Let's do the following Gedankenexperiment: Suppose I show you a movie of a swimming pool full of waves and splashes; suddenly (in the movie) all the waves come together and squirt a diver out of the pool. She flies gracefully through the air to land on the diving board while the pool's surface has miraculously returned to mirror smoothness. What is wrong with this picture? Wait! Before you answer, you also get the following movie: A box full of 100 black and 100 white marbles sits on a table; the marbles are arranged randomly. An anonymous assistant picks up the box, closes the lid, shakes the box for a while, puts it down and opens the lid. All the white marbles are now on the left side and all the black marbles are on the right side. Why do you keep thinking there is a problem? Try this: The same box, the same assistant, the same story; except this time there are only 4 marbles, two of each. Not so sure, hmmm? How about 2 marbles, one black and one white? Now we can't tell a thing about whether the movie is being shown forward or backward, right? What is going on here?
Our concept of the "arrow of time" is somehow bound up with statistical mechanics and is alarmingly fragile -- we can lose our bearings completely just by confining our attention to too small a system! As we will see later, the "fundamental" laws governing the microscopic interactions of matter will be no help at all in clarifying this mystery.