This excerpt taken from an MSNBC interview with Professor Stephen Hawking by Cosmic Log's Science Editor, Alan Boyle ahead of the Seattle Science Festival.
Q: John Gribbin recently argued that we are almost certainly the only intelligent life in the Milky Way – do you think he’s right or wrong, and why? Also, SETI astronomer Seth Shostak argues that even if there are other intelligent civilizations out there, it’s too late for us to keep quiet about our existence, because it’s possible to pick up the signals we’ve sent out over the past 70 years. So isn’t it too late for us to keep quiet, and shouldn’t we be thinking about upgrading our defenses against the alien hordes?
A: "We think that life developed spontaneously on Earth, so it must be possible for life to develop on suitable planets elsewhere such as the Earth. But we don't know the probability that a planet develops life. If it is very low, we may well be the only intelligent life in the galaxy. Another frightening possibility is, intelligent life is fairly common, but that it destroys itself when it reaches the stage of advanced technology.
"Evidence that intelligent life is rare or short-lived is that we don't seem to have been visited by extraterrestrials.I am discounting claims that UFOs contain aliens. Why would they appear only to cranks and weirdos? Nor do I believe that there is some government conspiracy to conceal the evidence, and keep for themselves the advanced technologies the aliens have. If that were the case, they aren't making much use of it. Further evidence that there isn't any intelligent life within a few hundred light-years comes from the fact that SETI, the search for extraterrestrial intelligence, hasn't picked up their television quiz shows.
"It is true that we advertise our presence by our broadcasts. But given that we haven't been visited for 4 billion years, it is unlikely that aliens will come anytime soon."
Q: What would it take to make time travel a reality, and how would that affect our present reality?
A: "We are all traveling forward in time anyway. We can fast-forward by going off in a rocket at high speed, and returning to find everyone on Earth much older or dead. Einstein's general theory of relativity seems to offer the possibility that we could warp space-time so much that we could travel back in time. However, it is likely that the warping would trigger a bolt of radiation that would destroy the spaceship, and maybe the space-time itself.
"I have experimental evidence that [backward] time travel is not possible. I gave a party for time travelers, but I didn't send out the invitation until after the party. I sat there a long time, but no one came."
Q: If M-theory is the only candidate for a complete theory of the universe, what’s the best evidence that you think will be found to support the theory? Lacking that evidence, isn’t M-theory merely another kind of religion?
A: "M-theory is the only theory that seems to have all the properties that we would expect of a complete and consistent theory of everything, but that may just reflect our lack of imagination. If M-theory is correct, it predicts that every particle should have a superpartner. So far we have not observed any superpartners, but the hope is that they will be found at the LHC. If they are discovered, that will be strong evidence for M-theory. On the other hand, if they are shown not to exist, that will be exciting, because then we'll learn something new.