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By Root 589 0

much more energy to produce the orderly things than the amount of order they produce or store. Each region of local order is part of a bigger region in which there is almost always a net increase in entropy. Take, for example, a reference library whose books are put back in the right place every night. The amount of energy needed to restore order is always greater than the amount used up in creating disorder, but thanks to the input of organized energy by librarians, the next day it’s ready to be disordered all over again. As we’ll see, most biological order is preserved for long periods, but at the cost of vast increases in disorder elsewhere.

Now, it is crucial to note a significant feature of the second law. Unlike the other basic laws of nature, it’s asymmetrical. It tells us that when it comes to the amount of entropy in a region, and in the universe as a whole, the future is different from the past. Future states of the universe will almost always be different from past ones because entropy (disorder) very, very, very probably increases as time goes on. The second law is fundamentally different from all the other basic laws of physics that govern reality. All the other fundamental laws of the universe are time symmetrical. They work just as well to determine the order of events back in time as they do forward in time. Every process in the universe is reversible—except entropy increases.

Here are a couple of simple examples of this symmetry: You can predict all the solar eclipses to come. But you can also “post-dict” or “retrodict” all the solar eclipses there have ever been. Film the motions of the balls on a billiard table after the initial break: one is struck and starts a chain of balls successively striking other balls. Now run the film backward. Nothing unexpected, funny, or weird will leap out at you. After a while, you won’t even remember which was the rewind and which was the forward direction of events. Any trajectory, or path, taken by a set of atoms or molecules is reversible. It can run back the other way in accordance with the same laws of motion. Except for the second law, all the basic laws of physics allow the past states of things to be fixed as much by their future states as they allow future states to be fixed by past states. And if the facts about atoms and molecules fix all the other facts, then any process can go both ways—including the painting of the Sistine chapel. When viewed as a whole bunch of trajectories of individual paint molecules, they could have gone from the ceiling back into the paint pots—could have except for the second law. All the other laws of physics will allow it. The same goes for our example of the extinction of the dinosaurs in Chapter 1. Except for its entropy increase, it could have gone in the other direction.

Hard to believe, but the second law is where the direction of time, its asymmetry, comes from. It cannot come from anywhere else in physics. By process of elimination, the time order of events from earlier to later is a consequence of the second law. The universe started in a highly improbable state—an incredibly hot, highly energetic sphere smaller than a Ping-Pong ball—and then expanded into a ball that was no bigger than an orange when it was a millionth of a second old and has been spreading out ever since. With each second, it has attained a more and more probable state, and so have almost all of the regions of space within the universe, no matter how small they are. Had the universe started in a highly disorganized state, one of the most probable distributions of matter, energy, and heat, it would have moved to other states of equally high probability, states in which all the mass in the universe is spread out evenly, along with all the energy and heat.

All this means that the second law is really the result of applying the symmetrical “forward or backward” laws to the universe at its starting place, which just happens to have been very orderly, very concentrated, and very small. The result was the spreading out and cooling off that is, and will be,