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Physicists who unravelled mysteries of black holes win Nobel prize

Mathematical physicist Roger Penrose shares the prize with Andrea Ghez and Reinhard Genzel, who discovered a giant black hole in the centre of the Galaxy.

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Sir Roger Penrose, Andrea Ghez and Reinhard Genzel

Roger Penrose, Andrea Ghez and Reinhard Genzel (left to right) received the 2020 Nobel physics prize for their research on black holes.Credit: David Levenson/Getty, Christopher Dibble, ESO/M. Zamani

A mathematical physicist and two astronomers have won the 2020 Nobel Prize in Physics for discoveries relating to the most massive and mysterious objects in the Universe — black holes.

British mathematical physicist Roger Penrose, 89, receives half the prize for theoretical work that showed how Albert Einstein’s general theory of relativity should result in black holes, which have a gravitational pull so strong that even light cannot escape.

US astronomer Andrea Ghez, 55, and German astronomer Reinhard Genzel, 68, share the other half of the 10-million-kronor (US$1.1-million) award for their discovery of the Universe’s most famous black hole — the supermassive object at the centre of the Milky Way.

Since the 1990s, Ghez and Genzel have each led groups that have mapped the orbits of stars close to the Galactic Centre. These studies led them to conclude that an extremely massive, invisible object must be dictating the stars’ frantic movements. The object known as Sagittarius A* (Sgr A*) is the most convincing evidence yet of a supermassive black hole at the centre of the Milky Way, said the Royal Swedish Academy of Sciences, which awards the prize.

‘Giant of physics’

Astrophysicist Monica Colpi at the University of Milan Bicocca in Italy says the prizes were highly deserved. “The observational data by Genzel and Ghez are splendid and truly unique in their ability to monitor star motions around this object.” Their data proved that Sgr A* has a density consistent with being a supermassive black hole.

Astrophysicist Heino Falcke agrees. “They made fundamental contributions to establishing that these dark hearts of the galaxies exist,” says Falcke, who is at Radboud University in Nijmegen, the Netherlands.

Penrose, meanwhile, is “a giant in theoretical physics”, who has influenced generations of scientists, says Carole Mundell, an astrophysicist at the University of Bath, UK. He is “a genuinely creative thinker with immense imagination, sense of fun and a passion for curiosity in everything he does,” she adds.

Ghez, at the University of California, Los Angeles, is just the fourth woman to win the physics prize — the Nobel award with the fewest female winners (see ‘Nobel imbalance’). In 2018, laser physicist Donna Strickland ended a 55-year drought when she became the third woman to win the physics prize.

“I take very seriously the responsibility associated with being the fourth woman to win the Nobel [physics] prize,” Ghez said at the press conference. “I hope I can inspire other young women into the field. It’s a field that has so many pleasures.”

Of the 216 Nobel medals awarded for physics between 1901 and 2020, just four have gone to women.

General relativity to geometry

In a seminal 1965 paper, Penrose demonstrated how, according to general relativity, black holes could form given the right conditions — the formation of a surface that traps light. Inside this surface, mass enters an irreversible gravitational collapse, producing a region of infinitely dense energy called a singularity. Previous researchers had demonstrated this inevitability only under conditions that were considered physically unrealistic.

Penrose’s contributions span many areas of mathematics and physics. He communicated with the graphic artist M. C. Escher and inspired some of his drawings of impossible geometrical objects. In the 1970s, he developed a geometrical theory: a non-repeating 2D pattern now called Penrose tilings. These patterns occur in nature in ‘quasicrystals', which were the subject of the 2011 Nobel Prize in Chemistry.

Penrose introduced sophisticated mathematical techniques into several branches of physics, says Matilde Marcolli, a mathematical physicist at the California Institute of Technology in Pasadena who is currently collaborating with Penrose. “It was a completely new way of thinking,” she says.

In particular, in the late 1960s Penrose developed the theory of ‘twistor spaces’, an attempt to reconcile general relativity with quantum mechanics, says mathematical physicist Asghar Qadir at Pakistan’s Government College University in Lahore. Twistor spaces would change the very nature of space-time. “He developed the idea that one can talk about space and time not as a fundamental thing in itself but an emergent thing,” says Qadir, who did his PhD with Penrose on twistor theory.

The mathematical physicist also did further foundational work on singularities together with the late Stephen Hawking. “I would guess that giving the prize to Penrose is indirectly also giving it to Stephen Hawking and it’s honouring the immense effort these two people and their teams put into theoretical physical interpretation of the black hole phenomenon,” says Andreas Eckart, an astrophysicist at the University of Cologne in Germany.

Dark heart

While Penrose laid the theoretical foundations for the existence of black holes, Ghez and Genzel’s teams produced powerful experimental evidence that such a void sits at our Galaxy’s heart.

Since the 1960s, astronomers suspected that a supermassive black hole — with a mass more than a million times that of the Sun — might lie at the centre of most galaxies. The Milky Way was a prime candidate. Radio observations had revealed energetic emissions from its centre, the object known as Sgr A*. Other observations showed a centre packed with stars and gas hurtling around at high speeds.

Time-lapse gif from the NACO instrument on ESO's VLT in Chile showing stars orbiting the supermassive black hole over 20 years.

A time-lapse of observations from Reinhard Genzel’s group shows stars orbiting the Milky Way’s central black hole over 20 years.Credit: ESO/MPE

But peering closely at these stars was a challenge, because gas and dust obscured the stars’ emissions. Beginning in the 1990s, rival teams led by Ghez and Genzel used some of the world’s biggest telescopes — the Keck Observatory on Mauna Kea, Hawaii, and the Very Large Telescope on Cerro Paranal, Chile, respectively — and cutting-edge observational techniques, to overcome this challenge.

Crucial to their work was finding ways to boost their resolution and sensitivity to the faint light, says Eckart, a former member of Genzel’s team at the Max Planck Institute for Extraterrestrial Physics, Garching, Germany. First using a technique known as speckle imaging, the groups took data in snapshots to avoid the blurring caused by the turbulence in Earth’s atmosphere. Later, both teams used adaptive optics, which uses a mirror to correct for the distortion. This allowed for longer exposures, to capture more light and boost sensitivity, also allowing them to track the motion of stars in three dimensions.

Over decades, these techniques allowed the teams to measure thousands of stars near the Galactic Centre and plot the orbits of about 30. The two teams eventually determined the mass of the object — around 4 million solar masses — and agreed on an upper limit to its size.

The conclusion that there is a supermassive black hole at the Milky Way’s centre was the culmination of team efforts and “many papers and many projects”, says Eckart. Genzel is known for being a hard worker, says Eckart, who still collaborates with the laureate. “He’s very concise, and a very good scientist,” he says. Ghez, meanwhile, is known for having a cheerful intensity and fierce devotion to her work, according to a 2013 Nature profile of the astronomer. “She’s a very focused person who goes at the problems in a very direct way,” Eckart adds.

Nature 586, 347-348 (2020)

Additional reporting by Nisha Gaind and Holly Else.

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