[4] On February 11, 2016, three months after the centenary of Einstein’s theory of gravity, nature disclosed two of its deepest secrets. Astronomers working at

[4] On February 11, 2016, three months after the centenary of Einstein’s theory of gravity, nature disclosed two of its deepest secrets. Astronomers working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had verified one of the theory’s signature predictions – the existence of gravitational waves. These waves are ripples in space-time analogous to electromagnetic waves, which constitute light. Fifty years before, this project had been regarded as a scientific moon-shot, and several leading research institutions had steered clear of it, but the gamble had paid off handsomely for its courageous investors, notably the National Science Foundation in the US. The astronomers’ observation was a tour de force of experimentation: using their detector, they measured a physical disturbance that was breathtakingly small, less than a millionth of the width of an atom. According to Einstein’s theory, the measurements indicated that the disturbance had been caused by gravitational waves emitted in a cataclysmic collision over a billion years ago between two black holes, each more massive than two dozen Suns. The “holes” will have merged with such violence that they will have shaken the very fabric of space and time: the astronomers on Earth had observed a minuscule after-effect of the event. The Nobel authorities know an epoch-making scientific discovery when they see one. To no one’s surprise, they awarded their 2017 physics prize to three of the leading lights of the project. They were rewarded for their decisive contributions to the discovery of gravitational waves, but they had also supplied the most compelling evidence yet for black holes. There was plenty of other evidence for these cosmic objects, too, especially the discovery of a massive and compact object at the centre of our galaxy. Although no one had observed a black hole, virtually no scientists were prepared to deny the reality of these elusive objects. The smart money was on the first black holes Nobel Prize going to Roger Penrose and to Stephen Hawking, who dearly wanted to win it, but who died in March 2018. If he had lived only another thirty-one months, he would almost certainly have realized his Nobel dream. Stephen Hawking is one of many physicists who made widely acclaimed contributions to their subject but who died before their work passed the stringent prize criteria set by the Nobel authorities. These days, data from the most ambitious experiments emerge after decades of planning and development – the LIGO project and the latest particle accelerator at CERN are classic cases of this. One consequence is that several world-class theorists fear that their predictions are likely to be testable only after they are dead. For the past forty years, many of the world’s finest theorists have worked on what many of them regard as the central challenge of modern physics: to bring together quantum mechanics and Einstein’s theory of gravity in a way that comprehensively describes all the fundamental particles and forces. Progress is slow, but most leading theorists now believe that the solution will entail a radically new understanding of space and time, and perhaps of reality itself. Black holes emerged long ago as the best laboratory for exploring this programme. The brilliant young black-hole theorist Douglas Stanford put it well recently: “these exotic objects are the ground zero in the war between quantum mechanics and Einstein’s theory of gravity”. Theorists today routinely imagine scenarios around and inside black holes by doing “thought experiments” based on well-established theories, mathematics and intuition. Astronomers will probably not be able to make detailed observations of these objects for several years. In the meantime, it is gratifying that the Nobel authorities have given Roger Penrose their physics prize, the first they have awarded specifically for the modern theory of gravity. (They have rightly given dozens of them for developing and applying quantum mechanics.) Penrose richly deserved the prize. No human mind has done more to enrich our understanding of black holes, the wonderfully simple objects now poised to teach us profound lessons not only about their own space-times but also about the fundamental laws underlying the entire universe.