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Nobel Prize in Physics honors research on black holes

6 October 2020

Roger Penrose’s theoretical work demonstrated the objects could form. Reinhard Genzel and Andrea Ghez independently discovered a supermassive one at the center of the Milky Way.

Roger Penrose, Reinhard Genzel, and Andrea Ghez
From left: Roger Penrose, Reinhard Genzel, and Andrea Ghez. Credits, from left: Cirone-Musi, Festival della Scienza, CC BY-SA 2.0; Max Planck Institute for Extraterrestrial Physics; Christopher Dibble

Roger Penrose, Reinhard Genzel, and Andrea Ghez are to be awarded the 2020 Nobel Prize in Physics for their theoretical and observational work on black holes, the Royal Swedish Academy of Sciences announced on Tuesday. Penrose will receive half the 10 million Swedish krona (roughly $1.1 million) prize; Ghez and Genzel will share the other half.

Penrose, of the University of Oxford, helped place the previously idealized concept of a black hole on sound theoretical footing in the 1960s by applying topology to general relativity and thus connecting the collapse of matter to the formation of a trapped surface and an inevitable singularity.

Several decades later, Genzel (Max Planck Institute for Extraterrestrial Physics and the University of California, Berkeley) and Ghez (UCLA) each led a team that advanced the techniques of speckle imaging and adaptive optics to obviate atmospheric turbulence and analyze the motion of stars tightly orbiting Sagittarius A*, the radio source at the Milky Way’s center. The researchers concluded that only a black hole, weighing in at about 4 million solar masses, could be responsible for the orbits they observed.

Ghez is the fourth woman to be named a Nobel physics laureate, after Donna Strickland (2018), Maria Goeppert Mayer (1963), and Marie Curie (1903).

A mathematical perspective

By 1964, when Penrose started thinking about black holes, the problem was well established. A mathematical oddity appeared in Karl Schwarzschild’s 1916 solution to Albert Einstein’s field equations for the curved spacetime around a mass of radius r: Some terms of the Schwarzschild solution vanish or diverge for r = 0 and r = 2GM/c2.

Twenty years later, J. Robert Oppenheimer tried to make sense of that observation by studying the collapse of a spherical cloud of matter down to a single point. He and his student Hartland Snyder were the first to realize that the second notable r value was the radius within which starlight, retarded by gravity, would no longer reach outside observers: the event horizon. But Oppenheimer’s assumption of spherical symmetry aroused skepticism about whether such a circumstance would occur in real life.

Penrose became interested in gravitational singularities early in his career after he attended a lecture by David Finkelstein on the Schwarzschild solution. Penrose’s background was in mathematics, and he had never taken a formal physics course. But he devoted himself to learning physics on his own and through discussions with astrophysicist Dennis Sciama and others.

In 1964 he devised a topological picture of gravitational collapse without assuming spherical symmetry. Doing so required some new mathematical methods, including the notion of a trapped surface, in which all light orthogonal to the two-dimensional surface converges. He found that inside the event horizon, the radial direction becomes time-like, reversing out of the black hole becomes impossible, and all matter ends up at the singularity. He also identified a point of no return—the formation of a trapped surface—after which the collapse into a black hole is inevitable.

Diagram in Penrose's 1965 paper.
Roger Penrose included this spacetime diagram in his pivotal 1965 paper. Credit: R. Penrose, Phys. Rev. Lett. 14, 57 (1965)

Penrose encapsulated all that information graphically using a technique he introduced based on conformal transformations, or Penrose diagrams. In those diagrams, time is one axis, and space is the other axis (or axes). The scales can be adjusted, even to the extent that infinite values are plotted, as long as the angle stays the same. In that framework, light travels along a line at 45 degrees.

After Penrose’s analysis, black holes became the prevailing explanation for quasars, the point-like extragalactic objects whose luminosity and variability had puzzled researchers. His research helped turn the opinion on black holes from unlikely-to-occur theoretical entities to a plausible explanation of quasars, blazars, and other active galactic nuclei. His subsequent research elucidated black holes’ structure and rotational energy.

“I think this prize is long, long overdue,” says theoretical physicist Lee Smolin, for Penrose and for general relativity. (Cosmologist Sean Carroll tweeted, “Even Einstein won for quantized light, not general relativity.”) Smolin emphasizes the influence Penrose has had on the mathematics that physicists use in general relativity, including the introduction of spinors and tensors to track curvature.

“It was important for me always,” Penrose said in a 1989 interview, “if I wanted to work on a problem, to think I had a different angle on it from other people.”

