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By Root 599 0

the motion of molecules—does any useful work, some of the heat is wasted. In fact, wasting energy is the ruling principle of this universe, and that is the second law. To see why, suppose you squeeze all the air molecules in the pump into a single layer at the bottom of it, or as close as you can get to a single layer. Now let go of the handle. It goes up, of course, as the air molecules come bouncing off the bottom. As they do, the molecules will spread out evenly throughout the cylinder, or at least it’s fantastically probable that they will do so. Suppose there are a million air molecules at the bottom of the pump. When they push the pump piston up and spread out evenly in the cylinder, there will be many more than a million times a million different ways the 1 million air molecules can be arranged throughout the cylinder. All of these arrangements are equally possible. And of course there is no reason why the molecules will end up distributed evenly throughout the cylinder except that an even distribution is the most probable one. The starting point, when all the molecules are flat against the bottom of the cylinder, is one of the most improbable distributions of air molecules in the pump. The second law tells us that in our universe, the arrangement of everything goes from more improbable distributions (with more useful energy) to less improbable ones (with less useful energy). And the same goes for any self-contained part of the universe.

The more improbable distributions are the ones that are more neatly arranged, better organized, than others. The less improbable distributions are the more disorderly, less organized ones. The most probable distribution of matter and energy in the universe is the completely even distribution of everything. This is the one in which there is no region left with more energy, more order than others. The even distribution of energy and disorder in the universe is the state toward which, according to the second law, everything is moving, some places slower, some places faster, but almost inexorably. This evening-out of things—from molecules to galaxies—from less probable to more probable distributions is the rule of the universe. Increasing disorganization is inevitable, or almost so.

The physicist’s term for disorder is entropy. The entropy of a region measures how much order there is in the region. But it measures orderliness backward, because the more disorganized a region, the higher its entropy. The second law says that entropy, or disorder, almost always increases. The “almost” is important. Lord Kelvin thought that the second law was a strict statement of invariable entropy increase. Thanks to the work of three nineteenth-century geniuses—James Clerk Maxwell, Ludwig Boltzmann, and J. Willard Gibbs—the increase in entropy has been recognized to be only very, very, very probable, not everywhere and not always inevitable. It’s barely possible (but extremely improbable) that the cream poured into a cup of coffee will stay right where it hits the coffee and not spread out. All that is needed is that the cream molecules collide with the coffee molecules and with each other in such a way that one after another they bounce back in the direction they came from instead of wandering out into the hot liquid. Possible, but very, very, very improbable.

The second law can’t be a statement of the absolute inevitability of increasingly disordered states everywhere and always. It requires only the extremely probable increase of entropy from moment to moment in a closed system—the whole universe or some isolated part of it. Of course, in this universe there are some areas of very low entropy: the insides of stars, the cylinder heads of gas-guzzling internal combustion engines, and of course the highly organized biological systems on this planet—from single cells to plants to us, for example. These are not counterexamples to the second law. Nor are they just some of the improbable outcomes that it permits. These are regions of the universe in which the maintenance of order is being paid for by using