Physics of Life (Script) (Patreon)

Our universe is prone to increasing disorder and chaos. So how did it generate the extreme complexity we see in life? Actually, the laws of physics themselves may demand it.

How did life begin? We can seek the answer in the chemistry of the early earth, or in the biology of the first cell. In fact our friends at PBS Eons and It’s OK to be Smart will do just that in companion videos to this one. But we all know that chemistry and biology are just applied physics. So can we approach the question of the origin and the very nature of life from the point of view of physics? We’re sure going to try.

To understand life we need to understand entropy. The universe tends towards disorder, decay, and equilibrium. A hot cup of coffee will tend towards the same temperature as the room, and the hot, dense early universe must expand. Stars always burn out, black holes evaporate. The particles that make up any system all have some degree of random motion. That random motion tends to drive the system towards the most common arrangements of particles. Such a random, disordered, “unspecial” arrangement is a high-entropy state.

Interesting arrangements, like thermal energy being concentrated in your cup of coffee, or all matter in the observable universe being crunched into an infinitely dense point – are low entropy. They’re highly specific configurations that almost never happen by chance.  So entropy is sort of a measure of the boringness of a system – the commonness of the arrangement of its particles. The second law of thermodynamics tells us that a closed system will only increase in entropy. Boringness tends to increase.

But there’s one type of system that seems to resist the second law of thermodynamics and maintain low entropy. We call such a system life. Life has a very low internal entropy because its structure is extremely specific and non-random. The molecular machinery of even a single cell defies belief. You know what? I’d prefer to let a real biologist convince you. Hey, Joe, tell em how it is.

[Joe’s bit]

Not only is life stunningly complex, but that complexity increases over extremely time timescales. In fact over the Eons, right Blake?

[Blake’s bit]

Naively, this preservation and increase in order appears to contradict the second law of thermodynamics. Entropy appears to either stay constant or decrease. The earth’s biosphere, at least, becomes less boring over time. But let me be clear – there’s NO violation of the second law. It tells us that CLOSED systems must increase in entropy – so systems unable to exchange energy with the outside environment. But living organisms and indeed the Earth’s biosphere are not closed. Both receive energy from outside. Ultimately, that source of energy is the Sun. Its light warms the atmosphere and the oceans, and it powers photosynthesis at the bottom of the food chain, driving a complex chain of nutrient synthesis that ends with whatever you had for dinner last night. 

On the other hand, the system of the Earth plus the Sun IS increasing in entropy. Life acts to reduce its own internal entropy by increasing the entropy of its surroundings. This was first pointed out by Ludwig Boltzmann, who described life as a struggle for entropy. Well, more accurately against entropy, or for negative entropy. Erwin Schrodinger, in his … book  What is Life, describes life as a process feeding on negative entropy. Life absorbs order, and it ejects disorder into is surroundings.

The order that life feeds on is free energy. By free energy, I mean the special, out-of-equilibrium energy sources like your cup of hot coffee or the sun. Another way to say this is that life feeds on energy gradients. When two systems with very different energy densities come into contact, energy must flow. Life feeds on that flow. In fact, the importance of energy gradients to life can help us understand the actual origin of life and its precursors.

The origin of life on Earth isn’t known. We think it started with a self-replicating molecule similar to RNA. The companion episode over on It’s OK to be Smart will get into the nitty-gritty of that. Following that synthesis, evolution took hold and the first protocell and then true, living cell pulled itself together. PBS Eons will cover that part, as they explore LUCA: the last universal common ancestor. But where on Earth did this all happen? There are a few hypotheses. Perhaps it was in tidal pools, or around deep sea hydrothermal vents, or even on the undersurface of Earth’s ice caps.

These environments share a critical property. They sit at persistent energy gradients. The water of tidal pools are both cooled by the earth and ocean and warmed by the Sun. Around deep sea vents the searing gases from Earth's hot interior meet the frigid water of the ocean depths. Beneath the thick ice caps there's the transition between the solid and liquid phases of water.

These are places struggling to return to equilibrium. These systems are doing their best to obey the second law by redistributing energy as evenly and randomly as they can. Heat energy flows from hot to cold seeking a uniform temperature, but energy is also dispersed into every form it can take consistent with the laws of physics. Some of that energy gets distributed into chemical bonds as simple molecules form via every chemical reaction that is possible given the available raw materials. As those molecules form, new channels open up for distributing energy – into the chemical bonds of increasingly complex molecules.

Normally, this local rise in complexity would all cease when the system reaches thermal equilibrium - energy is perfectly evenly distributed and new molecules break apart exactly as often as they are formed. But when our energy source is flowing into a much larger reservoir – like the ocean for example – then equilibrium is never reached. Complexity can increase indefinitely as a byproduct of the system striving to redistribute the endless gradient in energy.

And at some point, natural selection takes over. Molecules self-catalyze – they help drive the very reactions that create more of the same. Molecules better at that process become more abundant, at some point they become true self-replicators, and eventually they become life. But even life and self-replication might be a very natural part the same thermodynamic drive to dissipate energy.  Think about it: living things are incredible heat-dissipation, entropy-maximizing machines.

The most random possible form for energy is thermal radiation – and the lower the energy of its component photons the higher the entropy. A plant absorbs the concentrated ultraviolet light  from the Sun and reprocesses it into a much higher entropy infra-red heat glow. Animals consume high-energy-density packets of matter called food and convert it to lower-energy-density waste as well as that same infrared heat-glow.

Life is great at dissipating energy, and more generally, it may be that self-replicating systems are be the best possible energy dissipaters of all. This is a new idea proposed by MIT biophysicist Jeremy England, who puts the thermodynamics of life on more solid theoretical grounds. He has demonstrated, mathematically that self-replicating molecules and simple single-cell life are extremely good at shedding heat in the act of reproduction. Self-replication randomizes the environment, even if each new replicators is highly ordered. 

And it’s not just life that does this. Consider a whirlpool. It’s a highly ordered flow of water; persistent and organized. In order for a whirlpool to disperse it needs to get rid of angular momentum and energy. One way it can do this is to generate eddies. These mini-whirlpools can carry away energy and angular momentum, but they also carry away order. Zoom in on a forming eddy and it seems that the structure in the eddy spontaneously appeared in the smooth flow. Eddies can even catalyze the formation of more of its kind. These eddies are lower in entropy than their immediate surroundings, but the larger whirlpool is lower in entropy still. The whirlpool is the ultimate source of the eddy’s low entropy state, and the formation of eddies increases the entropy of the whirlpool.

So given a much larger source of order, the global process of dissipation of that order results in eddies of low entropy. Life appears to be just such an eddy of low entropy. In the case of life, the “whirlpool” or original source of extreme low entropy is the big bang itself. In the process of redistributing energy into the most random possible state, little eddies of order – like galaxies, stars, planets, and life – naturally arise. These blips in order are actually serving the second law, helping the universe disperse its early, extreme low-entropy state. So, I guess that makes you a little eddy of order; a momentary fluctuation of interesting, but ultimately in service of the spread of disorder and dullness. An agent in the inexorable trend to maximize the entropy of space time.