Yesterday the LIGO scientific collaboration announced that they had detected the gravitational waves from the in-spiral and merger of two black holes, shown in figure 1. It would not be an overstatement to say that this result has changed science forever. As a gravitational physicist, it is hard for me to put into words how scientifically important and emotionally powerful this moment is for me and for everyone in my field. But I’m going to try. This is my attempt to capture some of the science—and the poetry—of LIGO’s gravitational wave announcement.

The Source

About 1.3 billion years ago and as many light years away, two spinning black holes, each about thirty times the mass of the sun (one a bit bigger, one a bit smaller) ended their lives as separate entities. These two monsters had probably lived out many separate lives together: first as a binary system of two massive stars and most recently as two black holes orbiting each other. Somewhere in between, each one probably briefly outshone the entire galaxy as a core-collapse supernova.

But nothing lasts forever. Einstein tells us that mass distorts spacetime, warping distance and duration. And an accelerating mass (like a black hole in an orbit) releases some of its energy in ripples of this distortion. And so, over the billions of years of their shared lives, our black holes lost energy to these gravitational waves and their orbit decayed. They slowly, inevitably, spiralled towards each other.

As the partners approached, their orbit sped up and their slow, stately waltz gradually transitioned into a frantic tarantella toward coalescence. Eventually the partners came within about 10 kilometres of each other (people RUN that distance!). By this time, they were orbiting each other about thirty-five times per second!

The black holes spiralled towards each other at roughly the same rate about five more times before they suddenly plunged together, spinning around their shared centre of mass 250 times per second. But this stage didn’t last that long. Before even one second had passed, the black holes’ event horizons overlapped, and they merged into a single rapidly rotating object. This new single black hole oscillated wildly as it settled down into its final configuration, emitting gravitational waves all the while.

In-spiral. Merger. Ringdown. After (possibly) millions of years in a slowly decaying orbit, the final plunge took less than a fifth of a second. In those last moments, gravitational waves carried away Joules. That’s three times the energy contained in our Sun. Three suns, released as ripples in spacetime.

This is a computer simulation of the in-spiral and merger of two black holes much like the ones I described, produced by my friends and collaborators in the Simulating Extreme Spacetimes (SXS) collaboration:

The SXS collaboration has written up their own explanation of the gravitational wave detection, including a ton of really awesome simulations and videos of the black-holes in question. Check it out here.

(Note, I previously had a rough back-of-the-envelope calculation of distances. However, the paper has the relevant information, which I am now using.)

Gravitational Waves

But what of the gravitational waves emitted by our ill-fated dance partners? These ripples in distance, in the very fabric of space and time, travel outwards from their source at the speed of light. Space is large and empty and it is mostly a lonely journey. Perhaps they pass through a cloud of gas and dust. Perhaps they don’t. If they do, the distortions of distance move the gas. Some gas particles move apart, some together. The gravitational waves might move a ring of gas particles, as shown in figure 2.

The effect is small; if the gas cloud were a few kilometres in width, the gas particles would move a distance less than one one-thousandth of the width of a proton. But they would move. And if they moved enough (they don’t) they would make a sound—the sound of the merging black holes:

Detection

Eventually, after about 1.3 billion years, on September 14th, 2015, the gravitational waves reached Earth. They were too weak to make a sound, but we could detect them. A gravitational wave is a distortion in distance, one that travels. So we can measure this distortion with a very precise ruler. And light is one of the best possible rulers.

Actually, we used two gigantic, perpendicular light-rulers, each several kilometres long. As a gravitational wave passed the rulers, it shrank distance in one direction and grew it in the other. The scientists who use these light-rulers call this discrepancy a “strain.” The paired light-rulers themselves are called “interferometers.”

We’ve built several interferometers to detect gravitational waves. There’s one in Livingston, Louisiana, which is shown in figure 3, and one in Hanford, Washington. There’s another in Sarstedt, Germany and another in Cascina, Italy. One, destined for India, is in storage. And another is under construction underground in Kamioka, Japan.

On that fateful day, only the detectors in Livingston and Hanford were active. (Some of the others aren’t even sensitive enough for their intended purpose. When people first started building gravity-wave detectors, it wasn’t clear how far away the sources would be.) The waves hit Livingston first, at exactly 3:50:45 AM local time. About seven-thousandths of a second later, they reached Hanford and distorted the light-ruler there, too. And a fifth of a second after that, they were gone. The sound of the black holes had passed us by and continued its journey into the void.

