Looking for aliens

It's one of the great questions of the age : where are all the aliens? We've been looking for them since the Sixties and we've found nothing, unless you count various disturbed Americans who claim to have been kidnapped by aliens but (of course) can't prove it, and a lot of dubious UFO pictures.

The standard theory says there ought to be plenty of aliens to find. It goes something like this. Take the absolutely largest number you can think of, multiply it by itself a few times, and you still won't have a big enough number to count all the stars in the universe. Even if only the tiniest percentage of them have planets, and even if only the tiniest percentage of those support intelligent life, there ought to be millions of alien civilisations, some of which should be at least as advanced as ours. So let's build a great big antenna and listen for their radio/TV/communications chatter. It's called SETI, the Search for Extra-Terrestrial Intelligence, and it's been running since the 1970s. Trouble is, it hasn't found anything.

Why not? Good question. Part of the problem is that there are a whole bunch of unproven assumptions in there. For one thing, we have no idea how common or otherwise life really is. Or planets, for that matter. We've only ever been able to study one solar system - the one you're sitting in - and although life is incredibly prolific and versatile on one of its planets (guess which), as far as we know it doesn't exist on any of the others at all, though since we haven't actually been to any of them we don't know for sure. So we're not just extrapolating from only one sample, which any scientist will tell you is very bad science indeed, but we're extrapolating massively from just a small part of that one sample. For all we know, we might be an outrageous fluke, never repeated anywhere else. Or the exact opposite, which is where SETI comes in. If we don't look, we'll never know, so SETI is far from being a waste of time. The very fact that we haven't found anything is data of a sort. But very frustrating.

Another unproven assumption in all this is that alien civilisations have electromagnetic communications chatter. We've been broadcasting powerful radio and TV signals in all directions for nearly a century, and we're assuming that other civilisations would be doing the same, but we're only guessing. Perhaps there is a better technology for moving information around, but we haven't discovered it yet, and so can't detect it. In fact, in a sense we have already discovered a better method. These days most TV and radio signals go via satellite or cable, using far less powerful signals than the original brute-force transmitters, which in many places are being phased out altogether. You certainly couldn't detect signals moving through a fibre-optic cable in a different solar system with any conceivable technology, and it's highly doubtful you could detect the weak, carefully-targetted signal to a satellite either. And of course the signal from a satellite is aimed straight down at the planet, so very little of it would escape to be detectable elsewhere. So perhaps technological civilisations are only detectable electromagnetically for a short period early in their history, and there doesn't happen to be another one at the right stage within our range at present.

But if such alien civilisations do exist, why haven't they detected us from all the TV and radio signals we are putting out, and come over to say hi? Well, if they are more than seventy-odd lightyears away, a tiny distance in interstellar terms, the signals simply haven't reached them yet. Then there is what you might call the Star Trek theory : they discovered us ages ago, but are waiting for us to independently develop warp drive or whatever before they invite us to join the Galactic Federation. Or maybe they've been watching our TV and don't fancy showing themselves to a species that always seems to end up at war with any aliens they meet!