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James Johnson hopes to drive a car again one day. If he does, he will do it using only his thoughts.

In March 2017, Johnson broke his neck in a go-carting accident, leaving him almost completely paralysed below the shoulders. He understood his new reality better than most. For decades, he had been a carer for people with paralysis. “There was a deep depression,” he says. “I thought that when this happened to me there was nothing — nothing that I could do or give.”

But then Johnson’s rehabilitation team introduced him to researchers from the nearby California Institute of Technology (Caltech) in Pasadena, who invited him to join a clinical trial of a brain–computer interface (BCI). This would first entail neurosurgery to implant two grids of electrodes into his cortex. These electrodes would record neurons in his brain as they fire, and the researchers would use algorithms to decode his thoughts and intentions. The system would then use Johnson’s brain activity to operate computer applications or to move a prosthetic device. All told, it would take years and require hundreds of intensive training sessions. “I really didn’t hesitate,” says Johnson.

The first time he used his BCI, implanted in November 2018, Johnson moved a cursor around a computer screen. “It felt like The Matrix,” he says. “We hooked up to the computer, and lo and behold I was able to move the cursor just by thinking.”

Johnson has since used the BCI to control a robotic arm, use Photoshop software, play ‘shoot-’em-up’ video games, and now to drive a simulated car through a virtual environment, changing speed, steering and reacting to hazards. “I am always stunned at what we are able to do,” he says, “and it’s frigging awesome.”

Johnson is one of an estimated 35 people who have had a BCI implanted long-term in their brain. Only around a dozen laboratories conduct such research, but that number is growing. And in the past five years, the range of skills these devices can restore has expanded enormously. Last year alone, scientists described a study participant using a robotic arm that could send sensory feedback directly to his brain¹; a prosthetic speech device for someone left unable to speak by a stroke²; and a person able to communicate at record speeds by imagining himself handwriting³.

James Johnson uses a brain-computer interface to control a photo editing software on a computer screen

James Johnson uses his neural interface to create art by blending images.

Credit: Tyson Aflalo

So far, the vast majority of implants for recording long-term from individual neurons have been made by a single company: Blackrock Neurotech, a medical-device developer based in Salt Lake City, Utah. But in the past seven years, commercial interest in BCIs has surged. Most notably, in 2016, entrepreneur Elon Musk launched Neuralink in San Francisco, California, with the goal of connecting humans and computers. The company has raised US$363 million. Last year, Blackrock Neurotech and several other newer BCI companies also attracted major financial backing.

Bringing a BCI to market will, however, entail transforming a bespoke technology, road-tested in only a small number of people, into a product that can be manufactured, implanted and used at scale. Large trials will need to show that BCIs can work in non-research settings and demonstrably improve the everyday lives of users — at prices that the market can support. The timeline for achieving all this is uncertain, but the field is bullish. “For thousands of years, we have been looking for some way to heal people who have paralysis,” says Matt Angle, founding chief executive of Paradromics, a neurotechnology company in Austin, Texas. “Now we’re actually on the cusp of having technologies that we can leverage for those things.”

Interface evolution

In June 2004, researchers pressed a grid of electrodes into the motor cortex of a man who had been paralysed by a stabbing. He was the first person to receive a long-term BCI implant. Like most people who have received BCIs since, his cognition was intact. He could imagine moving, but he had lost the neural pathways between his motor cortex and his muscles. After decades of work in many labs in monkeys, researchers had learnt to decode the animals’ movements from real-time recordings of activity in the motor cortex. They now hoped to infer a person’s imagined movements from brain activity in the same region.

In 2006, a landmark paper⁴ described how the man had learnt to move a cursor around a computer screen, control a television and use robotic arms and hands just by thinking. The study was co-led by Leigh Hochberg, a neuroscientist and critical-care neurologist at Brown University in Providence, Rhode Island, and at Massachusetts General Hospital in Boston. It was the first of a multicentre suite of trials called BrainGate, which continues today.

“It was a very simple, rudimentary demonstration,” Hochberg says. “The movements were slow or imprecise — or both. But it demonstrated that it might be possible to record from the cortex of somebody who was unable to move and to allow that person to control an external device.”

