News & Analysis Physics World  November 2020

Cosmic pioneers bag Nobel prize

Roger Penrose, Reinhard Genzel and Andrea Ghez win award for their work on black holes, as Michael Banks and Hamish Johnston report

New laureates Roger Penrose, Reinhard Genzel and Andrea Ghez have won the 2020 Nobel Prize for Physics for their work on black holes. (Courtesy: IOP Publishing/Tushna Commissariat; CC-BY-SA H Garching; UCLA/Christopher Dibble)

The 2020 Nobel Prize for Physics has been awarded to Roger Penrose, Reinhard Genzel and Andrea Ghez for their work on black holes. The prize is worth 10 million Swedish krona (about $1.1m) and half goes to Penrose, with Genzel and Ghez each being awarded a quarter each. The Nobel Committee honours Penrose “for the discovery that black-hole formation is a robust prediction of the general theory of relativity” while Genzel and Ghez are credited “for the discovery of a supermassive compact object at the centre of our galaxy”.

Physicists first began speculating in the 18th century about the existence of objects so massive that even light cannot escape their gravitational pull, but it was not until Albert Einstein created his general theory of relativity just over 100 years ago that scientists had the mathematical tools to investigate black holes with mathematical precision. Even then, however, there was confusion over whether a black hole could form in nature. One concern was the fact that rotating stars are not spherically symmetrical – and any departure from perfect spherical symmetry would prevent it from collapsing to a singularity, which is a single point in space and time.

Symmetrical thinking

It was in 1965 that Penrose, while working at the University of Oxford, developed new mathematical tools for describing how a star could collapse to a black hole and devised a rigorous proof that the formation of a black hole is entirely consistent with general relativity. In particular, he introduced the concept of the “trapped surface” – a closed 2D surface with the property that all light rays orthogonal to the surface converge when traced toward the future. A trapped surface is formed in the early stages of the gravitational collapse of a star and, once it has formed, the system must collapse to a singularity, creating a black hole.

Crucially, Penrose showed that this applies irrespective of the symmetry of the collapsing object. As well as being the first major contribution to general relativity since Einstein, Penrose’s work inspired generations of astrophysicists and astronomers to work towards observing black holes. “It was Penrose, more than anyone else, who triggered the renaissance in relativity in the 1960s through his introduction of new mathematical techniques,” said the UK’s Astronomer Royal, Martin Rees.

A million Suns

At around the same time that Penrose did his ground-breaking work, astronomers and astrophysicists were beginning to think that light emitted from bright regions at the centres of some galaxies might be created by matter falling into black holes that are millions or even billions of times more massive than the Sun. However, verifying that these active galactic nuclei (AGNs) do indeed contain black holes proved difficult because telescopes did not have the resolution to distinguish between a black hole and a tight cluster of stars, which could also be lurking at the centres of galaxies.

One way round this problem is to study the motions of stars that orbit close to the AGN. If the stars are orbiting a black hole, their speeds should have a specific relationship with their distance from the black hole – as do planets orbiting the Sun. However, if the stars are orbiting a cluster of stars, a different speed–distance relationship is expected. And it was Ghez and Genzel, each working for competing teams, who made precise observations that pointed to the existence of supermassive black holes.

With Ghez using the Keck Observatory in Hawaii and Genzel using telescopes in Chile, the two physicists studied a star that takes about 16 years to orbit the AGN at the centre of the Milky Way. The star, which has a highly elliptical orbit, gets to within 17 light-hours of the AGN. Independent analyses of the motion of the star by both teams suggested that it is orbiting an extremely compact object with a mass of about four million Suns. The only reasonable interpretation of this finding being that there must be a supermassive black hole at the centre of our galaxy.

Speaking in an online interview with the Royal Swedish Academy of Sciences immediately after the announcement was made, Ghez, who is based at the University of California, Los Angeles, recalled that she had not been sure at first if her observations had indicated a black hole. “The first thing was doubt that you’re really seeing what you think you’re seeing,” she noted. “Doubt and excitement – that feeling that you’re at the frontier of research.”

Ghez becomes only the fourth woman Nobel laureate in physics, following Donna Strickland in 2018, Maria Goeppert Meyer in 1963 and Marie Curie in 1903. “I’m thrilled to receive the prize and take very seriously the responsibility with being the fourth woman to win the Nobel prize,” Ghez told the world’s media. “I hope I can inspire other young women into the field – it’s a field that has so many pleasures. And if you’re passionate about science there are so many things that can be done.”

I’m thrilled to receive the prize and take very seriously the responsibility with being the fourth woman to win the Nobel

Reactions and responses

The 2020 Nobel prize was welcomed by many researchers, including Laura Nuttall, a cosmologist at Portsmouth University who studies black-hole mergers. “It’s great to see Penrose, Ghez and Genzel recognized with the Nobel prize,” Nuttall told Physics World. “Penrose is synonymous with black holes. His work in proving how black holes form, as well as their centre being a singularity, has opened so many fields, including that of searching for gravitational waves.”

The fact that no more than three people can be given the Nobel Prize for Physics in any one year always dominates the aftermath of the award – and this year was no exception. In particular, there is the question of whether the cosmologist Stephen Hawking, were he still alive, would have been in the running for a share of the prize. Hawking, who died in March 2018 following a long neurodegenerative illness, dedicated much of his life to explaining the existence and physics of black holes, especially the radiation they emit after evaporation.

“There would be a consensus that Penrose and Hawking have done more than anyone else since Einstein to deepen our knowledge of gravity,” said Rees on Twitter. “Sadly, this Nobel award was too much delayed to allow Hawking to share the credit.” Indeed, Penrose himself told the Times that a Nobel prize for Hawking would have been “well deserved”, even though Hawking’s contributions were not what Penrose was honoured for. “He made improvements and applied the ideas to cosmology. I think he should have got one,” Penrose said. “But you need something that people have observed.”

However, in an interview with the German news magazine Der Spiegel, Genzel – a director of the Max Planck Institute for Extraterrestrial Physics in Munich, Germany – hinted at a long rivalry with Ghez. “We have consistently been in competition with each other over the years,” said Genzel in comments that sparked a furious reaction on social media. He claimed that Ghez had initially “enjoyed the advantage of having access to the Keck [10 m] telescope in Hawaii”, but that when Genzel started taking data using the GRAVITY instrument on the Very Large Telescope in Chile, his group was “always ahead” of hers. When asked why both he and Ghez were therefore made Nobel laureates if his team was supposedly out in front, Genzel said he was “convinced she is the reason why we have been awarded this prize at all”.

US astrophysicist Sera Markoff from the University of Amsterdam, who is a member of the Event Horizon Telescope (EHT) team that last year revealed the first image of the shadow of a black hole, said she was “very disappointed” by the interview, in which Genzel also cast doubt on the EHT’s image itself. “It’s really too bad that someone as powerful and influential as [Genzel], whom I respect greatly as a scientist, had to show himself to be such a ‘sore winner’,” she wrote on Facebook. “I’d like to live in a world where people didn’t think that celebrating others’ achievements diminishes one’s own.”

That view was backed by Carole Mundell, head of astrophysics at the Univeristy of Bath, UK. “If one can’t be humbled by the privilege of being able to study black holes for a living and be rewarded for it with 25% of a Nobel [prize],” she wrote on Twitter, “I think you’ve kinda missed the point.” Ghez and Genzel will not, however, have to face each other at the usual Nobel-prize festivities in Stockholm, which have been cancelled due to the COVID-19 pandemic.