Nobel Prize in Physics 2020

6 October 2020

Roger Penrose headshot

As the winners of this year’s Nobel Prize in Physics are announced as Roger Penrose from the University of Oxford and Reinhard Genzel and Andrea Ghez from the Max Planck Institute for Extraterrestrial Physics and the University of California, Los Angeles respectively, we look at the significance of their ground-breaking work.

An authority on black holes

Sir Roger Penrose, Emeritus Rouse Ball Professor of Mathematics at the University of Oxford has been awarded the prize for the discovery that black hole formation is a robust prediction of the general theory of relativity. Professor Steven Balbus is Savilian Professor of Astronomy at Oxford’s Department of Physics and comments on the well-deserved win: ‘Roger Penrose’s Nobel Prize in Physics is long overdue, and I know that my enthusiasm and excitement for this recognition is shared throughout the physics community. More than any other person, Roger Penrose is responsible for putting black hole physics on a rigorous theoretical footing. His work has been transformational.

‘The global topological methods introduced into the mathematics of general relativity by Roger Penrose revolutionised the field, and helped inaugurate what has been termed a “golden age” of relativity. In the 1930’s, work by S. Chandrasekhar, R. Oppenheimer and others had indicated that the self-gravity of sufficiently massive stars might actually collapse these objects down to a single mathematical point-like singularity. But these calculations were of necessity highly idealised, assuming spherical symmetry to make the difficult calculations tractable. Whether these singular “black holes,” as they later came to be known, actually existed in nature or not was an extremely controversial question. In the early 1960’s rival physics camps argued either that the calculations should be taken seriously and such singularities are inevitable, or that the conclusion is so preposterous that something must intervene to stave off collapse.

‘In 1965, Roger Penrose published a remarkable three-page paper in Physical Review Letters (Vol. 14, No. 3, p. 57) in which he compellingly demonstrated, using then-novel mathematical methods, that gravitational collapse is inevitable under far more general conditions that anyone had previously envisaged. Provided that general relativity is correct, and that the matter involved obeys a precise but rather weak “positive energy criterion,” once a trapped surface forms, singular structure in the spacetime cannot be avoided. What is a trapped surface? Roughly speaking, a trapped surface is a (spacelike) two-dimensional surface on which the gravity is so strong that all future-bound light rays are heading inward. This may sound extreme, but it is really not. Take gas with a density comparable to the Earth’s atmosphere, and spread it throughout a sphere to the outer solar system: the outer boundary would then constitute such a trapped surface. The sphere of gas, in turn, must inevitably collapse to a black hole!

Stunning ideas

‘Penrose’s stunning ideas transformed black holes from speculative notions to tangible objects, fully in the domain of astronomical observations. The techniques he developed became the basis of Stephen Hawking’s first great achievement, showing that a singularity at the dawn of the Universe must also be present. Hawking and Penrose would go on to brilliantly elucidate conditions under which singular structure is present in relativistic spacetimes under very general circumstances. Black holes are now more than objects of astronomical fascination, they are also an integral part of modern attempts to develop quantum gravity theories, via Hawking’s discovery that Penrose’s trapped surfaces, as they emerge through time, can create particles.

‘I confess to having a pang of regret that Stephen Hawking, whom I inevitably associate with Roger Penrose, did not quite live long enough to be similarly recognised for his own great contributions, which made brilliant use of Penrose’s foundational ideas. But the spacetime continuum is not for us mere mortals to control. Today we recognise Roger for his magnificent accomplishments. This recognition is so richly deserved.’

Secrets from the centre

Reinhard Genzel and Andrea Ghez were awarded the prize for the discovery of a supermassive compact object at the centre of our galaxy. Professor Rob Fender, Head of Astrophysics at the Department of Physics at Oxford comments: ‘Reinhard Genzel and Andrea Ghez led teams that use the largest European and American optical and infrared telescopes, respectively, to study the very closest regions to the centre of our galaxy: a mysterious radio source known as Sagittarius A*. Through a combination of extreme tenacity, the utilisation of cutting-edge techniques, and physical insight, they have established that Sagittarius A* is a black hole of mass, 3-4 million times the mass of our own Sun.

‘These works, resulting in a number of the most astonishing movies of real data ever produced in astronomy, are among the strongest evidence that supermassive black holes exist at the centres of most, if not all, galaxies. They have tracked the orbits of individual stars which are moving at a staggering 20,000 kilometres per second over a 10-year orbit around the supermassive black hole.

Outstanding observational astrophysicists

‘Both Genzel and Ghez are absolutely outstanding observational astrophysicists, whose decades-long studies of the centre of the Milky Way have definitively established the existence of supermassive black holes and vastly enriched our understanding of the centre of our celestial home. Their work, combined, represents arguably the most important insight into the existence, formation and impact of black holes at the centres of galaxies. For those of us who work in related areas, their work is the poster child for illustrating the power of observations to drive forward black hole research.

‘We are especially delighted that Professor Ghez holds an honorary degree from the University of Oxford, awarded only last year during a visit in which she gave an enthralling talk on her research.’

Ian Shipsey, Head of Physics at Oxford concludes: 'The Department of Physics congratulates the 2020 Nobel Laureates for their outstanding contributions to astrophysics, and the understanding of our Universe. Black holes play a profound role in physics and in the Universe as a whole and they will remain an incredibly exciting area of research both theoretically and observationally for the foreseeable future at Oxford and globally. Black holes also capture the imagination of society at large; it is our hope that this award will inspire the next generation of physicists, young women and young men, whom we are committed to supporting at every level at Oxford Physics.'

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