The Direction of Time

Irreversible Processes

The second law of thermodynamics describes the fact that many physical processes never happen in reverse. A process is irreversible when the net change in entropy is positive—that is, when the total entropy of the system and its environment is raised in the process. According to the second law, the total entropy of a system and its environment never decreases, and therefore any process that results in a net increase can’t be reversed. Ice melting in hot coffee and air leaking out of a punctured bicycle tire are examples of irreversible processes. Ice cubes don’t spontaneously form in hot coffee, since that would require heat to flow spontaneously from cold to hot. Punctured tires don’t spontaneously re-inflate, since that would require air to flow from low pressure to high pressure.

Irreversible processes are occurring practically everywhere, all the time. Basically, anything that would look strange in a movie played backward is probably an irreversible process. For example, imagine watching a movie of an apple falling from a tree to the ground. Now play the movie backward: the apple leaps off the ground and flies upward toward the tree branch. The time-reversed scenario you’ve just imagined involves a violation of the second law of thermodynamics. In ordinary (non-reversed) circumstances, when a falling apple hits the ground its kinetic energy is converted to heat, as explained earlier in this chapter. That heat promptly disperses through the apple and nearby soil, raising their temperatures. In order for this whole process to be reversed, the temperature (average kinetic energy) of the molecules near the bottom of the apple would have to increase significantly, so that those molecules have enough energy to bounce against the apple—all at the same time—and propel it upward toward the tree branch. Thus, heat would have to flow spontaneously from cold to hot, violating the second law. Although violations of the second law are technically possible, as discussed in the previous section, obviously it’s not going to happen in real life. (And by the way, this isn’t what happens when an apple bounces off the ground after a hard impact. Objects bounce when elastic potential energy is converted into kinetic energy—a conversion that is fully reversible. We’ll come back to that point on the next page.)

Likewise, any process involving friction is irreversible, because friction converts kinetic energy to heat, which then flows away from the hotter surfaces where the friction is occurring. For example, as a hockey puck slides to a stop on an icy rink, its kinetic energy is converted to heat due to friction with the ice. The heat quickly disperses through the puck and the ice, raising their temperatures slightly. Imagine how strange it would be for the reverse of that process to happen! Could thermal energy flow from the ice into the puck in a way that makes the puck speed up rather than slow down as it slides across the ice? For such an unlikely phenomenon to occur, heat would have to flow spontaneously toward, rather than away from, the slightly warmer surfaces of the puck and ice. The faster-moving molecules near these surfaces would also have to bounce against each other in just the right way so that they exert a net force that accelerates the puck. The chances of that occurring are so overwhelmingly tiny that you can bet your britches it will never happen.