Today’s BCI users have much finer control and access to a wider range of skills. In part, this is because researchers began to implant multiple BCIs in different brain areas of the user and devised new ways to identify useful signals. But Hochberg says the biggest boost has come from machine learning, which has improved the ability to decode neural activity. Rather than trying to understand what activity patterns mean, machine learning simply identifies and links patterns to a user’s intention.

“We have neural information; we know what that person who is generating the neural data is attempting to do; and we’re asking the algorithms to create a map between the two,” says Hochberg. “That turns out to be a remarkably powerful technique.”

Motor independence

Asked what they want from assistive neurotechnology, people with paralysis most often answer “independence”. For people who are unable to move their limbs, this typically means restoring movement.

One approach is to implant electrodes that directly stimulate the muscles of a person’s own limbs and have the BCI directly control these. “If you can capture the native cortical signals related to controlling hand movements, you can essentially bypass the spinal-cord injury to go directly from brain to periphery,” says Bolu Ajiboye, a neuroscientist at Case Western Reserve University in Cleveland, Ohio.

In 2017, Ajiboye and his colleagues described a participant who used this system to perform complex arm movements, including drinking a cup of coffee and feeding himself⁵. “When he first started the study,” Ajiboye says, “he had to think very hard about his arm moving from point A to point B. But as he gained more training, he could just think about moving his arm and it would move.” The participant also regained a sense of ownership of the arm.

Ajiboye is now expanding the repertoire of command signals his system can decode, such as those for grip force. He also wants to give BCI users a sense of touch, a goal being pursued by several labs.

In 2015, a team led by neuroscientist Robert Gaunt at the University of Pittsburgh in Pennsylvania, reported implanting an electrode array in the hand region of a person’s somatosensory cortex, where touch information is processed⁶. When they used the electrodes to stimulate neurons, the person felt something akin to being touched.

Gaunt then joined forces with Pittsburgh colleague Jennifer Collinger, a neuroscientist advancing the control of robotic arms by BCIs. Together, they fashioned a robotic arm with pressure sensors embedded in its fingertips, which fed into electrodes implanted in the somatosensory cortex to evoke a synthetic sense of touch¹. It was not an entirely natural feeling — sometimes it felt like pressure or being prodded, other times it was more like a buzzing, Gaunt explains. Nevertheless, tactile feedback made the prosthetic feel much more natural to use, and the time it took to pick up an object was halved, from roughly 20 seconds to 10.

Implanting arrays into brain regions that have different roles can add nuance to movement in other ways. Neuroscientist Richard Andersen — who is leading the trial at Caltech in which Johnson is participating — is trying to decode users’ more-abstract goals by tapping into the posterior parietal cortex (PPC), which forms the intention or plan to move⁷. That is, it might encode the thought ‘I want a drink’, whereas the motor cortex directs the hand to the coffee, then brings the coffee to the mouth.

Andersen’s group is exploring how this dual input aids BCI performance, contrasting use of the two cortical regions alone or together. Unpublished results show that Johnson’s intentions can be decoded more quickly in the PPC, “consistent with encoding the goal of the movement”, says Tyson Aflalo, a senior researcher in Andersen’s laboratory. Motor-cortex activity, by contrast, lasts throughout the whole movement, he says, “making the trajectory less jittery”.

This new type of neural input is helping Johnson and others to expand what they can do. Johnson uses the driving simulator, and another participant can play a virtual piano using her BCI.

Movement into meaning

“One of the most devastating outcomes related to brain injuries is the loss of ability to communicate,” says Edward Chang, a neurosurgeon and neuroscientist at the University of California, San Francisco. In early BCI work, participants could move a cursor around a computer screen by imagining their hand moving, and then imagining grasping to ‘click’ letters — offering a way to achieve communication. But more recently, Chang and others have made rapid progress by targeting movements that people naturally use to express themselves.

The benchmark for communication by cursor control — roughly 40 characters per minute⁸ — was set in 2017 by a team led by Krishna Shenoy, a neuroscientist at Stanford University in California.

Then, last year, this group reported³ an approach that enabled study participant Dennis Degray, who can speak but is paralysed from the neck down, to double the pace.

