As Huxley and Hooker were defending Darwin in the second half of the nineteenth century, physicists were elucidating the laws of thermodynamics. The first law states only that energy is conserved and remains constant, but it changes and flows from state to state. This set up the need for the second law, which provided the "motive force" for energy flow. An early observer, Sadi Carnot, compared a steam engine to a mill wheel; the fall of heat from higher to lower temperatures drove the steam engine, just as the falling of the water turned the mill wheel. If the energy in the system remained constant, what explained the change in the system? Something had to be driving the change in energy from one system to another and physicists called that force entropy. In Carnot's mechanical systems, entropy was dissipated potential. If there is an imbalance between states of energy, something called a field is set up. The potential difference of that field generates entropy—the need to equalize and de-organize that potential difference. Natural processes occur because of the "need" to dissipate the potential difference of energy (and of matter, which Einstein showed us was just a different form of energy).

The second law of thermodynamics states that entropy is always positive, meaning that in a closed system it always increases. Stated most simply by Clausius in 1865, "The energy of the world remains constant. The entropy of the world strives to a maximum." Most of us understand it as the notion that systems tend toward disorganization, which is the interpretation Boltzmann used to explain the behavior of gases.

To describe it scientifically is harder, but worth trying. We know that when two solutions—one hot and one cold—are placed together, the molecules of each will mix, and the heat will dissipate. On a microscopic level the molecules spread out and mix randomly, becoming disorganized. Macroscopi-cally, the random mixing makes the mix uniform—the whole solution comes to the same, lukewarm temperature.

This law of increasing entropy has implications for evolution. At first glance, life seems to contradict entropy because life is organized. Natural theology was all about how precisely well adapted living things are for their environments; this requires organization and the clever hand of a creator. Thus, that life could '"just happen" without external intervention seemed impossible, and this seemed even more substantiated by the second law of thermodynamics.

On the other hand, it was increasingly clear that living organisms followed the law of entropy very well. Chemists were working out the steps of the respiration of plants and animals. They compared the process to combustion; all organisms did was burn the molecules available to them, slowly, step by step, and gather energy from this slow combustion. Metabolism was known to be a series of steps of molecular change, with each providing a dollop of energy, like Car-not's stream dropping into each cup of the mill wheel. If the organism loses these molecular cascades, the energy escapes and without it the complex organization crumbles. The system reaches maximal entropy—the organism dies.

So how could life occur in a universe where entropy ruled? How could organization come about without outside input? One answer to this came quickly in entropy's history from quantum physicist Erwin Schrodinger. In his book What Is Life?, Schrodinger pointed out that although entropy is irresistible for a closed system, it was possible to divert it, for it to spin off into what he called "eddies of order." That, he proposed, was life—from organisms to ecosystems; as long as the whole system was far from equilibrium (and the sun still providing energy made it so), these pools and eddies could hold entropy at bay, at least temporarily. What an organism feeds on, then, is negative entropy, what Schro-dinger called "negentropy." He wrote, "the essential thing in metabolism is that the organism succeeds in freeing itself from all the entropy it cannot help producing while alive."

By not just subduing entropy but co-opting it, Schro-dinger succeeded in keeping life within the framework of physical laws. In doing this he also started to free us from a static perspective. Instead of seeing the world as resting at some equilibrium point, he persuaded us to see that our world is far from equilibrium. The apple has not yet hit the ground; the pendulum is still swinging. Schrodinger showed that this is how life could exist in a world driven by entropy.

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