[By the way: are we going too slowly? If there is overwhelming sentiment to move to two chapters per week, that would be no problem. But if sentiment is non-overwhelming, we'll stick to the original plan.]
While it’s true that the presence of the Earth beneath our feet picks out an “arrow of space” by distinguishing up from down, it’s pretty clear that this is a local, parochial phenomenon, rather than a reflection of the underlying laws of nature. We can easily imagine ourselves out in space where there is no preferred direction. But the underlying laws of nature do not pick out a preferred direction of time, any more than they pick out a preferred direction in space. If we confine our attention to very simple systems with just a few moving parts, whose motion reflects the basic laws of physics rather than our messy local conditions, there is no arrow of time—we can’t tell when a movie is being run backward…
The arrow of time, therefore, is not a feature of the underlying laws of physics, at least as far as we know. Rather, like the up/down orientation space picked out by the Earth, the preferred direction of time is also a consequence of features of our environment. In the case of time, it’s not that we live in the spatial vicinity of an influential object, it’s that we live in the temporal vicinity of an influential event: the birth of the universe. The beginning of our observable universe, the hot dense state known as the Big Bang, had a very low entropy. The influence of that event orients us in time, just as the presence of the Earth orients us in space.
This chapter serves an obvious purpose — it explains in basic terms the ideas of irreversibility, entropy, and the arrow of time. It’s a whirlwind overview of concepts that will be developed in greater detail in the rest of the book, especially in Part Three. As a consequence, there are a few statements that may seem like bald assertions that really deserve more careful justification — hopefully that justification will come later.
Here’s where I got to use those “incompatible arrows” stories I blogged about some time back (I, II, III, IV). The fact that the arrow of time is so strongly ingrained in the way we think about the world makes it an interesting target for fiction — what would happen if the arrow of time ran backwards? The straightforward answer, of course, is “absolutely nothing” — there is no prior notion of “backwards” or “forwards.” As long as there is an arrow of time that is consistent for everyone, things would appear normal to us; there is one direction of time we all remember, which we call “the past,” when the entropy was lower. It’s when different interacting subsystems of the universe have different arrows of time that things get interesting. So we look briefly at stories by Lewis Carroll, F. Scott Fitzgerald, and Martin Amis, all of which use that trick. (Does anyone know of a reversed-arrow story that predates Through the Looking Glass?) Of course these are all fantasies, because it can’t happen in the real world, but that’s part of the speculative fun.
Then we go into entropy and the Second Law, from Sadi Carnot and Rudolf Clausius to Ludwig Boltzmann, followed by some discussion of different manifestations of time’s arrow. All at lightning speed, I’m afraid — there’s a tremendous amount of fascinating history here that I don’t cover in anywhere near the detail it deserves. But the real point of the chapter isn’t to tell the historical stories, it’s to emphasize the ubiquity of the arrow of time. It’s not just about stirring eggs to make omelets — it has to do with metabolism and the structure of life, why we remember the past and not the future, and why we think we have free will. Man, someone should write a book about this stuff!