But they did not pass without a trace. No, the Livingston and Hanford detectors recorded their passage, shown beautifully in figure 4. The 1.3 billion-year-old waveform passed through our world and changed us forever.

Learning from the Waves

We already knew gravitational waves exist. That measurement took 30 years and won the Nobel prize. And we had a pretty good idea of what they should look like. But the only way to confirm that they looked like we expected was to observe them. So the first thing the LIGO team did was to use sophisticated statistical techniques, without any assumption about the final waveform, to extract the true wave from the noisy signal shown in figure 4.

They then compared that waveform to the wave predicted by general relativity. The two agree spectacularly. Score one for Einstein! Of course, there are possible modifications of general relativity such that a black hole in-spiral wouldn’t look any different. So only time, and more gravitational waves, will tell if those modifications are wrong. But for now, this result is a triumph of relativity.

Independently, the LIGO team matched the raw data to a “template bank” of possible gravitational waves, each generated for a different configuration of the black holes—different masses, different rotation rates, different orientations, et cetera. Eventually, they found a match. (Actually they found several, all of which were very similar.) And, fantastically, this match agreed perfectly with the wave extracted using the statistical technique. The extracted waveforms from the two detectors, calculated in both ways, are shown in figure 5.

As a huge bonus, matching the waveform in this way told the LIGO team the masses and rotation rates of the initial black holes and the final black hole that they became.

From the ripples in spacetime, we had extracted astrophysics!

Two Detections

I want to emphasize that one reason we can be so confident in the LIGO detection is that it happened twice, once for each detector. Both detectors are extremely sensitive—they could easily see an earthquake or a car driving down the highway and misinterpret it as a gravitational wave. But the gravitational wave was seen at both detectors, and the odds of them both getting exactly the same false positive are extremely low.

What We’ve Learned

In this one detection, we’ve learned a tremendous amount…some of it very definitive, some of it not. But at the very least, we now know the following:

Gravitational waves look very much like we expected.

Black holes definitively exist. No other two objects in the universe could have been so close before colliding. Of course, we had pretty good evidence that black holes existed before now.

Binary black hole systems definitely exist. A few years ago, it was not obvious that these systems formed. To get a pair of black holes orbiting each other, you need a pair of supernovae. And that could easily destroy the orbit.

What We Stand to Learn

For most of the history of astronomy, humans relied on their unaided eyes to look at the stars. In the early 1600s, telescopes were invented and the universe opened up. Suddenly the twinkle of stars and planets resolved into gas giants and moons, clusters and nebulae and galaxies. In the 1930s, we discovered a new kind of telescope: the radio telescope. Once again, we saw space in literally a whole new light. Suddenly objects we thought we understood looked very different. And wild new things appeared, like radio pulsars. Every advance in telescope technology sparked a huge leap in our understanding of the universe. We could, essentially, see a whole new side of the universe.

This is just as big. Now we can hear the universe. We’re going to learn so, so much.

Related Reading

If you enjoyed this post and want to learn more about general relativity and gravitational waves, you may be interested in my series on how GR works:

Further Reading

Here are some nice lay resources on the recent LIGO discovery. (Thanks to Jonathan Chung on Google+ for finding some of these.)

Scholarly Reading

For the very brave, here are my academic sources.

This is the LIGO detection paper. Already peer reviewed. Kudos to the LIGO collaboration for going through peer-review before announcing their result!

This is the LIGO paper describing how they extracted the mass and spin of the black holes.

This paper describes the LIGO team’s investigation of whether or not the December detection could have been a mistake. (Obviously, they concluded it was real, or I wouldn’t be writing this blog post…)

This paper describes the LIGO team’s model-agnostic approach to measuring the wave. This is how they know they’re not falling victim to wishful thinking.

This technical paper describes how the LIGO team estimated their noise and error.

This paper discusses how we’ve tested general relativity with this observation.

This is an assessment of the rates of black hole binary mergers in the universe based on the measurements LIGO has made so far.

This is a related paper on what that means for detectors.

This paper is a search for neutrinos from the black hole merger that LIGO observed. (None were found.)

This is the population model for binary black holes which may be wrong.

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