NASA/CXC/Univ. of Wisconsin/Y.Bai, et al.

This year's Nobel laureates in physics were honored for their discoveries pertaining to black holes, and indicating that there is a black hole at the center of the Milky Way, called "Sagittarius A*."

October 12, 2020

2020 Nobel Prize in Physics Highlights Black Holes, Mentors and Collaboration

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When massive stars undergo gravitational collapse, they sometimes become black holes, with such extreme gravity they prevent even light from escaping. Three researchers investigating black holes — Sir Roger Penrose, Reinhard Genzel and Andrea Ghez — received the 2020 Nobel Prize in Physics on Oct. 6.

Half of the award went to Penrose, emeritus professor at the University of Oxford, whose mathematical proofs show that Albert Einstein’s theory of general relativity predicts the existence of black holes. The other half was awarded jointly to Ghez, the Lauren B. Leichtman and Arthur E. Levine Professor of Astrophysics at University of California, Los Angeles, and Genzel, director of the Max Planck Institute for Extraterrestrial Physics and emeritus professor at University of California, Berkeley.

In the 1990s, Ghez and Genzel’s separate research teams observed and analyzed star movements near the center of the Milky Way and found compelling evidence that a black hole was responsible for the stars’ movement patterns.

Engineering Innovation Makes Observing Deep Space Easier

One of the many obstacles facing scientists who are trying to see into deep space is the same phenomenon that makes stars seem to twinkle — turbulence in Earth’s atmosphere  that can smear stellar images.

To correct for this turbulence, Ghez and Genzel used adaptive optics, an observational technique that utilizes carefully calibrated mirrors to correct for image distortions. The mirrors bend the light to make it all come from the same place, which results in sharp images of specific stars.

“We look out of the atmosphere and the atmosphere blurs the images,” said Stefan Gillessen, one of Genzel’s colleagues at the Max Planck Institute in Germany. “The idea behind these adaptive optics systems is that one actually introduces a mirror in the beam telescope, which exactly compensates the blurring the atmosphere does.”

Through adaptive optics, Ghez and Genzel could track the orbits of stars more easily than they could have with past methods. One of the main methods previously used is speckle interferometry, which involves taking many pictures of individual flecks of light at high speeds and consolidating these images into a clear image, according to Prof. Gordon Stacey, astronomy.

The Researchers Found The Theoretical Underpinnings For and Evidence Of Black Holes

Before Penrose’s 1965 proof, black hole formation had only been considered under the unrealistic condition that the collapsing star was a perfect sphere. Penrose showed that black holes actually could form from non-spherical stars by introducing “trapped surfaces” where light can only move toward a central singularity — the infinitely dense point at the center of a black hole.

While his proof did not show that black holes are inevitable, the only conditions in which black holes never form are considered to be unrealistic. Many years after Penrose’s singularity theorem, Ghez and Genzel’s research teams found overwhelming evidence of the existence of a supermassive black hole at the center of the Milky Way.

From observatories in Hawaii and Chile, respectively, Ghez and Genzel found stars moving very quickly near a bright object they named Sagittarius A*. By focusing on one star, S2, which orbits near the galactic center, the two researchers found significant evidence that at the center of the Milky Way there is a supermassive black hole weighing more than 4 million suns.

Making an Impact Through Research and Teaching

Penrose, Ghez and Genzel have been not only groundbreaking researchers, but also teachers, mentors and role models for many in their field, including Cornell students and faculty. Penrose was a visiting professor at Cornell University in the early 1960s. Christopher O’Connor grad worked for Ghez’s lab the summer after his sophomore year at UCLA, and Stacey worked for Genzel as a postdoctoral researcher.

Stacey described Genzel as a brilliant and energetic researcher, with a good sense of humor. Stacey said he was grateful to have had a chance to work for Genzel early in his career, and has since co-authored papers with him, focusing on the Galactic Center and star formation both in the Milky Way galaxy, and in nearby galaxies.

“He’s generous to younger people he mentors,” Stacey said. “He will give them their recognition as it’s due.”

Ghez is the fourth woman to win the Nobel prize in Physics, and the first female astrophysicist to win. Thankful Cromartie, a post-doctoral researcher in astronomy, said she was both inspired by Ghez’s example and expressed mixed feelings about what it means for women winning the Nobel Prize in physics to still be so rare.

“Andrea Ghez is only the fourth woman to win the Nobel prize in Physics, and the first female astrophysicist to win it,” Cromartie said. “It’s inspiring and encouraging to see her work be properly recognized and rewarded but it’s also a testament to how overlooked and undervalued the work of female astrophysicists has been in the past.”

While the lack of gender equity in physics remains an issue, many, including Cromartie and O’Connor, are happy that Ghez’s accomplishments have been recognized. According to O’Connor, Ghez is an inclusive professor and researcher, encouraging her students and team members while managing the complex logistics of large scale research projects.

“When it was announced that Ghez had won the Nobel, everyone I know in astronomy was thrilled, but no one was surprised,” O’Connor said. “It’s very gratifying to see someone who is so brilliant and accomplished receive the recognition she deserves.”