I would like to believe that crop circles are evidence of visitation. But there have been too many people who have admitted to creating these crop circles, and too many people who have shown how to make one on TV programs, so I have my doubts.

An event horizon, or the point of no return, is only a byproduct of the bending of space. However, electricity and magnetism, by themselves, have no event horizon. It gets complicated, however, if a black hole has charge, and then this new solution does have an event horizon.

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.)

In fact, all of us have a piece of Chernobyl in our bodies going back to 1986.

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.

Beyond work and love, I would add two other ingredients that give meaning to life. First, to fulfill whatever talents we are born with. However blessed we are by fate with different abilities and strengths, we should try to develop them to the fullest, rather than allow them to atrophy and decay. ... Second, we should try to leave the world a better place than when we entered it.

I agree, along with Carl Sagan, that we should eventually become a two planet species. Life is too precious to place on a single planet.