Just a quick thought…
The Anthropic Principle – from an intellectual point of view, it’s largely useless. It makes few genuine testable predictions – Fred Hoyle is about the only person I’ve heard of who managed to get something usable out of it. And not only is the Anthropic Principle a useless bit of psychological fluff, at its worst extreme it can actually lead to some very, very silly intellectual places. So, on the face of it, the Anthropic Principle seems really a bit pointless.
It occurs to me that it might, just might, conceivably tell us something about the possibility of intelligent extraterrestrial life. The argument is this – we as human beings are an intelligent species, and we exist. Therefore, the logical inference is that intelligent life is definitely possible, and a basic scientific principle is that if it can happen once, it can happen again, given the right conditions.
So, this means that the debate regarding ‘aliens’ shouldn’t be ‘can they exist?’. Rather, we should be asking questions about the likelihood – and thus the spatial and temporal distribution. (10,000 civilisations at any given time in a galaxy the size of ours? Or one every billion or so years on average?)
And the nice thing about questions like this is that it should be possible to put at least some upper bounds on these numbers, even with today’s technology. The argument would be, if we can reliably detect (say) radio wave signals out to X parsecs, and we don’t, then we can argue an immediate upper bound as ‘no more than one civilization, i.e. us’ per X^3 cubic parsecs.
Divide the volume of the galactic disk by that number, and you have your estimate.