Web edition: June 4, 2010
Print edition: June 19, 2010; Vol.177 #13 (p. 30)
Scientists have never been able to clearly explain why the laws of physics, on paper, work equally well forward or backward in time (see essay on Page 26) yet real life offers only a one-way street into the future. Innumerable books have been written about the conundrum of time’s direction, or “arrow,” but none have succeeded in answering the question to everyone’s satisfaction. So now there’s another one.
Caltech physicist Sean Carroll’s articulate exposition avoids any pretension of solving the problem. Instead, he tells a rich story of the various attempts to track time’s arrow to its source, which clearly has something to do with the second law of thermodynamics. That law requires the amount of disorder, or entropy, in a closed system to stay the same or increase over time until reaching its maximum possible level. Time marches forward because entropy in the past was lower than now. But that explanation merely restates the problem by defining “the past” as a time of lower entropy. Explaining time’s arrow, Carroll asserts, requires explaining why entropy was so low at the Big Bang.
He suggests that unknown universes exist, some with an arrow of time pointing in the opposite direction. Thus the whole “multiverse” of universes has overall time symmetry, while the universe that humans inhabit travels along its entropy-driven one-way street. In describing how this might happen, Carroll provides many entertaining and intuitively graspable descriptions of bizarre phenomena, from the weirdness of quantum entanglement to the birth of baby universes out of nothingness. To be sure, time’s arrow remains something of a mystery, but much of the physics surrounding the paradox is demystified for the diligent reader by Carroll’s expressive account.
Dutton, 2010, 438 p., $26.95.