The 2020 Nobel Prize in Physics, announced on 6 October, has recognised seminal achievements in the theoretical and experimental understanding of black holes. One half of the SEK 10 million ($1.15 million) award went to Roger Penrose of the University of Oxford “for the discovery that black-hole formation is a robust prediction of the general theory of relativity”. The other half was awarded jointly to Andrea Ghez of the University of California, Los Angeles and Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics “for the discovery of a supermassive compact object at the centre of our galaxy”, after the pair led separate research teams during the 1990s to identify a black hole at the centre of the Milky Way.
You might ask where the greatest entropy is in the universe – by an absolutely enormous factor it is in black holes
Roger Penrose
As soon as Einstein had completed his theory of general relativity in 1915, it was clear that solutions in the vicinity of a spherically symmetric, non-rotating mass allow space–time to be “pinched” to a point, or singularity, where known physics ceases to apply. Few people, including Einstein himself, however, thought that black holes really exist. But 50 years later, Penrose invented a mathematical tool called a trapped surface to show that black holes are a natural consequence of general relativity, proving that they each hide a singularity. His groundbreaking article (Phys. Rev. Lett. 14 57) is heralded as the first post-Einsteinian result in general relativity.
Penrose is also known for the “Penrose process”, whereby a particle–antiparticle pair that forms close to the event horizon of a black hole can become separated, with one of the two particles falling into the black hole and the other one escaping and carrying away energy and angular momentum. He also proposed twistor theory, which has evolved into a rich branch of theoretical and mathematical physics with potential relevance to the unification of general relativity and quantum mechanics, among many other contributions.
“I really had to have a good idea of the space–time geometry. Not just 3D, you had to think of the whole 4D space–time… I do most of my thinking in visual terms, rather than writing down equations,” said Penrose in an interview with the Nobel Foundation following the award. “Black holes have become more and more important, also in ways that people don’t normally appreciate. They are the basis of the second law of thermodynamics… You might ask where the greatest entropy is in the universe – by an absolutely enormous factor it is in black holes.”
