Hawking's 60th birthday party goes off with a Big Bang ? despite a broken hip

They came to talk about the big questions of life, the universe and everything. They brooded on the Big Bang, black holes and cosmic superstrings and pored over equations describing how time and space began – and when it will all end.

Some of the most famous cosmologists of our age came to Cambridge yesterday to celebrate the 60th birthday of Stephen Hawking, perhaps the best known mathematical theorist since Einstein. To mark the occasion, they dissected some of the most pressing issues in quantum spacetime -- and even found ordinary time to share a joke or two.

It was billed as "the future of theoretical physics and cosmology" and took place in the university's new Centre for Mathematical Sciences, a collection of funnel-shaped buildings constructed to house some of the leading scientific thinkers of the 21st century.

Professor Hawking, the university's Lucasian professor of mathematics, joked about his own flirtation with the end of time when he described how he crashed his wheelchair a few weeks ago and spent the following three days in hospital with a broken hip.

His talk, called "Sixty years in a nutshell", was a quantum moment away from being missed. "It was nearly 59.97 years in a nutshell. I had an argument with a wall a few days after Christmas, and the wall won," he explained.

Joining him yesterday were old friends, such as the cosmologist Kip Thorne from the California Institute of Technology (Caltech), who talked about warping spacetime and wormholes, and Sir Roger Penrose, the Oxford mathematician who worked with Professor Hawking on the knotty problem of how black holes radiate energy.

Sir Martin Rees, the Astronomer Royal and a contemporary of Hawking, started the proceedings with an overview on the current understanding of the complexities of how the universe evolved into what it is today. "We are indeed beginning to make some sense of the cosmos," Professor Rees said.

It is now accepted that our universe began with the Big Bang, a huge explosion that created all matter and possibly time itself, he said. But whether there was just one Big Bang or many of them is still open to debate.

Indeed, if there has been more than one Big Bang then there could be more than one universe, Professor Rees said. There may even be another universe just a few millimetres away from us that we are unable to see because it exists in a fourth dimension, whereas we are trapped in the three dimensions of space.

Cosmology has never been big on small talk, a fact that became more evident as the day wore on. Cosmologists, as the old joke goes, are often in error but never in doubt. (A version of this goes along the lines that there are "lies, damn lies and cosmology".)

One of the most elusive goals of contemporary cosmology is to unify all the laws of nature, sometimes called the theory of everything. Professor James Hartle of the University of California at Santa Barbara explained the theory of everything may turn out to be the theory of nothing. "To be discoverable, it can't be a theory of everything. If it is short enough to be discoverable then it's probably too short to predict everything," said Professor Hartle, who remembers his own physics tutor telling him that if he found the theory of everything then he would be a rich man.

Perhaps one of the biggest questions in cosmology -- how did the universe start? -- has already been answered, even if the answer is not to everyone's satisfaction. Only 40 years ago there was intense debate over whether there was ever a beginning.

"The big question in cosmology in the early Sixties was, did the universe have a beginning?" Professor Hawking said.

"Many scientists were instinctively opposed to the idea, because they felt that a point of creation would be a place where science broke down. One would have to appeal to religion and the hand of God to determine how the universe would start off."

One of the most important findings in cosmology over the past 40 years is the experimental verification of the Big Bang theory, sealed by the discovery of the background microwave radiation, the "echo" of the Big Bang, which now permeates space.

One of Professor Hawking's greatest achievements was in the realm of black holes, cosmic structures of such immense density and grav- itational attraction that nothing, not even light, is supposed to be able to escape.

His work on black holes began with a "Eureka moment" in 1970, a few days after the birth of his daughter, Lucy. "While getting into bed, I realised that I could apply to black holes the causal structure theory I had developed for singularity theorems," he said.

In short, he realised that when two black holes collide and merge, the area of the final black hole is greater than the sum of the areas of the original holes.

The research culminated in the famous Hawking mathematical equation that describes the temperature at which a black hole radiates, an equation that Professor Hawking would like to have written on his tombstone.

"I used to have a bumper sticker, 'Black holes are out of sight', on the door of my office," Professor Hawking said.

"This so irritated the head of department that he engineered my election to the Lucasian professorship, moved me to a better office on the strength of it and personally tore off the offending notice.

"It has been a glorious time to be alive, and doing research in theoretical physics. Our picture of the universe has changed a great deal in the last 40 years, and I'm happy if I have made a small contribution.

"I want to share my excitement and enthusiasm. 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."