From Eternity to Here (36 page)

DOES LIFE MAKE SENSE?

It should come as no surprise that these ideas connecting entropy and information come into play when we start thinking about the relationship between thermodynamics and life. Not that this relationship is very straightforward; although there certainly is a close connection, scientists haven’t even yet agreed on what “life” really means, much less understood all its workings. This is an active research area, one that has seen an upsurge in recent interest, drawing together insights from biology, physics, chemistry, mathematics, computer science, and complexity studies.
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Without yet addressing the question of how “life” should be defined, we can ask what sounds like a subsequent question: Does life make thermodynamic sense? The answer, before you get too excited, is “yes.” But the opposite has been claimed—not by any respectable scientists, but by creationists looking to discredit Darwinian natural selection as the correct explanation for the evolution of life on Earth. One of their arguments relies on a misunderstanding of the Second Law, which they read as “entropy always increases,” and then interpret as a universal tendency toward decay and disorder in all natural processes. Whatever life is, it’s pretty clear that life is complicated and orderly—how, then, can it be reconciled with the natural tendency toward disorder?

There is, of course, no contradiction whatsoever. The creationist argument would equally well imply that refrigerators are impossible, so it’s clearly not correct. The Second Law doesn’t say that entropy always increases. It says that entropy always increases (or stays constant) in a closed system, one that doesn’t interact noticeably with the external world. It’s pretty obvious that life is not like that; living organisms interact very strongly with the external world. They are the quintessential examples of open systems. And that is pretty much that; we can wash our hands of the issue and get on with our lives.

But there’s a more sophisticated version of the creationist argument, which is not quite as silly—although it’s still wrong—and it’s illuminating to see exactly how it fails. The more sophisticated argument is quantitative: Sure, living beings are open systems, so in principle they can decrease entropy somewhere as long as it increases somewhere else. But how do you know that the increase in entropy in the outside world is really enough to account for the low entropy of living beings?

As I mentioned back in Chapter Two, the Earth and its biosphere are systems that are very far away from thermal equilibrium. In equilibrium, the temperature is the same everywhere, whereas when we look up we see a very hot Sun in an otherwise very cold sky. There is plenty of room for entropy to increase, and that’s exactly what’s happening. But it’s instructive to run the numbers.
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Figure 50:
We receive energy from the Sun in a concentrated, low-entropy form, and radiate it back to the universe in a diffuse, high-entropy form. For every 1 high-energy photon we receive, the Earth radiates about 20 low-energy photons.

The energy budget of the Earth, considered as a single system, is pretty simple. We get energy from the Sun via radiation; we lose the same amount of energy to empty space, also via radiation. (Not exactly the same; processes such as nuclear decays also heat up the Earth and leak energy into space, and the rate at which energy is radiated is not strictly constant. Still, it’s an excellent approximation.) But while the amount is the same, there is a big difference in the
quality
of the energy we get and the energy we give back. Remember back in the pre-Boltzmann days, entropy was understood as a measurement of the uselessness of a certain amount of energy; low-entropy forms of energy could be put to useful work, such as powering an engine or grinding flour, while high-entropy forms of energy just sat there.

The energy we get from the Sun is of a low-entropy, useful form, while the energy we radiate back out into space has a much higher entropy. The temperature of the Sun is about 20 times the average temperature of the Earth. For radiation, the temperature is just the average energy of the photons of which it is made, so the Earth needs to radiate 20 low-energy (long-wavelength, infrared) photons for every 1 high-energy (short-wavelength, visible) photon it receives. It turns out, after a bit of math, that 20 times as many photons directly translates into 20 times the entropy. The Earth emits the same amount of energy as it receives, but with 20 times higher entropy.

The hard part is figuring out just what we mean when we say that the life forms here on Earth are “low-entropy.” How exactly do we do the coarse-graining? It is possible to come up with reasonable answers to that question, but it’s complicated. Fortunately, there is a dramatic shortcut we can take. Consider the entire biomass of the Earth—all of the molecules that are found in living organisms of any type. We can easily calculate the maximum entropy that collection of molecules could have, if it were in thermal equilibrium; plugging in the numbers (the biomass is 10
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kilograms; the temperature of the Earth is 255 Kelvin), we find that its maximum entropy is 10
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. And we can compare that to the minimum entropy it could possibly have—if it were in an exactly unique state, the entropy would be precisely zero.

So the largest conceivable change in entropy that would be required to take a completely disordered collection of molecules the size of our biomass and turn them into absolutely any configuration at all—including the actual ecosystem we currently have—is 10
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. If the evolution of life is consistent with the Second Law, it must be the case that the Earth has generated more entropy over the course of life’s evolution by converting high-energy photons into low-energy ones than it has decreased entropy by creating life. The number 10
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is certainly an overly generous estimate—we don’t have to generate nearly that much entropy, but if we can generate that much, the Second Law is in good shape.

How long does it take to generate that much entropy by converting useful solar energy into useless radiated heat? The answer, once again plugging in the temperature of the Sun and so forth, is: about 1 year. Every year, if we were really efficient, we could take an undifferentiated mass as large as the entire biosphere and arrange it in a configuration with as small an entropy as we can imagine. In reality, life has evolved over billions of years, and the total entropy of the “Sun + Earth (including life) + escaping radiation” system has increased by quite a bit. So the Second Law is perfectly consistent with life as we know it—not that you were ever in doubt.

