At the moment these are all very speculative ideas. The situation is rather like it was in the 1930s and 1940s when people like George Lemaître, George Gamow, and Alexander Friedman had basic ideas about the Big Bang even though no one could really test them. In the same way, inflation and superstring theories of the ultra-early universe are really ahead of any testable predictions. The question is whether in ten or 20 years we will have ways of testing them just as for the last ten years we have had very good tests of the Big Bang theory back to the stage when the universe was a second old. If these ideas could never be tested, then of course one could argue that they are no more than 'ironic science', in the disparaging sense of that phrase introduced by John Horgan. But I hope that within ten or 20 years we'll know which, if any, of them is on the right track, either because one of them will be part of a general unified theory explaining the basic forces and laws of nature, or because some astronomical test capable of discriminating between the different ideas will have taken place. Just as it would have been unfair to criticize Gamow in the 1940s for working on the Big Bang because we couldn't test it then, so it would be unfair to criticize these people now. Once again, theorists are leading, goading and stimulating the observers and experimenters.

A near-generic feature of the inflationary models—which Alan Guth, Andrei Linde, Alexander Vilenkin and many others have discussed—is that the cosmos extends far more than the horizon of our observations (perhaps even any conceivable future observations) and that there may even be many Big Bangs. What astronomers call our universe, the part we can observe within the horizon of our telescopes, is just a tiny fraction of everything there is, and could be an atypical part. For instance, Vilenkin has studied some explicit models, trying to estimate within what fraction of their total volume the conditions would be propitious for life. This meshes well with the so-called anthropic reasoning ­ the idea that although life may be possible only in a tiny part of the total cosmic domain, we are in that part. Some physicists foam at the mouth at any mention of 'the A-word'. However, if the cosmos contains domains as vast and varied as many theories suggest, then some features of our universe will have no better explanation than an anthropic one.

I'm interested in some fundamental questions about the uniqueness of physical laws. I've always been impressed by so called 'fine tuning arguments' ­ that our universe seems to be rather special, and the laws have an unusual character to allow such a complex cosmos to develop. How this happened is a genuine mystery, since you could easily imagine a set of laws that would lead to a sterile or a stillborn universe. The most natural answer to the mystery would be if our Big Bang weren't the only one—if there were many universes, and the different universes ended up governed by different laws, some which allow structures and eventually life to evolve. I'm attracted to these cosmological models that allow not just one Big Bang but many. That is one feature of the eternal inflation scenario pioneered by Linde, and also of some of these universes with extra dimensions. What I'd like to know is whether these universes are based on physics and turn out to be correct, and whether the different universes would be governed by different physical laws. Would they be governed by laws with different forces? Would they contain different kinds of particles? If there's a big variety among the different universes, then it should occasion no surprise if there were at least one universe of the kind that we inhabit.

Another perspective comes from David Deutsch, who has refined the so-called "many worlds" theory of quantum mechanics. He's thinking of these universes as being somehow superimposed on each other, which is not the same idea as Lisa Randall's parallel universes. I am very attracted to the idea that a clearer understanding of quantum theory and of quantum computation can be arrived at by thinking along the lines of David Deutsch. His is a much clearer way to think about what quantum computers can do. Incidentally, it may also be that some new theory like string theory might give us a deeper understanding of the nature of quantum theory. There's truth in John Polkinghorne's remark that 'your average quantum mechanic is no more philosophical than your average motor mechanic' ­ most physicists just use the theory in a rather mindless way. It might give you the answers, but there are still mysteries about it, and we shouldn't assume we've got the right way of looking at it yet. People like David Deutsch are perhaps heading us in a productive direction.