Two of the three winners of the Nobel Prize in Physics helped discover supermassive black holes that contained millions or even billions of solar masses.
ESO / ESA / Hubble / M. Kornmesser
By Adrian Cho, Daniel CleryOct. 6, 2020, 1:15 p.m.
This year’s Nobel Prize in Physics recognizes groundbreaking studies into the nature of black holes, including the discovery of the gigantic one that lurks in the heart of our Milky Way Galaxy.
Half of the award goes to Roger Penrose, a mathematician at Oxford University, for his work in the 1960s on the formation and stability of black holes. The other half is shared by two astronomers: Reinhard Genzel from the Max Planck Institute for Extraterrestrial Physics and Andrea Ghez from the University of California in Los Angeles. Since the 1990s, they have led competing research groups that tracked stars in the center of the Milky Way and showed that their orbits were bent by what is known as a supermassive black hole (SMBH).
The concept of a black hole – an object so massive that its gravity prevents light from escaping – has emerged in pieces over the decades. Albert Einstein published his theory of gravity, the general theory of relativity, in 1915. It says that gravity arises when mass and energy distort the structure of space and time and bend the trajectories of freely falling objects such as the earth’s elliptical orbit around the sun. Just a year later, German physicist Karl Schwarzschild worked out the shape of the pit in spacetime that would create a point mass and showed that it predicts an event horizon. This marks the edge of a sphere around the point mass, from which light can still escape.
However, the whole idea that burned-out stars could actually lead to these bizarre cavities in space didn’t really catch on until 1939. Then physicists J. Robert Oppenheimer and George Volkoff calculated that an overly massive neutron star should collapse its own weight to an infinitesimal point, leaving only its ultra-intense gravitational field. Their work heralded the astrophysicists’ current understanding of stellar mass black holes that arise when sufficiently massive stars burn out and their nuclei collapse.
Oppenheimer and his colleagues did not prove that the imploding star must form an event horizon. It was conceivable that the matter could somehow swirl away – or that the dead star’s gravitational field could not get stuck. In the 1960s, Penrose showed with the utmost mathematical accuracy that the formation of a black hole was essentially inevitable and that if it gobbled up more mass it would be indestructible and grow. “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 just grow.”
Will suggests that the award could be seen as a kind of award for Stephen Hawking, who passed away in 2018 and worked with Penrose. In fact, Penrose’s most important predictions are summarized in what are known as Hawking Penrose’s theorems. Penrose notes that Hawking applied his ideas about the formation of horizons around black holes to cosmology and the birth of the universe. “There was clearly progress in terms of what I had done,” says Penrose.
In short, Penrose showed general relativity and implied that the 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 when we look for them.'”
Roger Penrose (left) has proven that black holes are real objects. Andrea Ghez (center) and Reinhard Genzel (right) showed that someone who is 4 million times as heavy as the sun lurks in the heart of our galaxy.
(from left to right): TOMMASO BONAVENTURA / CONTRASTO / Redux; Christopher Dibble / UCLA / Sipa USA / Newscom; Matthias Balk / Image Alliance / dpa / AP Images
Since Penrose’s advances, astronomers have found a wealth of evidence of black holes. They found stars orbiting invisible companions, and they could see superheated gases glowing hot as they disappeared into suspected black holes. Gravitational wave detectors provided the clincher for such star-sized black holes, but not for 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 it’s not only black, it’s also quite small: its event horizon would fit in Mercury’s orbit. In addition, the galactic center is protected from gas and dust from curious telescopes.
When the sparring teams of Ghez and Genzel were pushing the observation techniques to their limits, they carried out a very simple study: they mapped the progress of a single star orbiting close to Sgr A * and showed, using simple Newtonian mechanics, that the object they were orbits must have a colossal mass. “With high school physics, you can understand broadly that there has to be something super massive there that we can’t see,” says Selma de Mink, a theoretical astrophysicist at Harvard University.
Their studies were made possible by infrared detectors. Wavelengths of around 2 micrometers turned out to be the sweet spot: These infrared photons could penetrate the haze and were not disturbed by turbulence in the earth’s atmosphere. The infrared wavelengths were also small enough to locate stars with relative accuracy.
