Why won't the blighters land?

The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilisations ought to exist.

However, this hypothesis seems inconsistent with the lack of observational evidence to support it.

The Fermi Paradox

Why haven't we come across any aliens yet?

I'm talking about convincing scientific proof, not garbled "...them thar moon-critturs been done abducted me ag'in..." reports from the likes of Cletus Hickson in Bumfuck, Arkansas. It feels right that aliens should exist. Surely by now we should have come across some hard evidence?

A belief in aliens is older than our knowledge of the wider universe. Even before we had any idea that there existed worlds other than our own we had a concept of non-human intelligent beings. These concepts were quite distinct from our ideas of other humans. Even if Thugg the Caveman had taken a gap year before going to The University of Central Ugland and had trekked across Europe and down into Africa he would very probably still have recognised the tribes he came across down there as his cousins and quite distinct from the pesky woodland folk, demons or wights used back home to scare the cavekids into behaving.

Little Green Men (or increasingly The Greys) are simply the latest iteration of our irrational belief in other beings, this latest revised edition rewritten in the light of science, logic, learning and not wanting to sound quite so stupid when talking about them (K'sk Qazan from Omicron Ceti B sounds marginally less embarrassing than Tössaren the Meadow Elf from the Land of Faerie).

Unfortunately talk of UFOs and LGMs clouds the issue when trying to talk about real aliens. The ones that almost certainly do exist but just haven't come calling yet.

Some scientists of the Worst of Both Worlds (WOBW) persuasion do believe that we are alone in the universe. Strictly speaking they can't be proved wrong, yet. The only life we know exists is here on Earth, so it's possible that it's a fluke. Unlikely, but possible. I subscribe to the Mediocrity Principle myself; there's nothing special about us. Just as we're not at the centre of the universe, I firmly believe we're not at the centre of time and furthermore unlikely to be at the centre of life. I don't have any evidence for that yet, but absence of evidence isn't evidence of absence.

I would like to make it perfectly clear at this point that arguing in favour of the existence of aliens isn't the same as arguing in favour of the existence of a god or gods. Belief in aliens is merely saying that if something happened once and you have proof of it, it can happen again. Belief in God on the other hand is saying that something that for which there is no proof exists.

Evidence of the existence of aliens may be waiting to be discovered. Perhaps we simply haven't looked hard enough or in the right places. The Search for Extraterrestrial Intelligence (SETI) is a relatively new discipline, not even fifty years old.

In 1961 a bunch of boffins and bigwigs came together in Green Bank, West Virginia to discuss how best to go about finding aliens. In preparation for the meeting Astrophysicist Frank Drake sketched out a rough formula as an aide memoire for the conference's agenda. In subsequent years it has taken on a greater significance and would perhaps be more famous if it rolled off the tongue as easily as E=mc². As it is it's a bit of a mouthful:

N = R* × f_p × n_e × f_l × f_i × f_c × L

What does it all mean? N is the magic number, the number of intelligent alien civilisations now present in the galaxy with whom communication may be possible (for the purposes of the equation we can discount life in other galaxies). N is determined by totting up all the other numbers.

R* is fairly straightforward - it's the number of stars formed every year in the galaxy and is a surprisingly low 10. Still, given the size and age of the galaxy that gives us a stable stellar population of 100 billion.

f_p is the proportion of stars with planets. Back in 1961 Drake estimated this at "half" and then over the next thirty years got repeatedly shrieked at by rabid WOBW scientists some of whom doubted that any stars other than the sun were orbited by planets at all. This seems daft in retrospect of the discovery of hundreds of the buggers since 1995, but at the time that's what reputable scientists took the absence of evidence to mean.

n_e is the number of planets in these potential systems that could actually support life, and f_l is the fraction of these that actually go on to develop life. Far more conjecture here of course, although I find it interesting that within the first fifteen years of the discovery of extrasolar planets we've already detected one that seems suitable, Gleise 581 d. We've already fired off a high powered radio message there just in case. If there's anyone there and if they're listening (and reply straight away) we should hear back from them in around 2050.

The development of life is the crux of the matter of course. However I am prepared to go out on a limb here and say that recent discoveries within our own solar system such as the seasonal release of methane into the Martian atmosphere and the existence of the vast subglacial oceans of Europa could mean that life is as common as planetary systems, which we now realise are everywhere.

The remaining terms in the equation are all to do with the development of intelligent, civilised life, and as such are even more subject to conjecture:

• f_i - the fraction of planets with life that actually go on to develop intelligent life
• f_c - the fraction of intelligent civilisations that are detectable over an interstellar range
• L - the lifetime of such a civilisation

We just have to guess here. Does all life tend towards intelligence or are complex intelligent brains simply an evolutionary cul-de-sac like the dinosaurs' bigness? And does the development of intelligence mean a technological and therefore detectable civilisation?

"Man had always assumed that he was more intelligent than dolphins because he had achieved so much - the wheel, New York, wars and so on - whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man for precisely the same reasons."

Douglas Adams, The Hitch-hiker's Guide to the Galaxy

Plus we have to consider whether technological civilisations will always be detectable. Sure, we are at the moment, having been vomiting out TV and radio into space for the past century, but with the development of fibreoptics and laser beam transmission it's possible that we'll soon all go quiet; the same could apply to any extraterrestrial neighbours.

Unless of course they want to be heard. Perhaps they don't. Maybe aliens are antisocial.

L is the big one of course. How long do civilisations last? When I was a kid we were all going to "blow each other up" in a nuclear war. Now we're facing ecogeddon. In the face of all this am cautiously optimistic; I think we'll get past this current problem and hopefully enter the twenty second century in a more enlightened mental state on a healthier planet with a far lower human population. And no religion.

In 1961 Drake and friends were conservative and worked N out to be 10. Whilst this sounds low, it's still not to be sniffed at, and I'm prepared to bet that the true figure is higher. Come on, we've hardly started looking. I am confident that firm evidence of extra-terrestrial life will be discovered in my lifetime (perhaps only Martian methanogens, hopefully more), but proof of intelligent aliens would be nice as well.

How about something next year in time for the fiftieth anniversary of the formulation of the Drake Equation? We can all help.