Looking locally

Penrose’s insights had provided an explanation for quasars. But those luminous objects also brought up a question that hit closer to home: If many galaxies have central black holes, then what about the Milky Way? Although it was clear that the galactic center isn’t exactly spewing radiation like a quasar, it does broadcast x-ray and radio signals that are consistent with the presence of a supermassive black hole.

As a postdoc at Berkeley in the 1980s, Genzel worked with 1964 Nobel laureate Charles Townes to use IR spectroscopy to track gas clouds orbiting 26 000 light-years away near the galactic center. Though their finding of steeply rising velocities with decreasing distance to the center suggested a massive, compact source of gravitation, the evidence wasn’t definitive, as the gas clouds could conceivably be influenced by forces besides gravity. The clincher would be to resolve the motion of stars—which are essentially point sources—located so close to the center of mass that no other known object aside from a black hole could be responsible.

Both Genzel and Ghez set out in the mid 1990s to make such observations. The task required large ground-based telescopes operating in the near-IR, an optimal wavelength range for detecting photons that can escape the dust-filled galactic center. Genzel and colleagues began observing with the 3.6 m New Technology Telescope in Chile in 1992; Ghez and her team started three years later with a 10 m telescope at Hawaii’s Keck Observatory.

Large aperture alone wasn’t enough to spatially resolve individual stars in the galaxy’s crammed core. Both laureates independently developed new methods of speckle imaging, a technique that corrects for the distortions caused by Earth’s roiling atmosphere by stacking a series of rapid exposures in a way that brings the smeared light of individual stars into crisp alignment—enough to pinpoint their gradually changing locations. (See the Quick Study by Steve Howell and Elliott Horch, Physics Today, November 2018, page 78.) Previously, the technique had been limited largely to optical observing. In 1997, capitalizing on their speckle imaging–enhanced observations, both groups released measurements of the proper motion of stars at the galactic center that strongly favored the black hole explanation (see Physics Today, March 1998, page 21).

Although speckle imaging was crucial for their initial success, the addition of adaptive optics to their repertoire—Ghez at Keck and Genzel at the 8 m Very Large Telescope in Chile—enabled the precision observations that put the existence of the supermassive black hole beyond reasonable doubt. The technique entails using a bright reference object such as a laser-projected guide star to measure the distortion of light due to Earth’s atmosphere and then, in real time, adjust a deformable mirror to counteract that distortion.

Adaptive optics at the Very Large Telescope.
Adaptive optics at ESO’s Paranal Observatory enables an observation of the galactic center. Credit: ESO/Y. Beletsky

By applying adaptive optics, both groups not only improved spatial resolution, but they also conducted spectroscopic analyses to derive stellar velocities in three dimensions and chart precise orbits. In 2002 the researchers reported measurements of stars moving too quickly and tightly to be orbiting anything other than a black hole (see Physics Today, February 2003, page 19). “They worked independently with different instruments, different processes, and different systematics, but they always got consistent results,” says Alessia Gualandris, an astrophysicist at the University of Surrey, UK.

The standout object in the observations, dubbed S0-2 by Ghez’s group and S2 by Genzel’s, is a bright star that approaches within about 17 light-hours of Sagittarius A* every 16 years in a highly elliptical orbit. Aside from solidifying the black hole case, S0-2 and its imperiled stellar neighbors have stimulated other astrophysical research. Spectroscopic analyses indicate S0-2 is a hot young star, which has left theorists wondering how such a star could form so close to a supermassive black hole and its disruptive tidal forces. Genzel and Ghez’s work provides an essential case study for researchers trying to understand how a galaxy’s supermassive black hole regulates its evolution and star formation, says astrophysicist Rosemary Wyse of Johns Hopkins University.

Animation of stellar motion about Sagittarius A*.
Stars orbit Sagittarius A* (denoted by a star) in this animation. Credit: Andrea Ghez and her research team at UCLA; data sets obtained with the W. M. Keck telescopes

Having made an ironclad case for the Milky Way’s black hole, both laureates continue to zoom in closer toward Sagittarius A*’s event horizon. Earlier this year Genzel and the GRAVITY collaboration used the combined data from four telescopes to measure the precession of S2’s orbit about the black hole, a rare exploration of general relativity in the strong-field regime.

Genzel and Ghez could get additional validation from the Event Horizon Telescope collaboration, which is exploiting the resolving power of a worldwide network of radio telescopes to image the silhouettes of two nearby supermassive black holes. One is M87*, whose initial portrait was released last year. The researchers are currently processing observations of the other target: Sagittarius A*.

Editor’s note: This post was updated from an earlier summary.

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