The reactors in Japan are stable in the same way that a ticking time bomb is also stable. It wouldn't take much to light the fuse - a 6.6 earthquake, like what happened today in Japan, a pipe break, an over-pressurized containment vessel - anything could set it off, in which case we would have another Chernobyl, three times the magnitude of a Chernobyl accident.

Think of all the nonsense you had to learn in psychology courses. None of which was testable. None of which was measurable. We had behaviorism, Freudian psychology, all of these theories that you learn in psychology. Totally untestable. Now, we can test it, because physics allows us to calculate energy flows in the brain.

We're in 'Jurassic Park' territory. If we go to the zoo in the future, we'll have zoos for extinct animals.

Our best shot at finding life in our solar system might be to look at the moons of Jupiter and Saturn. Mars, increasingly, looks like a dead planet. But the oceans beneath the ice cover of the moons of Jupiter and Saturn may actually have more liquid water than the oceans of Earth.

The idea that excites me the most concerns the two greatest puzzles in science: the origin of the universe, and the origin of consciousness. The origin of the universe is what I do for a living, working on string theory. But I am also fascinated by consciousness.

Physicists often quote from T. H. White's epic novel The Once and Future King, where a society of ants declares, "Everything not forbidden is compulsory." In other words, if there isn't a basic principle of physics forbidding time travel, then time travel is necessarily a physical possibility. (The reason for this is the uncertainty principle. Unless something is forbidden, quantum effects and fluctuations will eventually make it possible if we wait long enough. Thus, unless there is a law forbidding it, it will eventually occur.)

Once again, my colleague Stephen Hawking has upset the apple cart. The event horizon surrounding a black hole was once though to be an imaginary sphere. But recent theories indicate that it may actually be physical, maybe even a sphere of fire. But I don't trust any of these calculations until we have a full-blown string theory calculation, since Einstein's theory by itself is incomplete.