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The black hole whisperer
Features
The black hole whisperer Asimina Arvanitaki is in pursuit of planet-sized particles and a gigantic dark-matter beacon. Daniel Cossins meets her
t first, we thought it was absurd,” Asimina Arvanitaki tells me when we meet in her office at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. I’m not surprised. How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? But this sort of thing is Arvanitaki’s speciality. The first woman to hold a professorship at the institute, she is making a name for herself by taking neglected ideas, however far out they might sound, and then devising ingenious, inexpensive experiments to test them out. At a time when many seem to find it increasingly hard to matchmake ideas in theoretical physics and experiments, her knack for that, allied to her tendency to stray from the beaten path, sets Arvanitaki apart. “I get bored easily,” she says. “I want to see stuff. Otherwise, what’s the point?” She can’t recall a moment she decided to spend her life grappling with the mysteries of the universe, but she does remember the time, as a child in mainland Greece, when she found out the value for the speed of light. “I calculated that it takes 8 minutes for light to get from the sun to Earth,” she says. “And I realised then that we always see the past of things, we can never see the present.” She liked space, but she also liked cars, and at high school had to decide between engineering and physics. “I realised I was more interested in understanding why things work rather than how.”
46 | New Scientist | 1 June 2019
These days, her goal is to spot something truly exotic. Although our best picture of matter and its workings is a magnificent achievement, describing as it does all known particles and three of the four fundamental forces in a neat set of equations, it is far from perfect. We call this picture the standard model, but it says nothing about gravity or dark matter. Nor does it explain why gravity is so weak that you overcome the pull of an entire planet every time you pick up a glass of water.
Smashing physics Hints to these mysteries were meant to have come from the flagship experiment of physics, the Large Hadron Collider (LHC) near Geneva, Switzerland. Historically, building machines that smash particles together at ever higher energies has always led to discoveries. Many in the field were convinced the LHC would follow suit by turning up evidence for supersymmetry, an elegant idea that would solve pretty much everything, including the mystery of dark matter, by introducing a heavy twin for every known particle. It hasn’t worked out that way, leading some to question the way we craft such conjectures – and others to demand an even bigger collider. But even back when the LHC promised the universe, Arvanitaki had other ideas. “We kind of knew, as young people, that most of the good ideas about what it might find were already worked out,” she says. “We didn’t want to just regurgitate stuff.” So Arvanitaki, then a graduate student at Stanford University in
California, started sniffing around entities proposed by physics that couldn’t possibly be revealed at a collider. She was drawn to axions, hypothetical particles suggested in the 1970s to solve a mystery known as the charge-parity problem. Ultralight, they have no electrical charge but generate an entirely new force. They are a good candidate for dark matter. The trouble was, the force they carry would interact so weakly with other particles that they would never show up at the LHC. That might explain why axions fell out of favour – that and the fact that heavyweight particles emerged naturally from supersymmetry, encouraging everyone to build elaborate underground detectors to try to flush them out. In 2010, however, Arvanitaki and her colleagues found a way to resurrect axions by throwing string theory into the mix. Some versions of this complete – albeit completely untested – theory of nature
BRIGITTE LACOMBE FOR BREAKTHROUGH PRIZE
A
“
“We think about particles being tiny but, theoretically, they can be as big as a galaxy” Daniel Cossins is a features writer for New Scientist, specialising in the physical sciences
operate in 10 dimensions. The six beyond those we know must be scrunched up in elaborate fashion to fit into unimaginably small spaces, or else we would have seen them. And it is this rich, extra-dimensional structure, according to Arvanitaki, that gives rise to all manner of ultralight axions – what she calls a string axiverse. Which brings us back to planet-sized particles. When Arvanitaki was writing up the string axiverse idea, a visiting colleague asked if she had heard of black hole superradiance. She hadn’t. And yet once she had spent a year wrapping her head around the idea, she came to realise it could give us a unique opportunity to spot axions, if indeed they exist. Superradiance is a well-established process used in certain types of laser to multiply photons. It works on astrophysical scales too. Basically, if you have a particle of light – simultaneously a wave, as per the quirks of quantum mechanics – and you fire it at a
