Why bother trying to explain gravitational waves?
                         Is spacetime really curved?

Detection Number 2 -- Black Holes Rule

Rumours, Secrets and Other Sounds of Gravitational Waves

One Year Anniversary (of being world famous)


1. We know that some black holes exist, of course. We can see other stars orbiting around them. Unfortunately, that only tells us that individual black holes exist, not whether they ever end up in binaries. We also have not previously found any especially big black holes. We know that black holes a few times more massive than our sun exist. But what about fifty times bigger? A hundred times bigger? We have no idea. We do know that extremely massive black holes exist, millions and billions of times more massive than our sun. But there was only minimal evidence for black holes in the "intermediate" range, tens of times the mass of the sun.

Tuesday, 9 February 2016

LIGO Announces an Announcement

Yesterday the LIGO and Virgo Scientific Collaborations announced that they would be making an announcement on Thursday.

I was not in the least surprised, because I am a member of LIGO. They had already announced to me that they would be announcing to you and that they would, eventually, be making an announcement.

"What exactly is the announcement?" you ask.

Weren't you listening to the announcement? The announcement stated that the announcement will be made on Thursday. So you can bloody well wait.

I will be watching the announcement in London, where it will be streamed live to a special UK press conference. The announcement will be at 10.30am Eastern Standard Time in the US, which is 3.30pm in the UK. If you are in my homeland of New Zealand it will be at 4.30am on Friday, and you will be extremely upset if the announcement turns out to be, "This year we filed our taxes early."

If you want to watch the announcement, there will be a live stream. I can announce that the announcement of the live stream of the announcement will be made one hour earlier on the LIGO website. (UPDATE: Here is a weblink to the broadcast.)

Some background: LIGO is a network of detectors that were built to directly detect, for the first time ever, gravitational waves. Gravitational waves are ripples in space and time. Einstein predicted them 100 years ago. For fifty years after that, people debated whether or not they were real, but eventually got sick of arguing and decided to just try and find the damn things. In 1974 Hulse and Taylor measured two orbiting neutron stars getting very very slowly closer together, in exact agreement with gravitational-wave calculations, and since then no-one has doubted that gravitational waves are real. (Except a few crackpots, of course, who regularly send us letters explaining the foolish error we have made, usually written in crayon.)

What would be much more exciting would be to measure the waves themselves. That is what LIGO was built to do. Not only would that allow us to test Einstein's prediction much more rigorously, it would also mean that we could use gravitational waves as a tool to observe the universe. We expect that the universe is crammed with objects that we can't see with ordinary telescopes, but might be massive enough and dense enough and moving violently enough, that they can produce gravitational waves strong enough that we could measure them. That is our hope. That is our dream.

The LIGO collaboration has over 1000 members, working on building the detectors, and on searching in the data, and a multitude of other projects. My little part in all of this, along with many collaborators (and competitors!), is to calculate what the gravitational-wave signal would look like from two black holes colliding. Such calculations could help us find signals but, much more importantly, if we found something we could use these theoretical models to disentangle the signal to work out what produced it.

For over a decade increasingly sensitive incarnations of the LIGO detectors have been run, and they have observed nothing. Last year we switched on the most sensitive detector yet, "Advanced LIGO", and took data from mid-September until January 12th of this year. Analysis of the data began the moment the detector switched on, and is still being completed. The announcement on Thursday will provide an update on where we are so far.

Related Posts:

Advanced LIGO switched on.

My one modest attempt to explain curved space and time.

Another explanation of Einstein's theory, and why it is so difficult.

How gravitational waves confused Einstein himself.

Wednesday, 3 February 2016

How to get your name on a paper

I am in a panic. An incredible paper is being written that could change my career forever. The problem: it is not being written by me. I desperately need to get my name on that paper.

I should explain.