Stephen Hawking is the most famous physicist in the world. Indeed, the sales of Hawking’s books and his appearances on The Simpsons and Star Trek have tended to overshadow his scientific achievements. But that was not the case in Cambridge last month when Hawking’s contributions to physics and cosmology were celebrated at a week-long conference to mark his 60th birthday. After graduating from what he told the meeting was “the very easy physics degree at Oxford”, in 1962 Hawking moved to Cambridge where he hoped to work on a PhD under Fred Hoyle. However, Hoyle already had too many students and Hawking reluctantly switched to the tutorship of Dennis Sciama, who had been one of Paul Dirac’s few graduate students.

During his PhD, Hawking was diagnosed as having motor neurone disease and was only given a few years to live. Instead he went on to become one of the leading theoretical physicists of his generation and, in 1979, was appointed to the Lucasian chair in mathematics at Cambridge – the same professorship that had been held by Newton and Dirac.

Looking back and forward

“We organized the meeting to look back on the immense contribution that Stephen has made to many areas of gravitational physics and cosmology,” said Gary Gibbons, one of Hawking’s colleagues at Cambridge. “We also wanted to look forward to what the future might hold for theoretical physics and cosmology, with special reference to the areas that Stephen has been most interested and most active in.” But Hawking almost did not make it to the conference – he crashed his wheelchair into a wall a few days after Christmas and had to have a metal plate inserted in his femur.

Hawking made his name with a series of papers in the 1960s on singularities in cosmology. Building on work by Roger Penrose, he showed that Einstein’s general theory of relativity implied that space and time would have a beginning in the big bang and would end in a singularity. “It was a glorious feeling, having a whole field virtually to ourselves,” Hawking told the meeting. “How unlike particle physics, where people were falling over themselves to latch onto the latest idea. They still are.” Hawking then switched his attention to black holes – regions of space where gravity is so strong that nothing can escape. He was also one of the first physicists to make progress in combining general relativity – the classical theory of gravity – and quantum mechanics.

First he showed that when two black holes collide and merge, the area of the “event horizon” around the resulting black hole is greater than the sum of the two original areas. This led Hawking and co-workers* to link the area of the event horizon, A, with the entropy of a black hole, S. Hawking told the meeting that he wants this simple equation (S = Akc³/4hG) to be on his tombstone.

Hawking then went on to predict that black holes have a temperature and are not, therefore, completely black. In simple terms, what is now known as Hawking radiation is produced when quantum fluctuations give rise to pairs of short-lived virtual particles near the event horizon. The gravity of the black hole pulls one of the particles from each pair into the black hole, while the other escapes. From a distance it appears as if the black hole is radiating. This effect was symbolized by the conference mug, which changes from black to white to reveal the equation for the Hawking temperature when it is filled with hot tea or coffee.

One speaker showed the meeting a list of Hawking’s papers in the SPIRES database at the Stanford Linear Accelerator Center. Six of Hawking’s papers have qualified for “renowned” status, having been cited more than 500 times by other papers in the database. The paper on Hawking radiation had the most citations, followed by a paper with Jim Hartle on the initial wavefunction of the universe, and a paper on the inflationary theory of the early universe.

The struggle to develop a quantum theory of gravity and to unify the four fundamental forces of nature was an overarching theme at the conference. Most theorists believe that the best approach is so-called M-theory, in which the fundamental particles are actually vibrations in tiny strings in a 11-dimensional space-time. So far theorists know that M-theory embraces a classical theory known as 11-dimensional supergravity and all five of the superstring theories that were previously candidates for a unified theory. However, they do not yet know how to apply M-theory to the real universe.

Close to the edge

While the meeting was dominated by black holes, M-theory and quantum cosmology, there were lighter moments. A Marilyn Monroe lookalike sang “I want to be loved by you” to Hawking at his birthday party, and The Edge – guitarist with the rock band U2 – attended the final day.

Hawking also confirmed himself as the master of the soundbite. “It has been a glorious time to be alive and doing research in theoretical physics, and I’m happy if I have made a small contribution,” he said in remarks that were widely reported in the media the following day. “There’s nothing like the eureka moment of discovering something that no one knew before. I won’t compare it to sex, but it lasts longer.”

* The following correction appeared in the April 2002 issue of Physics World (page 20):

The articles “From the big bang to the eureka moment” and “Black holes and beyond” (February p9 and p13) should have made clear the role played by Jacob Bekenstein in the development of the concept of black hole entropy and the formulae for it and the temperature of a black hole (see, for example, Lettere al Nuovo Cimento 4 737 (1972) and Phys. Rev. D 7 2333 (1973)).