spinning black hole, it will extract energy and angular momentum from the black hole. In other words, the black hole will give the photon a kick. Now, if you do the same thing, but swap the massless photon for a force-carrying particle with mass, such as an axion, gravity would confine it to the vicinity of the black hole. In this case, it would be almost as if these axion particles are stuck between the black hole and the surface of a perfectly spherical mirror. “So now the [axion] waves scatter from the spinning black hole, but then keep bouncing back and forth, and eventually the amplification becomes exponential,” says Arvanitaki. In this version of superradiance, a cloud of gazillions of axions would be created, which would arrange themselves in an orderly fashion, she adds, “a lot like those pictures of atomic orbitals, only on a massive scale”. The problem is, to make these “black-hole atoms”, the axion wavelength must be as long as the black hole is wide. Except that isn’t a problem here, as wavelength is inversely proportional to mass, and with axions we are talking about extremely light particles. “We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy,” says Arvanitaki. All of which was known, at least to a few people. What Arvanitaki and her colleagues have recently figured out is that these axion clouds could reveal themselves in gravitational waves, the faint ripples in space‑time first picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. And in this case, you don’t need black holes smashing together. Axions colliding in the cloud should annihilate one another to produce gravitons, the particles thought to comprise gravitational waves. Essentially then, axions and black holes combine to dramatic effect, to produce what Arvanitaki describes as “gravitational beacons” that shine out in every direction. Arvanitaki has been working with researchers from LIGO to prepare for the detector’s third run, which began in April and right away detected gravitational waves. It is expected to continue to find them every couple of weeks. Arvanitaki is one of a new generation of particle physicists seeking to make their mark not by inventing more elaborate theories, but by figuring out where – and how – to look if you want to find something new. “When nature tells us it doesn’t work the way we think it should,” she says, “we have to look in other directions.” ❚ 1 June 2019 | New Scientist | 47
The black hole whisperer Asimina Arvanitaki is in pursuit of planet-sized particles and a gigantic dark-matter beacon. Daniel Cossins meets her
t first, we thought it was absurd,” Asimina Arvanitaki tells me when we meet in her office at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. I’m not surprised. How else could you respond to the idea that black holes generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together? But this sort of thing is Arvanitaki’s speciality. The first woman to hold a professorship at the institute, she is making a name for herself by taking neglected ideas, however far out they might sound, and then devising ingenious, inexpensive experiments to test them out. At a time when many seem to find it increasingly hard to matchmake ideas in theoretical physics and experiments, her knack for that, allied to her tendency to stray from the beaten path, sets Arvanitaki apart. “I get bored easily,” she says. “I want to see stuff. Otherwise, what’s the point?” She can’t recall a moment she decided to spend her life grappling with the mysteries of the universe, but she does remember the time, as a child in mainland Greece, when she found out the value for the speed of light. “I calculated that it takes 8 minutes for light to get from the sun to Earth,” she says. “And I realised then that we always see the past of things, we can never see the present.” She liked space, but she also liked cars, and at high school had to decide between engineering and physics. “I realised I was more interested in understanding why things work rather than how.”
46 | New Scientist | 1 June 2019
These days, her goal is to spot something truly exotic. Although our best picture of matter and its workings is a magnificent achievement, describing as it does all known particles and three of the four fundamental forces in a neat set of equations, it is far from perfect. We call this picture the standard model, but it says nothing about gravity or dark matter. Nor does it explain why gravity is so weak that you overcome the pull of an entire planet every time you pick up a glass of water.
