Physicists worry about the contrast between the obviously irreversible, one-directional nature of time in the macroscopic world, and the apparent complete reversibility of all of the laws of mechanics. Any molecular collision or reaction can take place equally well in a forward or a reverse direction.

If we were able to make molecular scale "home movies" of any atomic process, we would have no clue from the movie alone as to whether it was being run forward or backward . If the forward process was a reaction of molecules of H2 and I2 to form two HI molecules, the reverse process would be equally possible collision of two HI molecules to form H2 and I2.

One would have no basis for defining a "positive sense" to the flow of time at the molecular level. This is known as the principle of microscopic reversibility in physics and chemistry: On the molecular scale, if a forward reaction is possible, the reverse reaction must be equally possible.

In contrast, the positive direction of time presents no problem at the level of the macroscopic world. Movies run backward are considered hilarious just because they violate common sense so completely. At the top of the next page is a series of frames representing the simple operation of dissolving ink in water.

At the bottom of the page are the same frames in reverse order. We have no difficulty in saying that the top order is right and the bottom order is wrong. But what is the basis for this decision? The basis is our deeply ingrained, intuitive realization that order can lead to chaos, but chaos never spontaneously produces order. Beginning students sometimes feel that entropy is an abstruse concept, but we all appreciate what it means even before we put a name to it.