LIFE IN MOTION

It’s good to know that life doesn’t violate the Second Law of Thermodynamics. But it would also be nice to have a well-grounded understanding of what “life” actually means. Scientists haven’t yet agreed on a single definition, but there are a number of features that are often associated with living organisms: complexity, organization, metabolism, information processing, reproduction, response to stimuli, aging. It’s difficult to formulate a set of criteria that clearly separates living beings—algae, earthworms, house cats—from complex nonliving objects—forest fires, galaxies, personal computers. In the meantime, we are able to analyze some of life’s salient features, without drawing a clear distinction between their appearance in living and nonliving contexts.

One famous attempt to grapple with the concept of life from a physicist’s perspective was the short book
What Is Life?
written by none other than Erwin Schrödinger. Schrödinger was one of the inventors of quantum theory; it’s his equation that replaces Newton’s laws of motion as the dynamical description of the world when we move from classical mechanics to quantum mechanics. He also originated the Schrödinger’s Cat thought experiment to highlight the differences between our direct perceptions of the world and the formal structure of quantum theory.

After the Nazis came to power, Schrödinger left Germany, but despite winning the Nobel Prize in 1933 he had difficulty in finding a permanent position elsewhere, largely because of his colorful personal life. (His wife Annemarie knew that he had mistresses, and she had lovers of her own; at the time Schrödinger was involved with Hilde March, wife of one of his assistants, who would eventually bear a child with him.) He ultimately settled in Ireland, where he helped establish an Institute for Advanced Studies in Dublin.

In Ireland Schrödinger gave a series of public lectures, which were later published as
What Is Life?
He was interested in examining the phenomenon of life from the perspective of a physicist, and in particular an expert on quantum mechanics and statistical mechanics. Perhaps the most remarkable thing about the book is Schrödinger’s deduction that the stability of genetic information over time is best explained by positing the existence of some sort of “aperiodic crystal” that stored the information in its chemical structure. This insight helped inspire Francis Crick to leave physics in favor of molecular biology, eventually leading to his discovery with James Watson of the double-helix structure of DNA.
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But Schrödinger also mused on how to define “life.” He made a specific proposal in that direction, which comes across as somewhat casual and offhand, and perhaps hasn’t been taken as seriously as it might have been:

What is the characteristic feature of life? When is a piece of matter said to be alive? When it goes on ‘doing something’, exchanging material with its environment, and so forth, and that for a much longer period than we would expect an inanimate piece of matter to ‘keep going’ under similar circumstances.
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Admittedly, this is a bit vague; what exactly does it mean to “keep going,” how long should we “expect” it to happen, and what counts as “similar circumstances”? Furthermore, there’s nothing in this definition about organization, complexity, information processing, or any of that.

Nevertheless, Schrödinger’s idea captures something important about what distinguishes life from non-life. In the back of his mind, he was certainly thinking of Clausius’s version of the Second Law: objects in thermal contact evolve toward a common temperature (thermal equilibrium). If we put an ice cube in a glass of warm water, the ice cube melts fairly quickly. Even if the two objects are made of very different substances—say, if we put a plastic “ice cube” in a glass of water—they will still come to the same temperature. More generally, nonliving physical objects tend to wind down and come to rest. A rock may roll down a hill during an avalanche, but before too long it will reach the bottom, dissipate energy through the creation of noise and heat, and come to a complete halt.

Schrödinger’s point is simply that, for living organisms, this process of coming to rest can take much longer, or even be put off indefinitely. Imagine that, instead of an ice cube, we put a goldfish into our glass of water. Unlike the ice cube (whether water or plastic), the goldfish will not simply equilibrate with the water—at least, not within a few minutes or even hours. It will stay alive, doing something, swimming, exchanging material with its environment. If it’s put into a lake or a fish tank where food is available, it will keep going for much longer.

And that, suggests Schrödinger, is the essence of life: staving off the natural tendency toward equilibration with one’s surroundings. At first glance, most of the features we commonly associate with life are nowhere to be found in this definition. But if we start thinking about
why
organisms are able to keep doing something long after nonliving things would wind down—why the goldfish is still swimming long after the ice cube would have melted—we are immediately drawn to the complexity of the organism and its capacity for processing information. The outward sign of life is the ability of an organism to keep going for a long time, but the mechanism behind that ability is a subtle interplay between numerous levels of hierarchical structure.

We would like to be a little more specific than that. It’s nice to say, “living beings are things that keep going for longer than we would otherwise expect, and the reason they can keep going is because they’re complex,” but surely there is more to the story. Unfortunately, it’s not a simple story, nor one that scientists understand very well. Entropy certainly plays a big role in the nature of life, but there are important aspects that it doesn’t capture. Entropy characterizes individual states at a single moment in time, but the salient features of life involve processes that evolve through time. By itself, the concept of entropy has only very crude implications for evolution through time: It tends to go up or stay the same, not go down. The Second Law says nothing about
how fast
entropy will increase, or the particular methods by which entropy will grow—it’s all about Being, not Becoming.
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