From Michael Venables’ new Ars Technica piece about Stephen Hawking, a passage about the possibility of intelligent life beyond Earth:

“Stephen Hawking: We think that life develops spontaneously on Earth, so it must be possible for life to develop on suitable planets elsewhere in the universe. But we don’t know the probability that a planet develops life. If it is very low, we may be the only intelligent life in the galaxy. Another frightening possibility is intelligent life is not only common, but that it destroys itself when it reaches a stage of advanced technology.

Evidence that intelligent life is very short-lived is that we don’t seem to have been visited by extra terrestrials. I’m discounting claims that UFOs contain aliens. Why would they appear only to cranks and weirdos? Do I believe that there is some government conspiracy to conceal the evidence and keep for themselves the advanced technology 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 Extra Terrestrial Life, hasn’t picked up their television quiz shows. It is true that we advertise our presence by our broadcast. But given that we haven’t been visited for four billion years, it isn’t likely that aliens will come any time soon.”