Tuesday, November 7, 2017

Thermodynamics of Life

The theoretical research I do with my colleagues tries to comprehend a new aspect of life’s evolution by thinking of it in thermodynamic terms. When we conceive of an organism as just a bunch of molecules, which energy flows into, through and out of, we can use this information to build a probabilistic model of its behaviour. From this perspective, the extraordinary abilities of living things might turn out to be extreme outcomes of a much more widespread process going on all over the place, from turbulent fluids to vibrating crystals – a process by which dynamic, energy-consuming structures become fine-tuned or adapted to their environments. Far from being a freak event, finding something akin to evolving lifeforms might be quite likely in the kind of universe we inhabit – especially if we know how to look for it.
Of time and entropy:
One of the most important contributions came from a theorist named Gavin Crooks, now at the Lawrence Berkeley National Lab in the United States. He asked the following question: given that I have a movie (say, of a piece of wood burning to ash or a plant growing) and the rewind version of that movie, how would I tell which one is more likely to happen?

By applying some basic assumptions, he was able to mathematically prove the following. If you have a system (a piece of wood or a plant, for example) surrounded by a ‘bath’ of randomly jiggling particles (say, the atmosphere), the more heat the system releases into its bath, the less likely it is to rewind itself. In a rigorous, quantitative sense, the dissipation of heat is the price we pay for the arrow of time.

Why? Another way of phrasing this insight is to note that the more a system increases the entropy of its surroundings, the more irreversible it becomes. Now, it must be said that in the grand contest for the most misunderstood idea in the history of physics, entropy is probably the winner. Even people who are normally averse to any mention of the natural sciences will sagely volunteer that entropy – read: messiness, dysfunction, chaos, disorder, who knows? – must increase, all the time. It’s the second law of thermodynamics, obviously. But this simple picture can’t be right. Living organisms, for one, seem to defy this misleading gloss on the second law. They take disorganised bits and pieces of matter, and put them together in fiendishly complex and refined ways. [...] the reason a heat-producing movie is more likely than its heat-absorbing re-run has to do with the number of ways you can disperse that heat in the surrounding bath. The more heat you throw into the bath, the less hope you have of getting it back from a freak fluctuation, and the less likely it is that you will have the energy you need to retrace your steps once the movie has run forward.
Living things clearly have energy to burn, and they get this energy from being worked on. Like heat, ‘work’ in thermodynamics involves units of energy. But instead of the uncoordinated wiggling of molecules, here it’s a measure of how much and how fast energy has been transferred to a system from its surroundings in a way that produces a change. There are a variety of versions, such as movement, volume change and chemical transformation. What unites these processes is that energy is being forced, pushed or driven into a system from the outside, in a way that modifies the system’s shape or location. When you hit a car, it might move, or you might dent it, or both. In any case, you’ve done work on it.

Life is superb at capturing energy through work. [...] Now we know why mighty trees don’t ungrow themselves: because life produces heat. From a physics perspective, a tree harvests energy from its surroundings – work is done on it – and in the process, it dissipates energy to the surrounding air as heat. The differences in probability between forward and reverse in such cases are staggering.
Reproduction: "self-replication is just one example of a more general class of processes that exhibit what we call positive feedback. Positive feedback can happen whenever there’s a quantity in a system whose increase brings about a rise in its own rate of growth." There's more, but you get the idea. Read the whole piece.

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