Shenoy’s colleague Frank Willett suggested to Degray that he imagine handwriting while they recorded from his motor cortex (see ‘Turning thoughts into type’). The system sometimes struggled to parse signals relating to letters that are handwritten in a similar way, such as r, n and h, but generally it could easily distinguish the letters. The decoding algorithms were 95% accurate at baseline, but when autocorrected using statistical language models that are similar to predictive text in smartphones, this jumped to 99%.

Turning thoughts into type. Explainer graphic showing how a brain-computer interface works.

“You can decode really rapid, very fine movements,” says Shenoy, “and you’re able to do that at 90 characters per minute.”

Degray has had a functional BCI in his brain for nearly 6 years, and is a veteran of 18 studies by Shenoy’s group. He says it’s remarkable how effortless tasks become. He likens the process to learning to swim, saying, “You thrash around a lot at first, but all of a sudden, everything becomes understandable.”

Chang’s approach to restoring communication focuses on speaking rather than writing, albeit using a similar principle. Just as writing is formed of distinct letters, speech is formed of discrete units called phonemes, or individual sounds. There are around 50 phonemes in English, and each is created by a stereotyped movement of the vocal tract, tongue and lips.

Chang’s group first worked on characterizing the part of the brain that generates phonemes and, thereby, speech — an ill-defined region called the dorsal laryngeal cortex. Then, the researchers applied these insights to create a speech-decoding system that displayed the user’s intended speech as text on a screen. Last year, they reported² that this device enabled a person left unable to talk by a brainstem stroke to communicate, using a preselected vocabulary of 50 words and at a rate of 15 words per minute. “The most important thing that we’ve learnt,” Chang says, “is that it’s no longer a theoretical; it’s truly possible to decode full words.”

The word 'Yes' appears on a screen as Eddie Chang and a colleague help a paralysed man speak through an implant in his brain

Neuroscientist Edward Chang (right) at the University of California, San Francisco, helps a man with paralysis to speak through a brain implant that connects to a computer.

Credit: Mike Kai Chen/The New York Times/Redux/eyevine

Unlike other high-profile BCI breakthroughs, Chang didn’t record from single neurons. Instead, he used electrodes placed on the cortical surface that detect the averaged activity of neuronal populations. The signals are not as fine-grained as those from electrodes implanted in the cortex, but the approach is less invasive.

The most profound loss of communication occurs in people in a completely locked-in state, who remain conscious but are unable to speak or move. In March, a team including neuroscientist Ujwal Chaudhary and others at the University of Tübingen, Germany, reported⁹ restarting communication with a man who has amyotrophic lateral sclerosis (ALS, or motor neuron disease). The man had previously relied on eye movements to communicate, but he gradually lost the ability to move his eyes.

The team of researchers gained consent from the man’s family to implant a BCI and tried asking him to imagine movements to use his brain activity to choose letters on a screen. When this failed, they tried playing a sound that mimicked the man’s brain activity — a higher tone for more activity, lower for less — and taught him to modulate his neural activity to heighten the pitch of a tone to signal ‘yes’ and to lower it for ‘no’. That arrangement allowed him to pick out a letter every minute or so.

The method differs from that in a paper¹⁰ published in 2017, in which Chaudhary and others used a non-invasive technique to read brain activity. Questions were raised about the work and the paper was retracted, but Chaudhary stands by it.

These case studies suggest that the field is maturing rapidly, says Amy Orsborn, who researches BCIs in non-human primates at the University of Washington in Seattle. “There’s been a noticeable uptick in both the number of clinical studies and of the leaps that they’re making in the clinical space,” she says. “What comes along with that is the industrial interest”.

Lab to market

Although such achievements have attracted a flurry of attention from the media and investors, the field remains a long way from improving day-to-day life for people who’ve lost the ability to move or speak. Currently, study participants operate BCIs in brief, intensive sessions; nearly all must be physically wired to a bank of computers and supervised by a team of scientists working constantly to hone and recalibrate the decoders and associated software. “What I want,” says Hochberg, speaking as a critical-care neurologist, “is a device that is available, that can be prescribed, that is ‘off the shelf’ and can be used quickly.” In addition, such devices would ideally last users a lifetime.

