[1] In 1918, three years after Einstein completed his new theory of gravity, he declared that “the supreme task of the physicist is to arrive at the elementary

[1] In 1918, three years after Einstein completed his new theory of gravity, he declared that “the supreme task of the physicist is to arrive at the elementary laws from which the cosmos can be built up by pure deduction”. For the physicists who follow that credo, the most productive sources of inspiration and information in the twentieth century were atoms, the building blocks of matter. In the past few decades, however, the gaze of many of these physicists has turned to outer space, especially to black holes, objects so dense that not even light can escape them. These mysterious objects are proving a fruitful way of addressing the central challenge of fundamental physics: to gain a better understanding of space, time, gravity and even reality itself. Much simpler than atoms, black holes can be regarded as the simplest objects in the universe, made not of matter but only of space and time. This extreme simplicity is a boon to theorists, who have studied these objects in meticulous detail for decades, long before astronomers had observed even one of them through their telescopes. The embarrassment ended in April 2019, when scientists at the Event Horizon Telescope (EHT) published the first image of a black hole, 40 billion kilometres across and 500 million trillion kilometres away. It made news all over the world. At long last, nobody could reasonably argue that black holes were figments of the theorists’ imaginations: these cosmic objects were as real as the stars in the sky. Although several theorists had done excellent work in this field, none of them had been awarded a Nobel Prize. This is probably because the authorities award their prestigious gong only to scientists who have undeniably explained a feature of nature that experimenters have observed. The EHT’s striking image licensed the Nobel physics committee to give their prize this year to the most accomplished black-hole theorists alive (the award is never awarded posthumously). One obvious candidate was Roger Penrose. Many of his peers revere him as the greatest living contributor to the development of Einstein’s theory of gravity, which superseded the account given by Isaac Newton over two centuries before. Penrose introduced new ways of handling Einstein’s theory, using new mathematics that Einstein himself knew nothing about. As a result, Penrose revolutionized the way theorists thought about the theory and applied it to the real world. Most influentially, by using the first of his mathematical theorems relating to the theory, he was first to establish that the existence of black holes is a robust prediction of Einstein’s theory, contrary to the view expressed by many physicists, including Einstein himself. Six weeks after the EHT team published their image, I had a relaxed dinner with Penrose at the Côte Brasserie in Oxford. Over dessert, I could not resist saying how pleased I was that the image of the black hole made him a shoo-in for a Nobel Prize, but he batted the notion away. Happily, he was wrong. At home on the morning of October 6, 2020, the eighty-nine-year-old Penrose climbed out of the shower and took the phone call from Stockholm that most top-tier scientists dream of. He heard that he had received half the prize, and so will receive a cheque for approximately £440,000. The remainder of the prize was shared equally by the astronomers Reinhard Genzel and Andrea Ghez who led the groups that discovered an extremely massive and compact object at the centre of our Milky Way galaxy, an object now widely agreed to be a black hole. This branch of physics has long been dominated by theory rather than experiment, pioneered using pure thought rather than through observations made through telescopes. In the 1770s, John Michell, the rector of Thornhill in Yorkshire and a leading English astronomer, began to imagine “dark stars” from which light could not escape and in 1783 published a paper about them. Thirteen years later, the French mathematician and natural philosopher Pierre-Simon Laplace, sometimes known as the French Newton, set out a similar idea in “Exposition du Système du Monde” and backed it up in a calculation he published in 1799. The concepts of Michell and Laplace attracted little attention among their peers, and for a century remained little more than a curiosity. The first modern analysis of what became known as black holes was written at the University of California, Berkeley, by the American theoretical physicist Robert Oppenheimer and his student Hartland Snyder. They discussed the collapse of heavy stars under their gravity, using the late German physicist Karl Schwarzschild’s solution to the central equation of Einstein’s theory. Published on September 1, 1939, the day the Nazis invaded Poland, the paper received little attention and neither author returned to its theme. An early admirer of Einstein’s theory of gravity was the physicist Max Born, who later described it as “the greatest feat of human thinking about Nature, the most amazing combination of philosophical penetration, physical intuition and mathematical skill”. Yet, for all its brilliance, the theory made little impact on the mainstream in the next thirty-five years. Part of the reason for this was that the basic equations of Einstein’s theory of gravity are formidably difficult to solve and it seemed that their solutions would be only tiny differences to the predictions made by Newton’s theory. The detectors in that era were not sufficiently advanced to test most of these tweaks or test it in some other way. It was only in the early 1950s that interest in gravity theory picked up. Observational astronomers began to develop better detectors and a few theorists gradually began to focus on gravity rather than the forces that shape atoms, governed by the laws of quantum mechanics, a revolutionary theory that European physicists had discovered in the mid-1920s. Anyone who wanted to develop Einstein’s picture of gravity needed to be armed with an abundance of creativity and mathematical skill, as well as a feel for the inner workings of nature that is the hallmark of all exceptional theoretical physicists. Penrose was blessed with all these qualities and had the courage to ignore the tramlines of academia and instead to plough his own furrow in interdisciplinary research. All in all, he was supremely well-equipped to become one of the pioneers of what is often described as the renaissance of gravity theory.