There's a tendency to use terms like "theory of everything" and "final theory" to denote what people like Edward Witten (and hundreds of other talented theorists) are seeking. The theory they're looking for would be the end of a quest that started with Newton and continued through Einstein and his successors. But of course it wouldn't be the end of science—it would just be end of a particular quest. It wouldn't help us to understand most of the complexities of the world. Most scientists, even most physicists, wouldn't be helped at all by a fundamental theory, because the difficulties that confront most scientists are not the result of not knowing the basic laws. The challenge of science is not just what the theoretical physicists are doing but also to understand how complexity emerges. This is just as fundamental as the challenge of the so-called "theory of everything," and it's independent of it. Steve Weinberg says that if you go on asking "Why, why, why?" you get back to a question in particle physics or cosmology. That's true to a degree, but only in a limited sense. It's a challenging question to ask why a fluid sometimes behaves in a regular way and sometimes in a chaotic way—to understand turbulence or dripping taps, for example—but the answer isn't going to come from analyzing the liquid right down to its subatomic constituents. It's going to come by thinking in a quite different way about complexity.

To give an example, Mitchell Feigenbaum's discovery that the same series of numbers come up in the transition from ordered to chaotic behavior is an important discovery about the world, but it's got absolutely nothing to do with particle physics, even though it is just as fundamental. So I'm sympathetic to people like Phil Anderson who want to deflate the hubris of the fundamental physicists who claim that their subject is the deepest and highest priority of all. It's just as important to understand complexity, to see it in the simplest form in the transition to chaos and in more complicated forms in all the rest of science: the genetic code, fluid flows, and all the rest of it. These are equal challenges.

It's important that alternatives to mainstream ideas are being explored—for example Lee Smolin's work on loop quantum gravity. Sociologically, the one thing that concerns me about string theory is the perhaps excessive concentration of talents in that one field compared to others. It's not only a suboptimal deployment of scientific effort, but is sure to lead to a lot of disillusionment when so many brilliant young people who are all chasing the same ideas. It would be a good thing if more people explored alternative approaches, or indeed moved away from that kind of fundamental physics to explore the challenges of the complex world. Incidentally, Lee published a nice article in New Scientist a few months ago about how the university system, in deciding who to give appointments to, tends to disfavor the people in the most original areas, because obviously if you're working in an area that's not yet appreciated by a large cohort of colleagues then you're not going to have such an easy time making a career. There's a lot of resonance in what he says there.

Paul Davies has expressed some interesting ideas on time loops. The issue of time is important conceptually because the debate among physicists is whether these are impossible in principle, or whether they are just impossible to make technically. There's now a consensus that you can't rule them out on some general principle. You can't just flatly say that they violate causality. They could lead to inconsistencies—You would have to have something move in a closed loop in time that is entirely consistent to avoid the obvious paradoxes and make sure the loop closes up in a consistent way—but I think everyone accepts that can be done while avoiding the well-known paradoxes. I rather like the way Igor Novikov puts it. He says that it's not absurd to have a law of nature that prevents you from shooting your grandfather if you go on a time loop. This indeed constrains our free will, but Novikov notes that the laws of nature already constrain our free will in many other ways. Physical laws prevent us from exercising our free will to walk on the ceiling; likewise there may be a physical law preventing you, if you were in a time loop, from doing something that was inconsistent, like shooting your grandfather in his cradle.