Nobel laureates sketch

Roger Penrose, Reinhard Genzel and Andrea Ghez have been awarded the 2020 Nobel Prize in physics, for their work on the theory and observation of black holes. [Image: ©Nobel Media. III. Niklas Elmehed]

The 2020 Nobel Prize in Physics has been awarded to Roger Penrose, Oxford University, U.K., for his mathematical proof that black holes are an inevitable consequence of general relativity; and to Reinhard Genzel, Max Planck Institute for Extraterrestrial Physics, Germany, and Andrea Ghez, University of California, Los Angeles, USA, for their discovery of the black hole at the center of the Milky Way galaxy. Penrose will receive half of the prize of 10 million Swedish kronor (more than US$1.1 million); Genzel and Ghez will share the other half.

The accomplishments of the observational teams led by Genzel and Ghez were enabled by infrared telescopes—and, in particular, by the emergence of adaptive optics. The latter technique dramatically sped up the process of making the intricate observations of stellar orbits needed to infer the presence of the supermassive black hole at the galactic center.

Piercing interstellar clouds

After the classic papers by Penrose in the mid-1960s that led to his award of half of this year’s Nobel Prize, scientists began to ponder the role of supermassive black holes in galactic nuclei as a potential answer to a number of astrophysical mysteries. But while such objects had been inferred theoretically, no observations had been made, as the telescopes of the time lacked sufficient angular resolution. The only way to observe such an object was indirectly—through inferences on mass density drawn from the motion of stars near the galactic center.

Such measurements could be made only in the near-infrared, as clouds of interstellar gas obscure observations at optical wavelengths. And the need to track the motions of stars over long periods meant that the observations had to be undertaken using Earth-based telescopes.

In the 1990s, teams led by Genzel, using telescopes in Chile operated by the European Southern Observatory, and Ghez, using the Keck Telescope in Hawaii, began employing improved optical instruments and techniques to stare at the galactic center over long periods and tease out the required observations.

Sharpening the picture

One big issue for these Earth-based scopes was the one that has dogged astronomers since the days of Galileo: the obscuring effect of atmospheric turbulence. As a first approach to resolving that problem, the teams led by both Genzel and Ghez developed a technique called speckle imaging. The technique involved taking numerous short exposures of the target star with high sensitivity, and then stacking the data to sharpen the image.

The result was an impressive increase in the sharpness of the observations by the two teams. That was important, as the teams needed to track stellar motions at a relatively fine scale (at least in astronomical terms) to make the requisite calculations. But the brief exposure times of speckle imaging meant that it could be used to sharpen up only the brightest stars in the galactic center. And the technique was slow, requiring surveys stretching over years to extract velocity information for only a handful of stars.

An adaptive-optics speedup

The advent of adaptive optics at the end of the 1990s changed the game and allowed both research teams to speed things up considerably.

Adaptive optics works by first taking an observation of a “guidestar”—either a nearby natural star, or an artificially created point source made by exciting sodium atoms in the upper atmosphere with a powerful laser. Then, the known position and brightness of that point source is used to calculate the effect of atmospheric turbulence at that instant. Using that information, the wavefront of the light from the astronomical target actually being observed can in turn be reshaped in real time, via equipment such as rapidly deformable mirrors, to compensate for those atmospheric distortions.

laser at observatory

Some adaptive-optics systems use lasers to excite sodium atoms in the upper atmosphere. The emissions from those excited atoms form a “guidestar”—an artifical point source of light that can be used to computationally back out the effect of turbulence in Earth’s atmosphere. [Image: Getty Images]

While the idea of adaptive optics had been proposed in the 1950s, it wasn’t until the late 1990s that such systems started to become available on large ground-based telescopes. These included the Keck Telescope, where Ghez’s team was working, and ESO’s Very Large Telescope (VLT), one site of the Genzel team’s effort.

A key advantage of this new image-sharpening technique was that it permitted long exposures, expanding the number of stars that could be observed and the imaging depth that was possible. It also opened up the prospect of using sensitive spectroscopes that could probe (through the Doppler effect) the radial velocities of the stars, limning out a more complete picture of their motion. And it allowed monitoring of stellar motion over a much shorter timescale than was possible with speckle imaging.

“Still important”

These optical advances allowed both teams to image and analyze a crucial short-orbital-period star near the galactic center. And the data from the two teams’ observations showed excellent agreement, effectively nailing the case that the object at the galactic center was indeed a supermassive black hole.

Since then, the exploits of these bizarre dark objects have made many a scientific headline. Some high points have included the collisions of black holes now detected almost routinely by the LIGO and Virgo gravitational-wave observatories, and the stunning first “picture” of a black hole by the Event Horizon Telescope, released in 2019.

At the press conference announcing this year’s Nobel physics prize, co-laureate Andrea Ghez, reached by phone, noted in response to a question that the prize for her groundbreaking work underscored the importance of science, at a time when some have sensed a more than a whiff of anti-science sentiment in the zeitgeist.

“Science is still important, and pursuing the reality of our physical world is critical to human beings,” Ghez said. “I think today I feel more passionate about the teaching side of my job … It’s so important to teach the younger generation that their ability to question, and to think, is crucial to the future of the world.”