I’d like to start with a message from the past. (Static voice recording – unintelligible) (Recording stops) That voice is a voice from history. It’s actually a voice with a song, but in truth it’s a voice with a story. What we heard is the sound of Édouard-Léon Scott de Martinville singing Au Clair De La Lune in about 1860, recorded using one of the earliest known sound recording devices, a phonautograph.
To record sound required a device that could sense vibrations – one that could take the invisible, mechanical vibrations of the air and record them using a stylus to trace them out on soot-blackened paper or glass, and preserve them, so that in future, they could be played back again and again, preserving the unique stories that those sound vibrations carry.
Last September, on 14 September 2015 in the morning, we recorded a story from the past on an astronomical scale. This time, not from 150 years or so ago, like the sound vibrations we just heard, but from 1.3 billion years ago – a story that had been traveling across the universe to us here on Earth ever since then. What we recorded was an entirely new kind of vibration. This time, not of air, but a vibration of space-time itself, a vibration of the fabric of the universe – gravitational waves.
Those first gravitational wave signals we recorded also carried a story, a story from cosmic history, a story of our violent universe. The story that they carried was one of two black holes, each tens of times the mass of our own sun sitting far out in the cosmos. Each black hole is itself the endpoint of a stellar collapse.
As a star got to the end of its life, ran out of fuel, its core collapsed under the influence of gravity and became compressed into a region of space where gravity was so strong that nothing could escape, not even light. Those two black holes were caught in a spiral, orbiting around, stretching and squashing space-time around them as they spiraled in ever closer, until eventually they merged in a catastrophic collision. The message of that collision was sent out across the universe as a pulse of gravitational waves. The energy of that pulse was equivalent to the output of all the stars in our galaxy shining for 500 years.
Now, we’ve looked at a beautiful animation, but in fact, it’s most likely that that entire event was completely dark. It gave out no visible signal, no optical light, no light of any wavelength and left no image of the collision, only the vibrations of space-time. But those vibrations carried with them information. In fact, they carried with them information that this collision happened about 1.3 billion light-years away, where a light-year’s the distance that light travels in a year. 1.3 billion years ago then is when that collision happened.
Now, at that point, here on Earth, multicellular life was just kind of getting going. When that signal reached a distance from us about twice the size of our Milky Way Galaxy, about 200,000 light-years away, evolution had got to the point where Homo sapiens, modern humans, had developed. By the time the signal hit stars in our local group, about 100 light-years away, Einstein was just coming up with his Theory of General Relativity and predicting that gravitational waves should exist.
Now of course the timeline of that story is also true for light reaching us from the most distant galaxies. However, we’ve been studying the universe through its optical signals ever since humans first went out and looked up at the stars. It took Einstein to come along and tell us that gravitational waves existed, that we should be searching for them at all.
Now, notions of where and when events happen in the universe are sometimes a bit complicated. Relativity tells us it depends where we sit on a reference frame. But from our perspective, here on Earth, receiving and interpreting these gravitational wave signals, allows us to learn about distant events in the past history of the universe. So eventually, that signal reached the Earth, and it reached the Earth about 9:50 am, 14 September, last year, and it hit first the LIGO observatory in Livingston, Louisiana in the United States. And then, about 7.3 milliseconds later, it hit the LIGO observatory in Hanford, in Washington state, and then it swept on out into space.
But, as it passed those observatories, they were able to sense and record the vibrations of space-time as the gravitational wave passed by. And having recorded those vibrations, we can turn them into sound, we can listen to the sound of two black holes colliding. (Recording of hushed sound) Okay, that was kind of hard to hear, so let’s try again. We’ll shift the frequency a little bit and clean it up a little. (Recording of louder sound) Okay, still, it wasn’t terribly impressive.
In fact, though, it tells us a lot, it really tells us a lot about the event that produced it. Not only encoded in that signal is the fact that two black holes collided, we can tell that one of those black holes was about 29 times the mass of our sun. The other, about 36 times the mass of our sun. We can tell the distance; we can tell that when they collided they merged to form a new black hole about 62 times the mass of our sun, about 370 kilometers across, which is about the size of Iceland, spinning so that a point on its event horizon was traveling at nearly half the speed of light. All of that encoded in that signal.
And then, on 26 December 2015, actually on Christmas day in the United States, a second signal arrived. (Muffled blip) (Muffled blip) (Muffled blip) (Muffled blip) (Sound raising in pitch) (Sound raising in pitch) (Sound raising in pitch) (Sound raising in pitch) So you can hear that those sound slightly different. That’s because the events that produced them were slightly different. That second event was also black holes colliding, but this time the black holes were a bit smaller than in the first case. It’s a bit like being a bird watcher. After a while, you don’t need to see the bird that produced a chirp to know what kind of bird it was. You just need to listen to the chirp and you can tell whether it was produced by a sparrow or by an eagle.