The Nobel Prize in Physics this year honors pioneering studies about the nature of black holes, including the discovery of the gigantic one lurking in the heart of our Milky Way Galaxy.
Half of the prize goes to Roger Penrose, a mathematician at the University of Oxford, for his work in the 1960s on the formation and stability of black holes. The other half is shared by two astronomers: Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics and Andrea Ghez of the University of California, Los Angeles. Since the 1990s, they led rival research groups that tracked stars at the center of the Milky Way and showed that their orbits were bent by what’s known as a supermassive black hole (SMBH).
The concept of a black hole—an object so massive that its gravity prevents light from escaping—emerged in pieces over the course of decades. Albert Einstein published his theory of gravity, the general theory of relativity, in 1915. It states that gravity arises when mass and energy warp the fabric of space and time, causing the trajectories of freely falling objects to curve like Earth’s elliptical orbit around the Sun. Only 1 year later, German physicist Karl Schwarzschild worked out the shape of the pit in spacetime that a point mass would create and showed that it predicts an event horizon. That marks the edge of a sphere around the point mass from which light can still escape.
However, the whole notion that burned out stars could actually lead to these bizarre voids in space didn’t arrive until 1939. That’s when physicists J. Robert Oppenheimer and George Volkoff calculated that, if a neutron star grew too massive, it should collapse under its own weight to an infinitesimal point, leaving behind only its ultraintense gravitational field. Their work foreshadowed astrophysicists’ current understanding of stellar-mass black holes, which form when sufficiently massive stars burn out and their cores collapse.
Oppenheimer and his colleagues did not prove the imploding star had to form an event horizon. It was conceivable that the matter could somehow swirl away—or that the dead star’s gravitational field might not stick around. In the 1960s, Penrose showed with extreme mathematical rigor that the formation of a black hole was essentially inevitable and that it would be indestructible, growing as it devoured more mass. “It didn’t matter what you did, the horizon was always there,” says Clifford Will, a general relativity expert at the University of Florida. “It wouldn’t break apart, it would only grow.”
Will suggests the award could be considered a prize of sorts for Stephen Hawking, who died in 2018 and with whom Penrose collaborated. In fact, Penrose’s key predictions are framed in the so-called Hawking Penrose theorems. Penrose notes that Hawking took his ideas regarding the formation of horizons around black holes and applied them to cosmology and the birth of the universe. “They were clearly advances on what I had done,” Penrose says.
In short, Penrose showed general relativity implied that black hole would be a real, stable astrophysical object, says Ulf Danielsson, a theoretical physicist at Uppsala University and a member of the Nobel physics committee. “Penrose laid a theoretical foundation so that we could say, ‘Yes, these objects exist, we can expect to find them if we go out and look for them.’”
Since Penrose’s advances, astronomers have found a wealth of evidence for black holes. They found stars orbiting invisible companions, and they could see superheated gases glowing hot as they disappeared into putative black holes. Gravitational wave detectors provided the clincher for such stellar size black holes, but not the galactic giants.
The one at the center of the Milky Way, known as Sagittarius A* (Sgr A*), weighs millions of solar masses and is only 26,000 light-years away. But in addition to being black, it is quite small: Its event horizon would fit within Mercury’s orbit. On top of that, the galactic center is cloaked from prying telescopes by gas and dust.
By pushing observing techniques to their limits, the sparring teams of Ghez and Genzel carried out a very simple study: They mapped the progress of a single star as it orbited close to Sgr A* and showed, via simple Newtonian mechanics, that the object they were orbiting had to have a colossal mass. “With high school physics, you can get a long way to understanding that there must be something supermassive there that we can’t see,” says Selma de Mink, a theoretical astrophysicist at Harvard University.
Their studies were enabled by infrared detectors. Wavelengths of about 2 micrometers proved to be a sweet spot: Those infrared photons could penetrate the haze and weren’t too disturbed by turbulence in Earth’s atmosphere. The infrared wavelengths were also small enough to locate stars relatively precisely.
In the 1990s, Genzel and Ghez’s groups both latched onto a single star, known as S2 or S0-2 by the two teams, which is the closest star to the galactic center yet detected. “Andrea and Reinhard have had a legendary competition over the years which has kept the field moving,” says astrophysicist Heino Falcke of Radboud University. To get an accurate fix on S2, the teams needed the largest telescopes available: the four 8-meter telescopes of Europe’s Very Large Telescope in Genzel’s case, and the twin 10-meter Keck telescopes for Ghez.
In 2002, S2’s elliptical orbit appeared to reach its closest point to Sgr A*. It came within 20 billion kilometers or 17 light-hours, and traveled at 5000 kilometers per second, 3% of the speed of light. The teams then had enough of an orbit to draw conclusions about the invisible object. They calculated it must weigh the equivalent of 4 million Suns and be a concentrated object: It could only be a black hole. “They proved through observation what Penrose had predicted with theory, that black holes actually do exist,” says Gerry Gilmore of the University of Cambridge.
The teams have continued to follow S2 through its first full orbit in 2008 and its second close approach in 2018. They have used those data to subject general relativity to ever more stringent tests. “They laid the foundations for supermassive black holes,” Falcke says.
As good as the S2 results were, researchers want even more direct evidence for the existence of SMBHs. And in 2019, the Event Horizon Telescope (EHT) succeeded in revealing the shadow of an even bigger monster at the center of M87, one of the Milky Way’s neighboring galaxies. That black hole holds billions of solar masses. The EHT collaboration has tried to image Sgr A* but so far has been thwarted in presenting conclusive results.
Ghez is just the fourth woman ever to win a Nobel Prize in Physics, and the second in the past 3 years. “That means a lot to me,” de Mink says. In recent years, the Nobel science prizes have been criticized for their lack of diversity.
At 55, Ghez is also a relatively young laureate. Penrose, 89, is among the oldest. But Penrose says he has no regrets about waiting so long to get the prize. “I know some people who got a Nobel too early, and it ruined their science,” he says. “I think I’m about old enough.”