ANYONE THERE?

Dick Pountain/06 June 1997/Idealog 34

Last month I touched on the possibility of life on other planets, and hoped to say more before running out of space. Now feels like a good time to say it, as I'm writing in a state of monster jet-lag following a 24-hour flight from Taiwan; after voyaging through a thin atmosphere and feeding on synthetic alien glop I have only the dimmest idea which planet I landed on. What's more the in-flight movie was 'Star Trek - First Contact' featuring the Borgs, whose lack of social graces and motor skills fully supports my scepticism of last month toward flesh-machine integration.

So is there life out there? Let's skip the thorny problem of defining exactly what 'life' means, and be content with calling a 'life form' anything that at the very least can reproduce itself, and can evolve in order to survive in a changing environment. A life form needs some medium in which to store the information to create copies of itself, and this medium needs to be strong enough to hold together - so that the information doesn't get lost or corrupted - yet not so inert that it cannot come apart to permit copying, error correction and mutation. For all the known life forms on our own planet the giant polymer DNA or its close relative RNA fulfill this role. DNA's famous double helix is held together by the cross-linking of four different nucleotide bases which fit like lock and key. The ordering of triplets of these bases constitutes a digital code that enables strands of DNA to encode recipes for millions of different proteins, just as ASCII can encode millions of text strings. Proteins are both the building bricks and - as enzymes - the construction tools that make organisms. Richard Dawkins has speculated that any conceivable life form will have to use a digital code, as it's the only way to get sufficiently error-free duplication for natural selection to stand a chance.

The self-replication of DNA depends on the exact strength of the attachment of its two strands, which is 'just right' for its purpose - any stronger or weaker and it could not work. The covalent bonds that hold most molecules together (formed by sharing electrons) are too strong, but an unusually weak kind of bond in which a hydrogen atom acts as glue is just right. These hydrogen bonds can only form in hydrogen-rich, polar, fluid solvents, of which water is the prime example. Liquid ammonia is another, but it doesn't exist in nature on our planet. If you want to imagine life on some other planet, then you'd better either imagine liquid water on that planet, or else some alternative self-organising information processing system based on other molecules, which proves surprisingly difficult to do.

Why LIQUID water? Because it's very difficult indeed to imagine any natural information processing system arising from the gaseous state; there's too much randomness and not enough structure for information to persist for long enough. So it's highly unlikely that life has evolved within stars (as opposed to planets) because only a few light nuclei like hydrogen and helium exist in the hot plasma and they can't stick together in sufficient numbers or for long enough for persistent information storage. It's equally difficult to imagine life arising in the solid state for the opposite reason, that movement of information through a solid by diffusion is just too slow to make a plausible computing system. But surely there is all the time in the universe, so a life form could evolve much more slowly than we did? Actually no, there is a time limit, namely the life cycle of the stars themselves - life depends on concentrating information and your local sun going supernova has rather the opposite effect. Any would-be life form based on silicate chains dissolved in solid granite just wouldn't have time to, er, get off the ground.

You might object that electrons can flow freely through some solids, as indeed they do within silicon chips, so a life form might arise made solely from electrons. But don't forget that an outside agency (the great god Intel) fashioned those silicon and aluminium pathways into the right configurations to store information, and the probability of these spontaneously evolving within solid rocks seems vanishingly small. 

It's very likely then that only the liquid state offers thermodynamic conditions favourable for life, which severely constrains the temperature range of the planets on which it might appear, and hence the type of star around which they might form. The various SETI projects that search for extra-terrestrial signals use an equation invented by the radio astronomer Frank Drake to estimate the number of technical civilizations that might have evolved within the Milky Way Galaxy, but this equation is so sensitive to its (unprovable) input values that estimates range from 1 to tens of millions, which is hardly better than guessing.

Perhaps a non-carbon-based life chemistry is possible; some other elements like sulphur or silicon can form long chains that might encode information, and solvents like liquid hydrogen sulphide might work like water under different temperature and pressure conditions. But given that just 92 elements make up the entire universe, it's no less conceivable that the carbon/water combination might be the only thermodynamically workable one, and we are it. Were that to prove the case it wouldn't worry me one bit. I sense that a morbid interest in alien civilisations and cyborgs is really just a symptom of loss of faith and interest in solving problems here on earth (cling on to a Klingon?), a toxic side-effect of years of political stasis. Perhaps a new government will bring us back to earth.