I've just finished reading Marcus Chown's new book We Need to Talk about Kelvin (in the shops in a couple of weeks time), which starts from everyday observations to probe different aspects of basic physics and cosmology.
The final chapter is a little different, in that the concept being probed doesn't have a good scientific theory to cover it - it's 'why aren't the aliens here?'
The point is that it's a huge universe that should contain many planets capable of supporting life. So you'd expect there to be lots of intelligent life out there. And dismissing most UFO sightings etc. as the errors and wishful thinking that they indubitably are, we are shockingly short of alien observations. Why aren't they on the street corners? Why don't we see their probes and receive their messages?
In practice, the 'street corners' question is easy to answer. Aliens are just as limited by the speed of light as we are - and it is a very big universe. So the only sensible way to explore is using self-replicating probes - but they should be here by now.
There seem to be three serious possibilities for this situation. One is that aliens just don't want to come here. Maybe we smell (in the cosmic sense). Or we're just too insignificant. The second is that they are here, but they're too clever to let us know. And the third as that we are the only intelligent life - at least in the Milky Way galaxy.
On the whole, scientists don't like special cases like 'there's only us' - but the circumstances to produce a 'Goldilocks planet' - one that's just right for intelligent life as we know it to form on - are quite specific, and it's possible to envisage a number of scenarios where we are the only ones.
Whichever possibility is right, it's worth remembering that the concept of alien life isn't just something for the science fiction shows. It can tell us quite a lot about our place in the universe.