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Many leading academics are now collaborating with companies to develop marketable devices. Chaudhary, by contrast, has co-founded a not-for-profit company, ALS Voice, in Tübingen, to develop neurotechnologies for people in a completely locked-in state.

Blackrock Neurotech’s existing devices have been a mainstay of clinical research for 18 years, and it wants to market a BCI system within a year, according to chairman Florian Solzbacher. The company came a step closer last November, when the US Food and Drug Administration (FDA), which regulates medical devices, put the company’s products onto a fast-track review process to facilitate developing them commercially.

This possible first product would use four implanted arrays and connect through wires to a miniaturized device, which Solzbacher hopes will show how people’s lives can be improved. “We’re not talking about a 5, 10 or 30% improvement in efficacy,” he says. “People can do something they just couldn’t before.”

Blackrock Neurotech is also developing a fully implantable wireless BCI intended to be easier to use and to remove the need to have a port in the user’s cranium. Neuralink and Paradromics have aimed to have these features from the outset in the devices they are developing.

These two companies are also aiming to boost signal bandwidth, which should improve device performance, by increasing the number of recorded neurons. Paradromics’s interface — currently being tested in sheep — has 1,600 channels, divided between 4 modules.

Neuralink’s system uses very fine, flexible electrodes, called threads, that are designed to both bend with the brain and to reduce immune reactions, says Shenoy, who is a consultant and adviser to the company. The aim is to make the device more durable and recordings more stable. Neuralink has not published any peer-reviewed papers, but a 2021 blogpost reported the successful implantation of threads in a monkey’s brain to record at 1,024 sites (see go.nature.com/3jt71yq). Academics would like to see the technology published for full scrutiny, and Neuralink has so far trialled its system only in animals. But, Ajiboye says, “if what they’re claiming is true, it’s a game-changer”.

Just one other company besides Blackrock Neurotech has implanted a BCI long-term in humans — and it might prove an easier sell than other arrays. Synchron in New York City has developed a ‘stentrode’ — a set of 16 electrodes fashioned around a blood-vessel stent¹¹. Fitted in a day in an outpatient setting, this device is threaded through the jugular vein to a vein on top of the motor cortex. First implanted in a person with ALS in August 2019, the technology was put on a fast-track review path by the FDA a year later.

The Stentrode is a tube-shaped mesh of wires and sensors designed to pick up brain signals from inside blood vessels

The ‘stentrode’ interface can translate brain signals from the inside of a blood vessel without the need for open-brain surgery.

Credit: Synchron, Inc.

Akin to the electrodes Chang uses, the stentrode lacks the resolution of other implants, so can’t be used to control complex prosthetics. But it allows people who cannot move or speak to control a cursor on a computer tablet, and so to text, surf the Internet and control connected technologies.

Synchron’s co-founder, neurologist Thomas Oxley, says the company is now submitting the results of a four-person feasibility trial for publication, in which participants used the wireless device at home whenever they chose. “There’s nothing sticking out of the body. And it’s always working,” says Oxley. The next step before applying for FDA approval, he says, is a larger-scale trial to assess whether the device meaningfully improves functionality and quality of life.

Challenges ahead

Most researchers working on BCIs are realistic about the challenges before them. “If you take a step back, it is really more complicated than any other neurological device ever built,” says Shenoy. “There’s probably going to be some hard growing years to mature the technology even more.”

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The ethics of brain–computer interfaces

Orsborn stresses that commercial devices will have to work without expert oversight for months or years — and that they need to function equally well in every user. She anticipates that advances in machine learning will address the first issue by providing recalibration steps for users to implement. But achieving consistent performance across users might present a greater challenge.

“Variability from person to person is the one where I don’t think we know what the scope of the problem is,” Orsborn says. In non-human primates, even small variations in electrode positioning can affect which circuits are tapped. She suspects there are also important idiosyncrasies in exactly how different individuals think and learn — and the ways in which users’ brains have been affected by their various conditions.

Finally, there is widespread acknowledgement that ethical oversight must keep pace with this rapidly evolving technology. BCIs present multiple concerns, from privacy to personal autonomy. Ethicists stress that users must retain full control of the devices’ outputs. And although current technologies cannot decode people’s private thoughts, developers will have records of users’ every communication, and crucial data about their brain health. Moreover, BCIs present a new type of cybersecurity risk.

