General relativity makes very specific predictions about what black holes are and how they should appear, as shown in this simulation.

GODDARD SPACE FLIGHT CENTER/JEREMY SCHNITTMAN

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?

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

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.

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.

In a simulation, seen from various angles, a black hole’s intense gravity warps the image of the disk of hot, glowing gas surrounding it.


GODDARD SPACE FLIGHT CENTER/JEREMY SCHNITTMAN

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.

How to probe a black holeAlbert Einstein’s theory of gravity, general relativity, predicts that the collapse of enough mass can leave a self-sustaining gravitational field so strong that, inside a distance called the event horizon, nothing can escape, not even light. But are black holes exactly the inscrutable things general relativity predicts? Observers may now have the tools to find out.3. Catch the wavesWhen two small black holes spiral together, they radiate gravitational waves, which could reveal whether the supposed black holes are instead material objects. The final black hole reverberates at frequencies and overtones that provide another test of whether its only properties are mass and spin.Supermassiveblack holePrecessing orbitCloselyorbitingstarDistorted imageof glowing gasShadow ofblack hole2. Take a pictureAn image of a supermassive black hole holds clues to whether, as general relativity predicts, it has an event horizon rather than a surface, and mass and spin are its sole properties.1. Trace the starsTracking the orbits of stars around the black hole in our Galaxy’s center can reveal whether the black hole warps space and time exactly as general relativity predicts.Inspiraling black holesMergerRing downStretching of spaceTime

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.

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.”