I am very grateful to St John's College and to the Cambridge Mathematics Faculty for inviting me here to speak in honour of Paul Dirac. I was much in awe of him when as a student I learned of his great achievements. Later I had the privilege of meeting Dirac a few times, and I still am very much in awe of him. It's really quite a challenge to give a talk in honour of so great a man, and in planning it I felt that it would not be appropriate to speak about anything less than a great subject. I didn't think it would be fitting to tell you about the latest wrinkle in elementary particle physics that we discovered last week. So, instead, I am going to jump over all details, and speak about what is for people working in my own area of physics the greatest question of all: ‘What are the final laws of physics?’

Well, not quite. Much as I would like to honour Dirac by presenting a transparency on which I have written the final laws of physics, in fact I am not going to be able to do that. My real topic must necessarily be more modest. It will have to be ‘What clues can we find in today's physics that tell us about the shape of the final underlying theory, that we will discover some day in the future?’

First of all, let me say what I mean by a final underlying theory. Over the last few hundred years scientists have forged chains of explanation leading downward from the scale of ordinary life to the increasingly microscopic. So many of the old questions-Why is the sky blue? Why is water wet? and so on-have been answered in terms of the properties of atoms and of light. In turn, those properties have been explained in terms of the properties of what we call the elementary particles: quarks, leptons, gauge bosons and a few others. At the same time there has been a movement toward greater simplicity.