There is also a risk to participants that their devices might not be supported forever, or that the companies that manufacture them fold. There are already instances in which users were let down when their implanted devices were left unsupported.

Degray, however, is eager to see BCIs reach more people. What he would like most from assistive technology is to be able to scratch his eyebrow, he says. “Everybody looks at me in the chair and they always say, ‘Oh, that poor guy, he can’t play golf any more.’ That’s bad. But the real terror is in the middle of the night when a spider walks across your face. That’s the bad stuff.”

For Johnson, it’s about human connection and tactile feedback; a hug from a loved one. “If we can map the neurons that are responsible for that and somehow filter it into a prosthetic device some day in the future, then I will feel well satisfied with my efforts in these studies.”

Nature 604, 416-419 (2022)

doi: https://doi.org/10.1038/d41586-022-01047-w

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While working on his doctorate in theoretical physics in the early 1970s, Saul Teukolsky solved a problem that seemed purely hypothetical. Imagine a black hole, the ghostly knot of gravity that forms when, say, a massive star burns out and collapses to an infinitesimal point. Suppose you perturb it, as you might strike a bell. How does the black hole respond?

Teukolsky, then a graduate student at the California Institute of Technology (Caltech), attacked the problem with pencil, paper, and Albert Einstein's theory of gravity, general relativity. Like a bell, the black hole would oscillate at one main frequency and multiple overtones, he found. The oscillations would quickly fade as the black hole radiated gravitational waves—ripples in the fabric of space itself. It was a sweet problem, says Teukolsky, now at Cornell University. And it was completely abstract—until 5 years ago.

In February 2016, experimenters with the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of huge instruments in Louisiana and Washington, reported the first observation of fleeting gravitational ripples, which had emanated from two black holes, each about 30 times as massive as the Sun, spiraling into each other 1.3 billion light-years away. LIGO even sensed the "ring down": the shudder of the bigger black hole produced by the merger. Teukolsky's old thesis was suddenly cutting-edge physics.

"The thought that anything I did would ever have implications for anything measurable in my lifetime was so far-fetched that the last 5 years have seemed like living in a dream world," Teukolsky says. "I have to pinch myself, it doesn't feel real."

Fantastical though it may seem, scientists can now study black holes as real objects. Gravitational wave detectors have spotted four dozen black hole mergers since LIGO's breakthrough detection. In April 2019, an international collaboration called the Event Horizon Telescope (EHT) produced the first image of a black hole. By training radio telescopes around the globe on the supermassive black hole in the heart of the nearby galaxy Messier 87 (M87), EHT imaged a fiery ring of hot gas surrounding the black hole's inky "shadow." Meanwhile, astronomers are tracking stars that zip close to the black hole in the center of our own Galaxy, following paths that may hold clues to the nature of the black hole itself.

The observations are already challenging astrophysicists' assumptions about how black holes form and influence their surroundings. The smaller black holes detected by LIGO and, now, the European gravitational wave detector Virgo in Italy have proved heavier and more varied than expected, straining astrophysicists' understanding of the massive stars from which they presumably form. And the environment around the supermassive black hole in our Galaxy appears surprisingly fertile, teeming with young stars not expected to form in such a maelstrom. But some scientists feel the pull of a more fundamental question: Are they really seeing the black holes predicted by Einstein's theory?

Some theorists say the answer is most likely a ho-hum yes. "I don't think we're going to learn anything more about general relativity or the theory of black holes from any of this," says Robert Wald, a gravitational theorist at the University of Chicago. Others aren't so sure. "Are black holes strictly the same as you would expect with general relativity or are they different?" asks Clifford Will, a gravitational theorist at the University of Florida. "That's going to be a major thrust of future observations." Any anomalies would require a rethink of Einstein's theory, which physicists suspect is not the final word on gravity, as it doesn't jibe with the other cornerstone of modern physics, quantum mechanics.

Even though it's very unlikely, it would be so amazingly important if we found that there was any deviation [from general relativity].

