The Arrow of Time

The mysteries of black holes and supernovas notwithstanding, the universe on the whole is a ­law-­abiding place. From galaxies of stars to the tiny particles that con­sti­tute atoms, objects interact with each other according to rules that scientists think they understand. But one aspect of the universe has them baffled. That component is ­time.

There is a satisfying symmetry to the physical universe. For every action there is an equal and oppo­site reaction; for every negatively charged electron there is (presumably, somewhere) a positively charged positron. But time marches on in only one direction. One way of looking at this idea is that it is the stuff of the Back to the Future movies: It’s fun to think about traveling to the past, but you can’t actually do it. And ­entropy—­random­ness or ­disorder—tends to increase with time. That’s the second law of thermodynamics. So the universe has been steadily growing more disorderly. When you add milk to your coffee, the milk spreads randomly throughout the cup; it doesn’t spontaneously separate into a layer on top. Humpty Dumpty didn’t sud­denly reassemble himself; not even all the king’s horses and all the king’s men could put him together ­again.

But why should time go in only one direction? If the universe is otherwise symmetrical, what’s so special about time? Sean M. Car­roll, a senior research associate in physics at the California Institute of Technology, offers one possibility: Maybe, just maybe, ours is not the only universe there is. Maybe a big bang of the sort that is thought to have given birth to our universe happens every now and then. And maybe the arrow of time points in our direction (that is, toward the “future”) in half the universes and in the opposite direction (toward the “past”) in the other half. What if “we see only a tiny patch of the big picture, and this larger arena is fully time symmetric?” ­Carroll asks.

Not to worry, he says. In a uni­verse in which the “past” was the “future,” people wouldn’t be born old and die as infants. In the con­fines of their universe, everything would proceed as in ours. It is only when they compared their uni­verse to ours that anything would seem unusual. And each universe would be en­tirely separate and unknow­able to denizens of the other. Carroll can probably never be proved right or wrong. Regardless, the fact that milk spreads randomly through your coffee is another way of saying that time is always going in one direction, at least in this universe. In Carroll’s formulation, it takes the existence of a parallel uni­verse to preserve the symmetry of time, and the evidence comes from something no more elabor­ate than a cuppa ­joe.

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THE SOURCE: “Does Time Run Backward in Other Universes?” by Sean M. Carroll, in Scientific American, June 2008.

Photo courtesy of NASA