Great Sex

Discovering the sound of the universe

Pete Wolf is a research physicist at the Paris Observatory, where he specialises in fundamental aspects of gravitation; general relativity and alternative theories; experimental tests of fundamental physics; searches for dark matter; and gravitational waves.

‘Physics is like sex: sometimes something useful comes of it, but that’s not why we are doing it'.

This quote gets attributed to Robert Feynman — probably incorrectly, but who cares? It’s what I use when people ask me what my work is good for. It captures the essence of what we’re doing, up there in the ivory towers, and why we bother. And because the kind of person who asks this question is usually also the sort of person to be impressed by Richard Feynman, I usually say that the words are his.

Who’s this 'we'? Physicists doing fundamental physics, studying the fundamental laws and constituents (from sub-nuclear particles to galaxies) of the universe. The vast majority of us are nowhere near Feynman calibre; I’m talking about everyone from your slightly-above-average PhD student upwards. Why do we bother? And why should you care?

Have you thought about why you are asking me this question?

Maths and physics, in themselves, are no more and no less ‘useful’ than any other cultural activity, including writing essays for The Metropolitan. Why do we do it? For the money, of course. But also, the sex.

Think about what it feels like when you listen to an album for the first time and discover something you always felt was there; a place inside you that you had never before managed to nail down (like Tobias’s experience with 'Frank’s Wild Years'). Think about those sensations: awe, excitement, a deep satisfaction. Now think about what it’s like when you gain some new understanding of the world, a piece of knowledge about the mechanism: of what it is that moves it, and moves you, and how it all moves together. You feel reassured. Some primitive anguish is partially soothed. This sometimes happens to me when I learn something in physics that I didn’t know before, or understand something I didn’t understand before. But it also happens when I watch a film that particularly touches some part of my soul. It’s about our place in the world, and the world’s place in us; it’s about the pleasure of pondering where previously we could not or did not ponder.

This will work better if we use a concrete example, so I’m going to tell you a story about what it was like for me when I found out about spectacular advances in the detection of gravitational waves. If you were about to head off to Wikipedia: please don’t. Right now, you don’t need to know exactly what gravitational waves are. Let me tell you the story first. If I do a good-enough job of that, you’ll still be reading when I get to the explanation, and you’ll be more interested in it.

Gravitational waves are special; they offer a completely new but complementary way of observing the universe (or what we think of as the universe, but let’s not get into that now). And they came out of the ‘90s Zeitgeist, after the Wall came down, when everything was possible and the world was wide open once again. But, unlike most of the possibilities that arose in the ‘90s, gravitational waves delivered on their promise.

When I began my undergraduate degree in September 1989, the world as I experienced it was full of certainties. In the course of the Cold War the young Gen X in the West had become used to – even comfortable with – a clear, if daunting, geopolitical reality. The good guys and the bad guys were easily identified; ‘68 had failed; punk had lost its bite; global warming was still only a possibility. We sank back into our sofas and turned on our TVs, a spliff in one hand and a glass of Bulgarian white in the other, not caring very much about the beginning of the rest of our lives.

At the beginning of my first term at university the same sort of consensus reigned in the lecture halls. Pretty much all of physics, we thought, was sorted. The great minds of the twentieth century – Feynman included – had worked it all out. We knew about general relativity and quantum field theory, and the resulting standard model of particle physics. Everything was done, finished, settled; the cosmos, from sub-nuclear particles to the universe as a whole, was entirely explicable and explained. There were a few stray details to fix here and there, but overall, going into fundamental research in physics felt like going into battle after the war had ended.

And then, almost immediately, everything changed. The Iron Curtain came down and the Cold War ended. Our certitudes and convictions were disrupted. Those were years of great promise and equally great expectations, of ambitions and deceptions. This was true politically, economically and artistically, and also scientifically. We found out that the things we ‘knew’ actually referred to about 5% of the universe; the rest of it was dark energy and dark matter, creative synonyms for 'I don’t know what the fuck is going on'. Suddenly, there was a lot of new stuff to discover and everyone was all excited about it, like a priest at the sight of a new flock of choirboys. We built the Large Hadron Collider at CERN, and in 2012 we discovered the Higgs Boson particle. But that wasn’t all.

Do you remember February 11 2016? I do. It was a Thursday, and I was in my office at the Paris Observatory watching the video feed of a press conference. The collaboration between LIGO and Virgo – two groups of scientists running gravitational wave detectors using ‘interferometers’ (an explanation of those is coming too) – had yielded an astonishing result. Four minutes into the press conference LIGO’s Director David Reitze stepped up to the mic and announced: ‘Ladies and gentlemen, we have detected gravitational waves.'