SEAN CARROLL

CALIFORNIA INSTITUTE OF TECHNOLOGY

Using multiple techniques, researchers are already gaining different, complementary views of these strange objects, says Andrea Ghez, an astrophysicist at the University of California, Los Angeles, who shared the 2020 Nobel Prize in Physics for inferring the existence of the supermassive black hole in the heart of our Galaxy. "We're still a long way from putting a complete picture together," she says, "but we're certainly getting more of the puzzle pieces in place."

CONSISTING OF PURE gravitational energy, a black hole is a ball of contradictions. It contains no matter, but, like a bowling ball, possesses mass and can spin. It has no surface, but has a size. It behaves like an imposing, weighty object, but is really just a peculiar region of space.

Or so says general relativity, which Einstein published in 1915. Two centuries earlier, Isaac Newton had posited that gravity is a force that somehow reaches through space to attract massive objects to one another. Einstein went deeper and argued that gravity arises because massive things such as stars and planets warp space and time—more accurately, spacetime—causing the trajectories of freely falling objects to curve into, say, the parabolic arc of a thrown ball.

Early predictions of general relativity differed only slightly from those of Newton's theory. Whereas Newton predicted that a planet should orbit its star in an ellipse, general relativity predicts that the orientation of the ellipse should advance slightly, or precess, with each orbit. In the first triumph of the theory, Einstein showed it accounted for the previously unexplained precession of the orbit of the planet Mercury. Only years later did physicists realize the theory also implied something far more radical.

In 1939, theorist J. Robert Oppenheimer and colleagues calculated that when a sufficiently massive star burned out, no known force could stop its core from collapsing to an infinitesimal point, leaving behind its gravitational field as a permanent pit in spacetime. Within a certain distance of the point, gravity would be so strong that not even light could escape. Anything closer would be cut off from the rest of the universe, David Finkelstein, a theorist at Caltech, argued in 1958. This "event horizon" isn't a physical surface. An astronaut falling past it would notice nothing special. Nevertheless, reasoned Finkelstein, who died just days before LIGO's announcement in 2016, the horizon would act like a one-way membrane, letting things fall in, but preventing anything from getting out.

According to general relativity, these objects—eventually named black holes by famed theorist John Archibald Wheeler—should also exhibit a shocking sameness. In 1963, Roy Kerr, a mathematician from New Zealand, worked out how a spinning black hole of a given mass would warp and twist spacetime. Others soon proved that, in general relativity, mass and spin are the only characteristics a black hole can have, implying that Kerr's mathematical formula, known as the Kerr metric, describes every black hole there is. Wheeler dubbed the result the no-hair theorem to emphasize that two black holes of the same mass and spin are as indistinguishable as bald pates. Wheeler himself was bald, Teukolsky notes, "so maybe it was bald pride."

Some physicists suspected black holes might not exist outside theorists' imaginations, says Sean Carroll, a theorist at Caltech. Skeptics argued that black holes might be an artifact of general relativity's subtle math, or that they might only form under unrealistic conditions, such as the collapse of a perfectly spherical star. However, in the late 1960s, Roger Penrose, a theorist at the University of Oxford, dispelled such doubts with rigorous math, for which he shared the 2020 Nobel Prize in Physics. "Penrose exactly proved that, no, no, even if you have a lumpy thing, as long as the density became high enough, it was going to collapse to a black hole," Carroll says.

Soon enough, astronomers began to see signs of actual black holes. They spotted tiny x-ray sources, such as Cygnus X-1, each in orbit around a star. Astrophysicists deduced that the x-rays came from gas flowing from the star and heating up as it fell onto the mysterious object. The temperature of the gas and the details of the orbit implied the x-ray source was too massive and too small to be anything but a black hole. Similar reasoning suggested quasars, distant galaxies spewing radiation, are powered by supermassive black holes in their centers.

But no one could be sure those black holes actually are what theorists had pictured, notes Feryal Özel, an astrophysicist at the University of Arizona (UA). For example, "Very little that we have done so far establishes the presence of an event horizon," she says. "That is an open question."

Now, with multiple ways to peer at black holes, scientists can start to test their understanding and look for surprises that could revolutionize physics. "Even though it's very unlikely, it would be so amazingly important if we found that there was any deviation" from the predictions of general relativity, Carroll says. "It's a very high-risk, high-reward question."