In the 1990s, Genzel and Ghez’s groups clung to a single star, referred to by the two teams as S2 or S0-2, which is closest to the galactic center previously discovered. “Over the years Andrea and Reinhard had a legendary competition that kept the field moving,” says astrophysicist Heino Falcke from Radboud University. To get an accurate solution for S2, the teams needed the largest telescopes available: the four 8-meter telescopes of the European Very Large Telescope in Genzel’s case and the two 10-meter Keck telescopes for Ghez.
In 2002, S2’s elliptical orbit appeared to be reaching its closest point to Sgr A *. It came within 20 billion kilometers, or 17 light hours, and moved at 5,000 kilometers per second, 3% the speed of light. The teams then had enough orbit to draw conclusions about the invisible object. They calculated that it weighs the equivalent of 4 million suns and must be a concentrated object: it could just be a black hole. “You have proven by observation what Penrose had predicted with the theory that there actually are black holes,” says Gerry Gilmore of the University of Cambridge.
Teams have continued to track S2 through its first full orbit in 2008 and its second narrow approach in 2018. They used this data to put the general theory of relativity to increasingly stringent tests. “They laid the foundation for supermassive black holes,” says Falcke.
As good as the S2 results were, the researchers want even more direct evidence of the existence of SMBHs. And in 2019 the Event Horizon Telescope (EHT) managed to reveal the shadow of an even larger monster at the center of M87, one of the Milky Way’s neighboring galaxies. This black hole contains billions of solar masses. The EHT collaboration has tried to get an idea of Sgr A * but has so far been thwarted by the presentation of conclusive results.
Ghez is only the fourth woman to ever win a Nobel Prize in Physics and the second in the past three years. “That means a lot to me,” says de Mink. In recent years, the Nobel Prize for Science has been criticized for its lack of diversity.
At 55, Ghez is also a relatively young award winner. Penrose, 89, is one of the oldest. But Penrose says he doesn’t regret waiting so long for the award. “I know some people who got a Nobel Prize too early and that ruined their science,” he says. “I think I’m about old enough.”
Science related content
Meyer et al., “The Shortest Known Period Star Orbiting the Supermassive Black Hole of our Galaxy,” Science 3386103 (October 5, 2012)
Tacchella et al., “Evidence of Mature Bulges and Inside-Out Quenching 3 Billion Years After the Big Bang,” Science 3486232 (April 17, 2015)
Do et al., “Relativistic Redshift of Star S0-2 Orbiting the Galactic Center’s Supermassive Black Hole,” Science 3656454 (August 16, 2019)
Related content from science news
A. Hammond, “Stellar Age: White Dwarfs, Neutron Stars, and Black Holes,” Science (March 12, 1971)
A. Hammond, “Stellar Age (III): Black Holes and Gravitational Collapse,” Science (March 26, 1971)
W. Metz, “Astrophysics: The Discovery and Omnipresence of Black Holes,” Science (January 21, 1977)
J. Glanz, “The Gathering Darkness,” Science (Jan. 24, 1997)
E. Stokstad, “Into the Animal’s Den”, Science (Jan. 7, 2000)
R. Irion, “The Dark, Starving Pit of the Milky Way,” Science (May 30, 2003)
J. Kling, “Changing Faces of Astronomy”, Science (January 20, 2006)
R. Irion, “A Surprising Stellar Nursery,” Science (June 2, 2006)
R. Zelkowitz, “Genius of the Year”, Science (September 23, 2008)
K. Croswell, “Warning: Black Hole Dead Ahead!” science
D. Clery, “Astronomers Identify Mysterious Objects in the Heart of the Milky Way,” Science (04 Nov 2014)
D. Clery, “The dark lab”, Science (March 6, 2015)
D. Clery, “Star’s encounter with the black hole tests Einstein’s theory of gravity,” Science (July 26, 2018)
Tuthill et al., “Brevia: Wind Turbines in the Quintuplet Cluster,” Science 3135789 (August 18, 2006)