It seems obvious that I could not have written my impressions of this book before reading it, but to astrophysicist Sean Carroll, of the California Institute of Technology in Pasadena, there’s something profoundly strange about that. Why, he asks, can we move in any direction in space, whereas time only shuttles us from the past into the future? The key to understanding time’s one-way arrow, according to Carroll, is the second law of thermodynamics, which insists that entropy, a measure of disorder in a complex system of objects, inexorably increases with time. Put more succinctly: you can’t unscramble an egg, even though there’s no law of physics prohibiting each atom in the scramble from spontaneously returning to its proper place in the shell. It’s just supremely unlikely.
Paradoxically, when you look at a single particle, time actually is reversible, for all intents and purposes. Imagine a billiard ball caroming off the side of a pool table: you couldn’t tell whether a film of the collision is playing forward or backward. Clearly, the same is not true for complex systems. A film of a cue ball breaking a formation of fifteen colored balls is readily distinguishable from the reverse: fifteen colored balls assembling into a perfect triangle and spitting a single white one out of their apex. While the latter scenario is physically possible, it is vanishingly improbable—there are a gazillion ways a cue could break the colored balls, but only one arrangement of the velocities of the colored balls that could bring them to a stop in triangle formation. Time, in other words, flows in the direction of maximum statistical likelihood.
Recognizing the second law in action, however, is just the beginning of understanding time, which is why Carroll has written more than 400 pages on the subject. Since the entropy of the universe has been increasing for more than 14 billion years, it must have been in an exquisitely uniform—some astrophysicists would say “orderly”—state at the time of the big bang. But how could that be, Carroll asks, if, at the atomic level at least, time can roll backward just as easily as it can roll forward? And where is all the entropy leading? Will the universe continue to become more and more diffuse and disordered until the stars all burn out, the galaxies dissipate, and the whole thing becomes cold and lifeless?
The simple answer to all those questions is that we don’t know for sure. But Carroll is not one for simple answers. He examines his subject from a variety of viewpoints, writing clearly and expressively about the temporal implications of black holes, time machines, and theories of the expanding universe. He quotes liberally from such literary lights as Marcel Proust, T.S. Eliot, and the playwright Tom Stoppard. But despite its avoidance of equations, From Eternity to Here is not a book for the mathematically faint of heart. It presupposes a patient reader who is willing and able to follow some pretty complex logical and philosophical arguments. Still, it is one of the most lucid popular overviews of modern theoretical cosmology that I have read in recent years. And though you may not come away from it knowing exactly why you read the book from start to finish rather than the other way around, you will doubtless regard the experience as time well spent.