SCIENTISTS HOPE TO ANSWER three specific questions: Do the observed black holes really have event horizons? Are they as featureless as the no-hair theorem says? And do they distort spacetime exactly as the Kerr metric predicts?

Perhaps the simplest tool for answering them is one that Ghez developed. Since 1995, she and colleagues have used the 10-meter Keck telescope in Hawaii to track stars around a radio source known as Sagittarius A* (Sgr A*) in the center of our Galaxy. In 1998, the stars' high speeds revealed they orbit an object 4 million times as massive as the Sun. Because Sgr A* packs so much mass into such a small volume, general relativity predicts it must be a supermassive black hole. Reinhard Genzel, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics, independently tracked the stars to reach the same conclusion and shared the Nobel Prize with Ghez.

Graphic illustrating methods of observing black holes.

C. BICKEL/SCIENCE

Much of the information comes from a single star, dubbed SO2 by Ghez, which whips around Sgr A* once every 16 years. Just as the orbit of Mercury around the Sun precesses, so, too, should the orbit of SO2. Ghez and colleagues are now trying to tease out that precession from the extremely complicated data. "We're right on the cusp," she says. "We have a signal, but we're still trying to convince ourselves that it's real." (In April 2020, Genzel and colleagues claimed to have seen the precession.)

If they get a little lucky, Ghez and company hope to look for other anomalies that would probe the nature of the supermassive black hole. Close to the black hole, its spin should modify the precession of a star's orbit in a way that's predictable from Kerr's mathematical description. "If there were stars even closer than the ones they've seen—maybe 10 times closer—then you could test whether the Kerr metric is exactly correct," Will says.

The star tracking will likely never probe very close to the event horizon of Sgr A*, which could fit within the orbit of Mercury. But EHT, which combines data from 11 radio telescopes or arrays around the world to form, essentially, one big telescope, has offered a closer look at a different supermassive black hole, the 6.5-billion-solar-mass beast in M87.

The famous image the team released 2 years ago, which resembles a fiery circus hoop, is more complicated than it looks. The bright ring emanates from hot gas, but the dark center is not the black hole itself. Rather it is a "shadow" cast by the black hole as its gravity distorts or "lenses" the light from the gas in front of it. The edge of shadow marks not the event horizon, but rather a distance about 50% farther out where spacetime is distorted just enough so that passing light circles the black hole, neither escaping nor falling into the maw.

A black hole.

Gravity from a supermassive black hole casts a dark circular “shadow” in the glow from surrounding gas in the iconic image captured by the Event Horizon Telescope collaboration.

EVENT HORIZON TELESCOPE COLLABORATION ET AL.

Even so, the image holds clues about the object at its center. The spectrum of the glowing ring could reveal, for example, whether the object has a physical surface rather than an event horizon. Matter crashing onto a surface would shine even brighter than stuff sliding into a black hole, Özel explains. (So far researchers have seen no spectral distortion.) The shadow's shape can also test the classical picture of a black hole. A spinning black hole's event horizon should bulge at the equator. However, other effects in general relativity should counteract that effect on the shadow. "Because of a very funky cancellation of squishing in different directions, the shadow still looks circular," Özel says. "That's why the shape of the shadow becomes a direct test of the no-hair theorem."

Some researchers doubt EHT can image the black hole with enough precision for such tests. Samuel Gralla, a theorist at UA, questions whether EHT is even seeing a black hole shadow or merely viewing the disk of gas swirling around the black hole from the top down, in which case the dark spot is simply the eye of that astrophysical hurricane. But Özel says that even with limited resolution, EHT can contribute significantly to testing general relativity in the conceptual terra incognita around a black hole.

Gravitational waves, in contrast, convey information straight from the black holes themselves. Churned out when black holes spiral together at half the speed of light, these ripples in spacetime pass unimpeded through ordinary matter. LIGO and Virgo have now detected mergers of black holes with masses ranging from three to 86 solar masses.

