Nobel Prize in Physics awarded to three astrophysicists for their work on black holes

The New York Times
·10-min read

The Nobel Prize in physics was awarded to three astrophysicists Tuesday for work that was literally out of the world, and indeed the universe. They are Roger Penrose, an Englishman, Reinhard Genzel, a German, and Andrea Ghez, an American. They were recognized for their work on the gateways to eternity known as black holes, massive objects that swallow light and everything else forever that falls in their unsparing maws.

Penrose, a mathematician at Oxford University, was awarded half of the approximately $1.1 million prize for proving that black holes must exist if Albert Einstein's theory of gravity, known as general relativity, is right.

The second half was split between Genzel and Ghez for their relentless and decadeslong investigation of the dark monster here in the centre of our own galaxy, gathering evidence to convict it of being a supermassive black hole.

Ghez is only the fourth woman to win the Nobel in physics, following Marie Curie in 1903, Maria Goeppert Mayer in 1963 and Donna Strickland in 2018.

"I'm so thrilled," she said in an email.

The Nobel Assembly announced the prize at the Royal Swedish Academy of Sciences in Stockholm.

More Einstein, Less Math

Black holes were one of the first and most extreme predictions of Einstein's general theory of relativity, first announced in November 1915. The theory explains the force we call gravity, as objects try to follow a straight line through a universe whose geometry is warped by matter and energy. As a result, planets as well as light beams follow curving paths, like balls going around a roulette wheel.

Einstein was taken aback a few months later when Karl Schwarzschild, a German astronomer, pointed out that the equations contained an apocalyptic prediction: In effect, cramming too much matter and energy inside too small a space would cause space-time to collapse into a point of infinite density called a singularity. In that place €" if you could call it a place €" neither Einstein's equations nor any other physical law made sense.

Einstein could not fault the math, but he figured that in real life, nature would find a way to avoid such a calamity.

In 1965, however, a decade after Einstein's death, Penrose slammed the door on Einstein's hopes.

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Born in 1931 into an intellectual family, Penrose is a professor at the University of Oxford. When he was a child, he recalled in a recent interview, his father and his brother would play mental chess on family hikes, and his job was to keep track of the board.

"My job was the runner," he said, "I would take the moves from one brother and race up to my father. And I just got exercise by running back and forth."

A talented mathematician, he invented a new way of portraying space-time, called a Penrose diagram, which bypassed most of the mathematical complexities of general relativity.

His diagrams are now the lingua franca of cosmology. He proved that if too much mass accumulated in too small a place, collapse into a black hole was inevitable. At the boundary of a black hole, called the event horizon, you would have to go faster than the speed of light €" the acknowledged cosmic speed limit €" to getaway. So you could never escape. Inside the boundary, time and space would switch roles and so all directions would lead downward, to the centre, where the density became infinite and the laws of physics, as we knew them, would break down.

He showed that the black hole would become a gateway to the end of time, the end of the universe.

As they hailed the news, some astronomers and physicists lamented the absence of Stephen Hawking, the Cambridge University cosmologist who was arguably the world's leading black hole theorist until he died in 2018, making him ineligible for the Nobel.

Shortly after Penrose made his breakthrough calculations, Hawking and Penrose collaborated using the same methods to prove that if general relativity was right, the universe must also have had a beginning €" a fairly big discovery.

John Preskill, a physicist at the California Institute of Technology, celebrated the accomplishment of Ghez and the other scientists in a tweet. But he added that the moment was poignant.

"I'm thinking of how much Stephen Hawking would have enjoyed sharing a Prize for advances in General Relativity," he said.

Monster of the Milky Way

Today, astronomers agree that the universe is speckled with monstrous black holes, including beasts lurking in the hearts of most galaxies that are millions and billions of times more massive than the sun. They've even taken a picture of one in a galaxy some 55 million light-years away.

But closer to home, at the centre of our Milky Way galaxy, 26,000 light-years from here, there is a faint source of radio noise called Sagittarius A*. In 1971 Martin Rees and Donald Lynden-Bell suggested that it was a supermassive black hole.

Working independently, Genzel and Ghez, and their teams, have spent the last decades tracking stars and dust clouds whizzing around the centre of our galaxy with telescopes in Chile and Hawaii, trying to see if that dark dusty realm does indeed harbour a black hole.

