Cracking the Einstein Code: Relativity and the Birth of Black Hole Physics

Einstein's General Theory of Relativity "is one of the least promising fields that one can think of for research", the 25-year-old Freeman Dyson wrote to his parents in autumn 1948. The problem was that the theory was "completely definite and in agreement with all experiments".

A few years later, Dyson settled at the Institute for Advanced Study in Princeton, where physicists would occasionally mention objects whose gravity is so strong that even light cannot escape their clutches. Several years before, Robert Oppenheimer, the institute's director, had shown that the theory predicts the existence of these objects, later dubbed "black holes", although he had no interest in talking about them.

The main problem for the physicists working on Einstein's theory was that its equations are extremely difficult to solve, except in trivial cases. By the early 1960s, however, the theory was the focus of quite a few research centres, including a first-rate one at the University of Texas at Austin. There, mathematician Roy Kerr, a 29-year-old New Zealander, made a great discovery when he found the exact solution of the Einstein equations in the region of rotating objects, including the predicted black holes. When Kerr first presented this remarkable solution at a conference in December 1963, the astrophysicists in the audience could not even be bothered to give him their attention, much to the annoyance of the relativity experts sitting alongside them. Soon, however, the importance of Kerr's work was universally acclaimed: Subrahmanyan Chandrasekhar, the Nobel prize-winning astrophysicist, later said that his realisation that Kerr had solved this problem was "the most shattering experience" of his 45-year career.

Kerr and his discovery are the subjects of this slim volume by Fulvio Melia, professor of physics and astronomy at the University of Arizona. Writing for a wide audience, Melia steers clear of mathematical notation and gives us an easy-to-read history of relativity and a brief biography of the protagonist. Kerr is not well known outside his field, although his native country has given him the distinction of setting his portrait in a stained-glass window at St Andrew's Chapel in Christchurch, alongside an image of New Zealand's most famous scientist, Lord Rutherford.

Although this book is not the place to learn the subtleties of the history of general relativity, Melia does give us some juicy anecdotes about its "golden age". In one story, Kerr identified an error in one of Stephen Hawking's calculations about the collapse of ultra-dense matter, but had to wait for some time to get the credit. Melia knows Kerr well, so it is disappointing that this account does not give us a rounded picture of its hero or a sense of how his personality and scientific style developed. The picture of him that emerges is rather bland and sketchy, and we are left wondering quite why Kerr did so little first-class research after his tour de force.

Apparently, he did not care for "the adversarial manner of the scientific community in America" and so moved back to his homeland in the late 1960s for a quieter life. Had his appetite for research declined, too? Our hunger for more information about Kerr is made all the keener when we read his charming afterword. Here we learn about his time as a mathematical prodigy in an environment where there were few resources to help cultivate his talent. Before doing his doctorate in Cambridge, he kicked his heels in New Zealand, where, in the absence of research facilities, he spent two years "mostly playing golf, tennis, badminton and finally bridge".

He tells us that he never had a mentor and spent the last year and a half of his doctoral studies without a supervisor. Perhaps as a result, he began his life as a professional physicist "untethered to any established ideas" and "free to ... pursue paths others might have shunned". Comments such as this make us wish that Melia had probed more deeply into Kerr, who, I suspect, is richer and more complex than the watercolour character presented here. Cracking the Einstein Code does give us a sense of the critical importance of Kerr's breakthrough, which he made 15 years after Dyson decided not to do research on general relativity. In the letter Dyson wrote in 1948, he told his parents that most physicists expected that the general theory would "remain much as it is until there are either some new experiments to upset it or a development of quantum theory to include it".

Today, experiments are slowly shedding light on Einstein's theory, and theoreticians are still trying to find a rigorous way to quantise it. Much of this research uses the results so brilliantly obtained by Roy Kerr, whose work is honoured by this modest but agreeable book.