Sheila Rowan – TRANSCRIPT
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.
So how do we sense, then, these tiny vibrations in space? Well, remember that general relativity tells us that we can think of gravity as the curvature of space-time caused by mass. And if mass moves, the curvature of space-time around it changes. And that change propagates, it travels out through the universe, like ripples on the surface of a pond but in all directions.
So the proper distance between two points in space is changed, it’s stretched and squashed as a gravitational wave passes by. However, despite the huge energy that goes into producing a gravitational wave signal, as it spreads out through space, by the time it arrives here on Earth, the effects are tiny. So, for two objects on the surface of the Earth, separated by about a meter, their separation would only be changed by about ten to the minus 22 of a meter, by a typical gravitational wave. So that’s less than a millionth of a millionth of the size of an atom. That’s why despite the fact that we are being squashed and stretched as gravitational waves pass through us, it doesn’t bother us at all. It’s also why we have to build incredibly sensitive detectors to measure this effect.
And in fact, gravitational waves have a very characteristic way of distorting space. They stretch space in one direction whilst simultaneously squashing it in a perpendicular direction. And more, they’re tidal in effect so that means the further apart we initially place two objects, the more their separation will be changed by a passing gravitational wave. That tells us we need to make detectors large.
Indeed, the LIGO observatories are the largest of a network of detectors either operating or being commissioned around the globe. In LIGO, what we do is we measure the relative position of mirrors, each suspended from delicate fibers at the ends of four-kilometer-long arms, and we use light to sense their position. We take a beam of light, split it into two, and reflect it from those mirrors so it adds up again at a beam splitter. If a gravitational wave passes through the detector, it changes the relative positions of the mirrors, so it changes how light adds up or cancels out again at the beam splitter.
So by checking the brightness of the spot at the beam splitter, we can search for and find the effects of gravitational waves. However, remember the effects are tiny, so the instrumentation required to do this is incredibly sophisticated. It requires superb isolation of the mirrors so that no background disturbances cause problems. So it’s taken decades of honing the technology to get to the point of being able to measure these tiny vibrations in space-time.
So having taken decades to get to this point, what comes next? Well, we soon are due to turn the LIGO observatories back on again, along with another observatory in Europe, the Virgo Observatory. And when we do, we expect to detect more colliding black holes, many more. In fact, over the next few years, as detectors are made more sensitive, perhaps even one a day, from even further out in cosmic time and space. When we have many of these events, we aim to use them to reveal the family tree of black hole evolution throughout cosmic time, perhaps filling in the missing links between the stellar mass black holes of the type that we talked about here and the supermassive black holes that we believe live in the centers of galaxies, including our own.
Now, we know from other studies that our universe is expanding, and that it’s not only expanding, it’s accelerating in its expansion. Galaxies are rushing apart ever faster. We say that that’s caused by dark energy; we do not know what dark energy is. To date, all the measurements of the expansion of the universe have been carried out using telescopes sensing electromagnetic signals, essentially, light. We may be able to use these black hole chirps to tell how gravity probes the expansion of the universe, a completely different way to study this phenomenon.
We also expect to detect gravitational wave signals from other sources: from supernovae, helping study how stars end their life. From colliding neutron stars, probing the mystery of their interior. And tantalizingly, as our detectors get more sensitive, detecting ever-fainter signals from further back in cosmic time. Until one day, not yet, but in future, with more sensitive detectors, we may be able to detect the faint signals coming from the first few moments after the big bang, accessing a time in the history of the universe that currently we can’t get to any other way. Sensing these tiny vibrations of space-time has given us a new sense.
A veil has dropped revealing dark cosmic phenomena. When we listened to those first recordings of the human voice, it sounded to us now, pretty crackly and crude compared to what we expect from modern recording technologies. But in fact, those first sound recording devices were cutting edge at the time, they were miracles of technological ingenuity.
Our ability to record and play back sound has had enormous impact in society. Today we take for granted our ability to record the spoken word, preserve information for posterity. In the same way, LIGO and its current gravitational wave detector cousins, are both simultaneously miracles of modern gravitational wave detector technology and at the same time, they’re like those original smoked glass plates or wax cylinders, they’re just the first detectors we’ve been able to build and make work.
In future, we’ll have more sensitive detectors; we’ll be able to pick up the suite of signals that the dark universe is sending us. So we’ll no longer be listening to a crackling voice in the noise, we’ll be listening to the symphony of the cosmos. Thank you.
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