The controversy is between those who think that time loops are prohibited by some basic law; and those who argue that they're "merely" technically difficult. To give an analogy, most of us would say that a rocket going faster than light is impossible, but a rocket going at 99% of the speed of light is possible, in principle, although that's of course equally impossible in practice. The question is whether the time loop is like going at 99% of the speed of light, or like going faster than light. In one case it's just technically impossible to make a time machine, but not impossible in principle; in the other case it would be impossible in principle. A lot of futuristic science does sound a bit like science fiction. The key point, though, is that as we explore more extreme environments—the very large, the very small, etc.—we have to be prepared to give up more of our common sense notions. That's the fascination of the subject.

When we were talking earlier you asked what computation has to do with all of this. Of course, even Feigenbaum's results would not have been possible without a post 1970 HP calculator on which you could actually perform simple iterations. Another example can be seen in fractal patterns like the Mandelbrot set—marvelous pictures that have layer upon layer upon layer of complexity. Before the age of computers you could never actually draw these patterns. You couldn't fully appreciate how a simple algorithm could result in such tremendous complexity. It's through the computer that we've been able to do this new kind of science—for which, of course, Wolfram is the highest-profile propagandist—which allows us to develop new intuitions about how simple patterns and simple algorithms can have extremely complex consequences. That's an example of a type of science that is fully on the level of particle physics and string theory intellectually but is quite disjoined from them. Steve Wolfram has given a very fine manifesto for this kind of science. Whether his way of looking at things is actually the key to understanding space, time, and particles, I don't know. I'm rather skeptical about that, to be honest, but it is important in illustrating how simple algorithms can generate a great deal of the complex structures in our world.

Apart from being a spectator of the exciting debates about the ultra-early universe, my preoccupation right now —as indeed it has been for more than 20 years—is to understand how the 'cosmic dark age' ended. After the initial brilliance of the big bang, the universe cooled and darkened, until it was lit up again when the first stars or galaxies formed. We're making great progress (with the aid of both observations and computations) in understanding how the universe went from being amorphous and structureless to becoming complex. This key transition happened quite late ­ perhaps a hundred million years after the Big Bang. The basic physics at the prevailing low densities and temperatures is uncontroversial but things get complicated for the same reason that all environmental science is. I'm trying to understand how the first structures evolved, how the first stars, black holes and galaxies developed, and how the universe changed from being an amorphous expanding fireball to consisting of stars and galaxies and the other things we observe.

Cosmologists are no less concerned than anyone else with what happens next week or next year ­ indeed their awareness of the vast eons that stretch ahead perhaps makes them especially mindful of life's future posthuman potential. One other thing I am doing right now is writing a book focusing on scenarios for the coming century. There's a lot of jubilation about the accelerating progress of certain sciences, and of course there are people—like Ray Kurzweil in particular—who think that technical progress is running away towards some kind of singularity or cusp, that could be reached in about 50 years. My concern is that each of these advances, particularly advances in biotechnology, leads to greater instability. It increases the leverage and power that a single disaffected person or a small group has. If will take only a few people, with the tremendous leverage that technology will offer, to cause disasters that could disrupt our whole society. The question is whether we will get through this period. The anthrax episode showed that it isn't necessary to do much to affect the psyche of a whole society, because the media, and the general hype, can amplify any scare. Any outrage or disaster is amplified by the media and by the fact that we're so connected, so networked. I can't see how we can avoid having episodes that just completely seize up society—or even cause it to collapse. I'm pessimistic because it seems to me it's going to be very hard to guard against these things. Look at what's happened in the world. 20 years ago we worried about confrontation between superpowers. In the 1990s we worried about nationalism and smaller scale conflicts. Now we worry about terrorists and other disaffected groups, and in the future we'll have to worry about disaffected individuals with the mindset of those who now design computer viruses, but the power to do far worse.

Unless we have absolute surveillance of everyone all the time, it's going to be very hard to guarantee that one or two people don't generate some kind of catastrophic event. People will accept a very high level of surveillance. We've certainly seen this already in Britain and the States with closed circuit TV in public places. We have to change our mindset a bit, before this would become acceptable. Not only the people who go on these "reality" television shows are exhibitionists. What's surprising is how many people put a lot of personal stuff on their Web sites. The reticence and valuation of privacy among people of my generation (and of those even more ancient than me) may be eroding away. Surveillance will become more acceptable if we can all choose to be voyeurs even as we are being watched. If people get to the stage when they are all prepared to accept this kind of intrusive surveillance, it will become a way of reducing the risk.

Such thoughts make me rather depressed about what's going to happen in the next ten or 20 years. If we can stave off that disaster, however, then I'm with Kurzweil in expecting that the rate of change in our life is going to be even faster in the coming 50 years than it was in the last 50. In particular, if they are right about getting computers with humanesque capacities then that will be a real quantum change. It's not enough to have computers with the processing power of a human brain: they've got to be able to sense and interact with the external world and not just be stuck in a box. They must relate to the external world as well as we do through our senses. When that happens we will have machines that are of a human level, and of course they may start solving problems for us. The real test will be if they can actually solve some of these scientific problems better than we've been able to up to now. To give one example, there's been great progress in high temperature superconductivity. People have been gradually raising the temperature, even though they don't really understand what's going on, to try out recipes of very complicated chemicals. But suppose you were working with a machine that spewed out the formulas and then gave you a superconductor that worked at room temperature. It may have done this by testing billions of alternatives, not by the kind of insights that could lead human theorists to the same result. But, just as we have to accept that Deep Blue plays chess very well (even though it doesn't think and analyze like Kasparov) we'd have to accept that it would deserve the Nobel physics prize. There could be a runaway increase in our scientific capability when machines get to that threshold—perhaps they will even solve the key cosmological problems.