Smashing physics Hints to these mysteries were meant to have come from the flagship experiment of physics, the Large Hadron Collider (LHC) near Geneva, Switzerland. Historically, building machines that smash particles together at ever higher energies has always led to discoveries. Many in the field were convinced the LHC would follow suit by turning up evidence for supersymmetry, an elegant idea that would solve pretty much everything, including the mystery of dark matter, by introducing a heavy twin for every known particle. It hasn’t worked out that way, leading some to question the way we craft such conjectures – and others to demand an even bigger collider. But even back when the LHC promised the universe, Arvanitaki had other ideas. “We kind of knew, as young people, that most of the good ideas about what it might find were already worked out,” she says. “We didn’t want to just regurgitate stuff.” So Arvanitaki, then a graduate student at Stanford University in
California, started sniffing around entities proposed by physics that couldn’t possibly be revealed at a collider. She was drawn to axions, hypothetical particles suggested in the 1970s to solve a mystery known as the charge-parity problem. Ultralight, they have no electrical charge but generate an entirely new force. They are a good candidate for dark matter. The trouble was, the force they carry would interact so weakly with other particles that they would never show up at the LHC. That might explain why axions fell out of favour – that and the fact that heavyweight particles emerged naturally from supersymmetry, encouraging everyone to build elaborate underground detectors to try to flush them out. In 2010, however, Arvanitaki and her colleagues found a way to resurrect axions by throwing string theory into the mix. Some versions of this complete – albeit completely untested – theory of nature
BRIGITTE LACOMBE FOR BREAKTHROUGH PRIZE
A
“
“We think about particles being tiny but, theoretically, they can be as big as a galaxy” Daniel Cossins is a features writer for New Scientist, specialising in the physical sciences
operate in 10 dimensions. The six beyond those we know must be scrunched up in elaborate fashion to fit into unimaginably small spaces, or else we would have seen them. And it is this rich, extra-dimensional structure, according to Arvanitaki, that gives rise to all manner of ultralight axions – what she calls a string axiverse. Which brings us back to planet-sized particles. When Arvanitaki was writing up the string axiverse idea, a visiting colleague asked if she had heard of black hole superradiance. She hadn’t. And yet once she had spent a year wrapping her head around the idea, she came to realise it could give us a unique opportunity to spot axions, if indeed they exist. Superradiance is a well-established process used in certain types of laser to multiply photons. It works on astrophysical scales too. Basically, if you have a particle of light – simultaneously a wave, as per the quirks of quantum mechanics – and you fire it at a
spinning black hole, it will extract energy and angular momentum from the black hole. In other words, the black hole will give the photon a kick. Now, if you do the same thing, but swap the massless photon for a force-carrying particle with mass, such as an axion, gravity would confine it to the vicinity of the black hole. In this case, it would be almost as if these axion particles are stuck between the black hole and the surface of a perfectly spherical mirror. “So now the [axion] waves scatter from the spinning black hole, but then keep bouncing back and forth, and eventually the amplification becomes exponential,” says Arvanitaki. In this version of superradiance, a cloud of gazillions of axions would be created, which would arrange themselves in an orderly fashion, she adds, “a lot like those pictures of atomic orbitals, only on a massive scale”. The problem is, to make these “black-hole atoms”, the axion wavelength must be as long as the black hole is wide. Except that isn’t a problem here, as wavelength is inversely proportional to mass, and with axions we are talking about extremely light particles. “We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy,” says Arvanitaki. All of which was known, at least to a few people. What Arvanitaki and her colleagues have recently figured out is that these axion clouds could reveal themselves in gravitational waves, the faint ripples in space‑time first picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. And in this case, you don’t need black holes smashing together. Axions colliding in the cloud should annihilate one another to produce gravitons, the particles thought to comprise gravitational waves. Essentially then, axions and black holes combine to dramatic effect, to produce what Arvanitaki describes as “gravitational beacons” that shine out in every direction. Arvanitaki has been working with researchers from LIGO to prepare for the detector’s third run, which began in April and right away detected gravitational waves. It is expected to continue to find them every couple of weeks. Arvanitaki is one of a new generation of particle physicists seeking to make their mark not by inventing more elaborate theories, but by figuring out where – and how – to look if you want to find something new. “When nature tells us it doesn’t work the way we think it should,” she says, “we have to look in other directions.” ❚ 1 June 2019 | New Scientist | 47