This was a guaranteed Nobel Prize, and it was also so much more than that. From Einstein onwards, fundamental physicists -- in particular those working on general relativity and gravitation -- had been thinking about this moment. Many physicists (mostly from other fields of physics) were sceptical, believing either that gravitational waves didn’t exist, or that they were much smaller and/or rarer than calculations and models had predicted. Funding for gravitational wave detectors (such as interferometers) had been scarce and difficult; most of the money was going to particle colliders and other experiments that had more 'consensual' aims, or might produce ‘something useful'.

The actual detection had taken place months before, on September 14 2015 at 09:50:45 UTC, and of course the news had leaked very quickly. By February 2016 only the most reclusive gravitational physicist could have been entirely unaware of what would be announced. But the details of what exactly had been seen (‘heard’ is actually a better way to think about it) remained a secret kept well away from anyone who, like me, had not been directly involved in the LIGO/Virgo collaborations. For most of the world of physics, this was a Sputnik moment.

The scientific paper was published in Physical Review Letters on the same day, and I got my hands on it that afternoon. I read it once, and then I read it again. Then I sat down and went through the calculations: working out the odds that this might be a statistical fluke, getting bogged down in details, understanding some of it, not understanding other parts, looking up references.

It was extremely exciting. I’m not sure that I could tell you exactly why. A little bit because this was history in the making; a little bit because I was curious; a little bit because I’d need to explain it to my students and discuss it with colleagues. But mostly out of excitement, a primitive urge. I was experiencing something for the first time, and I was literally at it all night. You can see where I’m going with this analogy.

I had been following the efforts to detect gravitational waves since the early ‘90s, and I understood some of the theory behind it, but it was only in 2017 that I was called to serve (yes, that’s the word we use) on the Scientific and Technological Advisory Committee (STAC) of the Virgo gravitational wave detector near Pisa. I have been on that committee ever since – I am now its chair – and have become quite fascinated not only by the science of what was discovered, but by the technology of the machine: the sheer madness of it, the fact that we dared to try, the sociology and collective psychology.

Incidentally, I absolutely guarantee that nothing useful will ever come of it.

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Still reading? Let’s avoid the Oppenheimer trap of relating everything except the physics (the physics of the atomic nucleus being the thing that brought Fermi, Feynman, Teller, Oppenheimer, Rabi, von Neumann and the rest to Los Alamos and Trinity; the very thing – not love of their country, or hate of communism, or money – that motivated their lives, and killed many of them).

It’s time to explain what gravitational waves are.

A long time ago, in a galaxy far, far away, about 1300 million light years from the Earth, two black holes are orbiting each other. They are driven by a mutual, irresistible attraction. This cosmic dance accelerates and becomes ever more frenzied. It culminates in a cataclysmic coalescence into a single even more massive black hole, after which everything calms down until there is no motion at all (the 'ring-down' phase). Again: you can see where I’m going with this analogy.

The resulting single black hole is about three solar masses lighter than the sum of the initial two.

Where did all that mass go? It turned into pure energy and radiated away: we know this, because E = mc². And as no light of any kind was emitted by the event – it was as black as its two protagonists – the energy, in this case, was emitted as some other form of radiation: not electromagnetic radiation (light, X-rays, gamma-rays, radio-waves and so on), but something else. The radiation took the form of gravity.

The event generated a gravitational shock wave: a perturbation of the gravitational field of the universe, of space-time itself. And that shock wave propagates across the universe, just like the ripples in a calm pond after you throw a pebble into it.

We call these perturbations 'gravitational waves'. They travel through the universe unhindered. Nothing can stop them because they barely interact – only very weakly – with the rest of the universe. And that makes them almost imperceptible. For many decades the consensual view was that no instrument that we could conceivably build would have been sensitive enough to detect them. When Einstein first predicted their existence, he initially dismissed the idea and then remained sceptical about them for decades. And ever since, most physicists had thought they would forever remain theoretical chimeras, travelling through the universe without ever being captured, their message forever lost to humanity.

Flash forward to September 14 2015. We are in Livingston, Louisiana; the local time is 04:50. The LIGO-Livingston interferometer records a tiny signal. 6.9 milliseconds later, in Hanford, Washington, the LIGO-Hanford interferometer records the same signal. Here’s what that signal sounded like.

A time-frequency map (spectrogram) of the noise-filtered signal. The sound is a true rendering, in the sense that the frequency is unchanged and that it is in real time. In other words: this is the space-time-boop produced by that particular cataclysmic coalescence of two massive black holes.
Credit for all: LIGO

A few months later, on December 26 2015, a second signal is captured, also from a coalescence of two black holes.

