Information theory: explaining life with physics
Physicist Paul Davies discusses an emerging area of research that aims to merge physics and biology, to explain how life began.
What has physics got to do with the origin of life?
Seventy-five years ago, the famous quantum physicist Erwin Schrödinger gave a series of lectures and then wrote a book called What Is Life?.
Schrödinger was one of the founders of quantum mechanics, which is probably the most successful scientific theory we have. At a stroke it explained the nature of matter, all the way from atomic nuclei up to stars. So here was an architect of a theory so powerful that it explained the nature of matter. Could it also explain the nature of life?
In his lectures, Schrödinger had to admit that, in spite of his brilliance, he still found life very baffling, looked at through the eyes of a physicist. At the level of individual atoms inside a living organism, it is just standard physics – nobody doubts that. But by the time you get to the level of a living cell, it looks like some sort of magic is going on.
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It’s so extraordinary, so baffling, the things that life does – all those stupid atoms getting together to do such clever things – that Schrödinger felt that some new type of physics must be involved. He said that we must be prepared to find a new type of physical law prevailing in it. Not just a new law, but a new type of law.
Even 75 years on, and in spite of the extraordinary advances in biology during that period of time, nobody knows what life is or how it began. But I think, just in the last few years, we’ve seen a little crack in the wall of mystery that shrouds life. I think we are now beginning to see what makes life tick.
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Why do we need a new type of law? What's wrong with the physics that we have now?
Biologists think of life in terms of information, that is, information storage and processing. They cast a narrative in terms of things like coded information. Your DNA, for example, is packed full of coded information – instructions, translation, transcription, editing, and signalling.
The biosphere is like the original world wide web. When you talk to physicists or chemists about life, they will talk in terms of intermolecular forces and shapes, entropy, binding energies, and forces, that sort of thing. So, we have twin narratives, but it’s clear that the physics narrative is adapted really just to the level of individual particles and molecules. It doesn’t adapt well to the system as a whole.
Information, I think, provides the bridge that can unite physics and biology. We need new physics because we need to incorporate information, not just as a way of speaking, but as a physical variable into fundamental physics.
A lot of us think of information as what's stored in computers. How can a living thing contain information?
It seems to me that a living cell is really a supercomputer, and of course a brain is also an incredible computational system. Maybe it isn’t too much like a digital computer but it’s certainly processing huge amounts of information. The computer analogy is a close one.
The difficulty is not so much being persuaded that information is the key to biology, it is trying to understand how information can be incorporated into physics. I believe that the way forward was already shown to us over 150 years ago by mathematical physicist James Clerk Maxwell and his famous demon.
What was Maxwell's Demon?
Maxwell envisaged a tiny being – this was, I should hasten to add, a thought experiment – who could perceive individual molecules in their motion, and use a device that wouldn’t expend any energy, but would be able to sort these molecules into fast and slow. So, he could put the fast-moving molecules on one side and slow-moving molecules on the other.
Because temperature is a measure of molecular speed, in effect that would create a temperature gradient, from which some sort of heat engine could be operated to do useful work. So, Maxwell’s demon is capable of creating a sort of perpetual motion machine, able to run a device to do useful work just from the heat energy of a gas, in flagrant violation of the cherished second law of thermodynamics.
This second law normally says that heat flows only from hot to cold. What Maxwell’s demon does is allow heat to flow backwards – reverse the arrow of time, if you like – by using information about the molecular speed.
What does Maxwell's Demon tell us about life?
Well, it turns out that living organisms are packed full of Maxwell demons. They are in your body as I speak to you. They are chuntering away, manipulating energy and garnering information, processing it and responding. You are full of tiny little molecular machines. These machines, while they don’t break the second law of thermodynamics, are playing the margins of it, achieving extraordinary levels of efficiency.
That’s one of the reasons why the human brain – though it’s got the capability of a supercomputer, which would consume megawatts of electricity – runs on the equivalent of just a small lightbulb.
Can we think of information as a driving force for life?
So, am I saying that information is a type of ‘life force’? Well, the word ‘force’ has bad connotations in connection with life because, in the 19th Century, it was widely believed that the secret to life was some sort of special substance or essence that infused matter and, sort of, brought it to life.
The life force was, I think, associated in many people’s minds with psychic phenomena or spiritualism or something of that sort, so it got a bad press. And unless you can measure this force, it doesn’t really help you with an explanation. So, information isn’t a force: it’s a more subtle concept.
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