The mergers can probe the black holes in several ways, says Frank Ohme, a gravitational theorist and LIGO member at the Max Planck Institute for Gravitational Physics. Assuming the objects are classical black holes, researchers can calculate from general relativity how the chirplike gravitational wave signal from a merger should speed up, climax in a spike, and then ring down. If the massive partners are actually larger material objects, then as they draw close they should distort each other, altering the peak of the signal. So far, researchers see no alterations, Ohme says.

The merger produces a perturbed black hole just like the one in Teukolsky's old thesis, offering another test of general relativity. The final black hole undulates briefly but powerfully, at one main frequency and multiple shorter lived overtones. According to the no-hair theorem, those frequencies and lifetimes only depend on the final black hole's mass and spin. "If you analyze each mode individually, they all have to point to the same black hole mass and spin or something's wrong," Ohme says.

In September 2019, Teukolsky and colleagues teased out the main vibration and a single overtone from a particularly loud merger. If experimenters can improve the sensitivity of their detectors, Ohme says, they might be able to spot two or three overtones—enough to start to test the no-hair theorem.

FUTURE INSTRUMENTS MAY make such tests much easier. The 30-meter optical telescopes being built in Chile and Hawaii should scrutinize the neighborhood of Sgr A* with a resolution roughly 80 times better than current instruments, Ghez says, possibly spying closer stars. Similarly, EHT researchers are adding more radio dishes to their network, which should enable them to image the black hole in M87 more precisely. They're also trying to image Sgr A*.

Meanwhile, gravitational wave researchers are already planning the next generation of more sensitive detectors, including the Laser Interferometer Space Antenna (LISA), made up of three satellites flying in formation millions of kilometers apart. To be launched in the 2030s, LISA would be so sensitive that it could spot an ordinary stellar-mass black hole spiraling into a much bigger supermassive black hole in a distant galaxy, says Nicolas Yunes, a theoretical physicist at the University of Illinois, Urbana-Champaign.

The smaller black hole would serve as a precise probe of the spacetime around the bigger black hole, revealing whether it warps and twists exactly as the Kerr metric dictates. An affirmative result would cement the case that black holes are what general relativity predicts, Yunes says. "But you have to wait for LISA."

In the meantime, the sudden observability of black holes has changed the lives of gravitational physicists. Once the domain of thought experiments and elegant but abstract calculations like Teukolsky's, general relativity and black holes are suddenly the hottest things in fundamental physics, with experts in general relativity feeding vital input to billion-dollar experiments. "I felt this transition very literally myself," Ohme says. "It was really a small niche community, and with the detection of gravitational waves that all changed."

RELATED STORY

Alternatives to black holes are scarce and strange

BY ADRIAN CHO

At one time, many physicists resisted the notion of a black hole, a ghostly, self-sustaining gravitational field so intense that not even light can escape. Now, theorists have few alternatives to these creepy holes in the universe.

Any alternative to a black hole must be some dark, dense material orb that’s slightly bigger than a black hole of the same mass. (Were the thing smaller, its own gravity would create a black hole around it.) Observers might detect the orb’s surface by spotting matter crashing onto it and heating up. Or they might deduce the greater size of the object as it swirls around a companion and tears into it before merging.

One hypothetical possibility is a gravastar: a thin material shell filled with dark energy, the mysterious space-stretching stuff that appears to be accelerating the expansion of the universe. A gravastar would have a solid surface rather than an event horizon, the point of no return that is the defining feature of a black hole. It satisfies the mathematical requirements of Albert Einstein’s theory of gravity, general relativity, says Carlos Palenzuela Luque, a gravitational theorist at the University of the Balearic Islands. However, nobody knows how a gravastar could ever form, he says.

Easier to produce might be a boson star, which would consist of exotic, massive Higgs bosons or nearly massless hypothetical particles called axions. Higgs bosons or axions could crowd into a single quantum wave, yielding a density approaching that of a black hole. However, Palenzuela Luque notes, boson stars would likely have masses in a narrow range, whereas a black hole can have any mass. Observed masses vary by factors of billions.

Black hole alternatives “is an area that’s maybe undertheorized,”says Sean Carroll, a theorist at the California Institute of Technology. But that would surely change if observations showed the current description of black holes isn’t quite right. “When we have something to guide our search, we will be able to say something new,” Palenzuela Luque says. “Right now, everything is so speculative.”