Two years later, this time with the help of the French-Italian detector Virgo, the gravitational wave emitted during a coalescence of two neutron stars is captured for the first time. Simultaneous detection by three detectors allows the triangulation of the source. It is located in a Galaxy called NGC4993, 130 million light years away from us, in the Hydra constellation.

Since then the three detectors have been operating intermittently with upgrade periods in between. From initially detecting a GW event every few months, they are now up to about three a week, with a total of close to 300 events on record. The vast majority are black hole mergers, but nevertheless there have been many surprises concerning such things as the masses of black holes and their distribution. Some of the mergers involved one or two neutron stars (e.g. the third sound you heard above). Neutron stars are extremely dense objects composed solely of neutrons, and they are interesting in their own right.

GWs are often referred to as the 'sound of the universe', and the analogy is a good one. It’s not sound, strictly speaking; there is no interstellar air in which sound could propagate. (Most sci-fi movies, with their whooshing space ships and banging laser-guns, ignore this; Kubrick is the exception, of course.) However, the frequency range of the GWs that we can detect today – and this is a pure historico-technological coincidence – is the same as the acoustic range of the human ear (30–10000Hz or so). That’s why we can 'listen' to the GW examples above. Also, the details of the objects that emitted the GWs are obtained from an analysis of all the harmonics of the signals, which is a bit like being able to tell the difference between a violin and a viola from their timbre.

More profoundly, so far as our analogy goes, GWs are a fundamentally new and independent way of observing our world. Since 2015 we have been a bit like someone who was born deaf and has started to recover a little bit of hearing. We could see the forests and the trees, but now we are beginning to hear the birds hiding in the foliage. Recovering just this tiny part of the gravitational 'sound of the universe' took a century of human effort, vision and imagination.

Einstein knew that in principle GWs should be observable. For example, we could measure the distance between two freely falling objects; this distance oscillates in unison with the GW. But an elementary calculation showed that this effect would be tiny, even for the most powerful GWs he could imagine. The distance between two objects separated by a few kilometres varies by less than one thousandth of the size of an atomic nucleus. This is totally unmeasurable, but that didn’t stop people trying. Joseph Weber’s effort in the 1960s is the most famous and is an interesting story in itself, with dramatic claims of detection and counter-claims. The detectors at the time were huge bars of metal weighing several tons; they were supposed to resonate acoustically when a GW passed. But these 'Weber bars' spent years listening without picking up the slightest signal. The universe persisted in its silence.

Those who came after Weber took a different route. They built giant laser interferometers.

What is an interferometer? This is most easily explained visually. But if you prefer words: an interferometer consists of two orthogonal arms, each several kilometres long. The light of a laser is sent into the arms, reflected by mirrors at the end, and recombined in a central hall. The passage of a GW will 'move' the mirrors at the end of the interferometer arms by a very, very, very small amount, which translates into tiny changes in the intensity of the recombined light. We’re talking about distances of a few attometres (an attometre is 10⁻¹⁸ of a metre), but that’s measurable using modern lasers and quantum measurement techniques.

An aerial view of the Virgo interferometer (Wikimedia Commons)

The first giant interferometers were built in the mid-1990s, in the USA and Europe. And here we return to where we started, that moment of change and possibility, because I don’t think the timing was a coincidence. Giant interferometers require investment, and the associated risks were huge. Much of the technology had never been imagined before, let alone built. And it was all speculative: there were doubts that detectable GWs existed at all. But the mid-‘90s were one of those periods in human history during which discovery was prioritised over destruction.

It took about 20 years of building and improving the interferometers before we 'heard' the first signals from the universe. And so far the GWs we have captured originate from processes that we already thought existed; they were, to some extent, expected. But we keep improving our gravitational hearing. And as we get better at listening, by building next generation detectors¹ on ground and even in space, we will find new patterns in the noise. We will hear signals we didn’t expect, originating from cosmic processes that we never imagined. I tell my students that the next decades will be exciting times. And they will be their times.

But I count myself lucky to have been around in the ‘90s, when it all began. It was one of those brief periods in history when certainties are doubted, and new opportunities open up. The detection of gravitational waves is one of the (few?) positive outcomes. I wish there were more.

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Some of the above is taken from the texts I wrote for https://ensemblecalliopee.com/cosmosono/. (Skip the texts and listen to the music.)

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I know! Actual physics in The Metropolitan! You weren’t expecting that were you? Here’s that piece from Rowan on Christopher Nolan’s Oppenheimer and the fallacy of the two cultures:

1. https://www.einsteintelescope-emr.eu/en/ https://www.einstein-telescope.it/en/home-e https://cosmicexplorer.org/