I had one encounter with Stephen Hawking. He came in the summer of 1989 to the Aspen Center for Physics and had the office next to mine. He travelled with an entourage with whom he could communicate with his voice synthesiser. His hands still worked well enough. He gave a full house public lecture and afterwards Sidney Coleman presided over a question session. Hawking had to type out all his answers on his voice synthesiser which took a lot of time. At one point Sidney said: 'You can have it fast or you can have it good.' If I had asked a question, it would have been: how did he come up with the idea of Hawking radiation? I have always found his paper hard going and have always marvelled at the simple result at the end. In A Brief History of Time he gives an account which explains the phenomenon but not the result.

In the neighbourhood of the surface of the black hole there are very large fluctuations of the vacuum. These fluctuations supply the energy to create pairs of electrons and positrons. One of them is created inside the hole and the other escapes – the Hawking radiation. What this does not explain is the simple formula for the temperature, which varies inversely to the mass of the black hole. There were probably small black holes that were created at the time of the Big Bang but these, because of the high temperatures, have radiated away. The large black holes in the centre of our galaxy and elsewhere still remain because they are cooler.

It is sometimes asked why Hawking did not win a Nobel Prize. Nobel Prizes are not given for speculation; even Einstein did not win his for relativity. There is no experimental evidence for Hawking radiation but everyone who has studied the theory is sure that it is there.

Sidney Coleman told me another story about Hawking. He came to his place in Boston for dinner and since there were stairs Sidney carried Hawking up. A neighbour asked: 'Sidney, what are you carrying?' To which he replied: 'It is the Lucasian Professor of Physics from Cambridge.'