So where are all the space aliens?, Guardian science writer asks

Good question, where are all the space aliens that Carl Sagan thought populated our galaxy by the thousands of civilizations? At one time, it was rude to express skepticism about their existence. That implied you weren't keeping up with the progress of science.

But now, even a science writer is permitted to wonder. "You never write, you never call," complains Tim Radford (August 25, 2005) - with considerable justice, because - as he engagingly points out - the idea that there could be alien civilizations inhabiting other parts of our universe was first proposed in 300 B.C.

Actually, though Radford doesn't mention it, in mediaeval times, people happily believed that life on other planets was much nicer than on Earth, a view that modern science unfortunately confutes. ("There's no life there, but if there is, it would be hell, not heaven.").

But then Radford goes on to say,

If life exists on Earth - a nondescript planet orbiting an undistinguished star in a neither-here-nor-there galaxy in an ordinary corner of the universe - then it ought to exist on at least some other planets around a proportion of other suns in at least a selection of other galaxies. There are at least 200bn galaxies, and each may be home to 200bn stars. Even if the evolution of a sentient, intelligent, technologically aware civilisation is rare, the firmament should still be fizzing with life.

Uh, wait a minute. As Peter Ward and Donald Brownlee have amply demonstrated in Rare Earth, Earth is quite an unusual planet. Not necessarily unique, but very unusual. As Guillermo Gonzalez and Jay W. Richards have demonstrated in Privileged Planet, Earth occupies an unusual position in the galaxy. If we start with questionable assumptions ("nondescript planet" "undistinguished star" "neither here-nor-there galaxy"), we may well wait forever to get good answers.

One explanation that Radford introduces for the fact that the aliens never return our calls is that our signals are not getting through. The aliens too far away. The signal gets lost.

That could be all it takes to keep the neighbours from getting the message or putting a call through, say engineers such as Christopher Rose of Rutgers State University, New Jersey, in the journal Nature, and biologists such as Clive Trotman at the University of Otago in New Zealand, who did a similar set of sums in his book The Feathered Onion last year. You can't just broadcast a message saying, "Is anybody out there?" The signal dissipates as the square of the distance. By the time you get to Pluto, it's already vanishingly faint.

Okay, Tim. If we need to believe, that's a good enough reason I guess. But why do we need to believe? Tell me again, okay?