No life forms detected

So far, we haven’t observed anything recognisable as ‘life’ anywhere in the universe but Earth (specifically, no further than 22 light-milliseconds from the Earth’s core). This is somewhat surprising, given the sheer, unthinkable expanse of the universe. There are probably more than 10 billion roughly Earth-like planets just in our galaxy. 10 billion! That’s:


And there are at least a hundred billion galaxies in the universe, for a total of 10²¹ ‘habitable’ planets in the observable universe. That’s:


Think about that number for a moment.

It’s just not possible that precisely one out of those sextillion planets, possibly more if our definition of ‘hospitable’ turns out to be wrong, hosts complex organisms such as ourselves. But yet we haven’t seen them yet: this is the Fermi paradox.

Perhaps we’re looking for the wrong thing?

Leaving aside the question of sentience, what can we assume about ‘life’ outside of our tiny 22-light-millisecond bubble? For that matter, what is ‘life’ within that bubble? There are all sorts of weird things we call ‘life’ on Earth: anaerobic microbes living by superheated steam vents at the bottom of the ocean, endoliths that live inside rocks kilometers below the surface and live for tens of thousands of years, and tardigrades which can survive the vacuum of space by slowing their metabolism to one ten thousandth of its normal rate. Even so, we haven’t found anything life-like that isn’t stuffed with amino acids. Viruses are made from RNA or DNA and generally some protein structures; even prions, self-replicating proteins with no DNA, are made from the same twenty or so amino acids that humans are made from.

But amino acids and DNA aren’t fundamental to the nature of life. It’s not too hard to conceive of a system we would call ‘living’ made of nothing like amino acids and DNA—for example, a computer program, or at a stretch, something like Lionel Penrose’s Automatic Mechanical Self Replication (pdf), in which specially crafted wooden blocks perpetuate their own structure.

So what is it that we mean when we say ‘life’? What, if we identified it on the far side of the galaxy, would make us say “Aha! That’s like here!” Clearly, complex chemical processes are insufficient—stars are incredibly complex chemical engines, but we don’t find them to be life-like. Chaos isn’t a signal, nor is regularity: quasars and pulsars with their highly regular emissions don’t pique our life detectors.

It turns out Schrödinger (yes, that Schrödinger) wrote a book about this question, aptly titled What is Life?. In it, he defines life as (spoiler alert) that which decreases or maintains its entropy, contrary to the second law of thermodynamics. That is, if a system seems to be maintaining or reducing entropy, unlike the general tendency we see everywhere else in the universe of steadily increasing entropy, then it’s life. Life is that which creates order from disorder. Of course, it does so by the influx of energy from outside the system, so it doesn’t in fact violate the second law.

What then can we say about systems of this sort in general? What must be true about life everywhere? We can’t assume that we will find DNA and amino acids and carbon chains everywhere we find life (though that may be). What can we assume?

Having laid the stage, I’ll write about that in my next post. For now, I’ll leave you with Charles and Ray Eames.

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