Ghez was born in New York on June 16, 1965. She is a professor at the University of California, Los Angeles, and one of the authors of the children's book "You Can Be a Woman Astronomer." Noting Tuesday that she was only the fourth woman to win the physics prize, she said that she hoped to inspire young women.

"It's a field that has so many pleasures, and if you're passionate about the science, there is so much that can be done," she said.

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Genzel is a director at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and a professor at the University of California, Berkeley.

He grew up in Freiburg, Germany, in the Black Forest. As a young man, he was one of the best javelin throwers in Germany, training with the national team for the 1972 Munich Olympics.

Over the years, their observations have crept closer to the conclusion that whatever is at the galactic centre is dark and must have a mass equivalent to 4 million suns, in order to exert enough gravitational pull to keep the stars and gas that circle it in check.

One of the stars, which Genzel calls S2 and Ghez calls S0-2, is a young blue star that follows a very elongated orbit and passes within just 11 billion miles, or 17 light-hours, of the mouth of the putative black hole every 16 years.

During these fraught passages, the star, yanked around an egg-shaped orbit at speeds of up to 5,000 miles per second, should experience the full strangeness of the universe, according to Einstein. That last happened in the summer of 2018, with both teams watching for deviation or surprise from the star.

To conduct that experiment, astronomers needed to know the star's orbit to a high precision, which in turn required decades of observations with the most powerful telescopes on Earth.

"You need 20 years of data just to get a seat at this table," said Ghez, who joined the fray in 1995.

In fall 2018, Genzel announced that they had detected the gas clouds circling the centre of the galaxy every 45 minutes or so at 30% the speed of light. Those clouds are so close to the suspected black hole that if they were any closer, they would fall in, according to classical Einsteinian physics, Genzel said.

The results provide "strong support" that the dark thing in Sagittarius "is indeed a massive black hole," Genzel's group wrote in the journal Astronomy & Astrophysics in 2018.

"Their pioneering work has given us the most convincing evidence yet of a supermassive black hole at the centre of the Milky Way," the Swedish Academy of Sciences said in its announcement.

Einstein might grumble, but he would also be proud.

Knowing that black holes exist, physicists say, only reminds us that we don't understand what goes on inside them and that we don't really understand gravity.

The black hole "teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as 'sacred,' as immutable, are anything but," John Wheeler, one of the leaders of general relativity as a professor at Princeton University and the University of Texas at Austin, said in his 1998 autobiography.

Most physicists believe that Einstein's theory of general relativity will need to be modified to cope with extreme situations such as the Big Bang or whatever does happen in black holes.

"We already know Einstein's theory of gravity is fraying around the edges," Ghez said in an interview a couple of years ago. "What better places to look for discrepancies in it than a supermassive black hole?"

Tuesday's award extends a recent streak of prizes for astrophysics.

Last year, cosmologist James Peebles split the prize with two astronomers, Michel Mayor and Didier Queloz, for work the Nobel judges said "transformed our ideas about the cosmos."

And in 2017 the committee honoured Rainer Weiss, Kip Thorne and Barry Barish for the discovery of gravitational waves from black holes.

"Astrophysics seems to own the Nobel physics prize these days," said Michael Turner, a cosmologist at the Kavli Foundation, adding "and rightly so with all that we are learning about the universe."

Who Else Won a Nobel Prize This Year?

Harvey J. Alter, Michael Houghton and Charles M. Rice on Monday received the prize in physiology or medicine for their discovery of the hepatitis C virus. The Nobel committee said the three scientists had "made possible blood tests and new medicines that have saved millions of lives."

When Will the Other Nobel Prizes Be Announced?

€" The Nobel Prize in chemistry will be announced on Wednesday in Sweden.

€" The Nobel Prizes in literature will be announced on Thursday in Sweden. The prizes for both 2018 and 2019 were announced last year after a postponement of the 2018 prize. That occurred after the husband of an academy member was accused, and ultimately convicted, of rape €" a crisis that led to the departure of several board members and required the intervention of the king of Sweden.

€" The Nobel Peace Prize will be announced on Friday in Norway.

€" The Nobel Memorial Prize in Economic Sciences will be announced Monday next week in Sweden.

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