The arrow of time

Andrew Grant writes: In T.H. White’s fantasy novel The Once and Future King, Merlyn the magician suffers from a rare and incurable condition: He experiences time in reverse. He knows what will happen, he laments, but not what has happened. “I have to live backwards from in front, while surrounded by a lot of people living forwards from behind,” he explains to a justifiably confused companion.

While Merlyn is fictional, the backward flow of time should not be. As the society of ants in White’s novel proclaimed, “everything not forbidden is compulsory,” and the laws of physics do not forbid time to run backward. Equations that determine the acceleration of a rocket or the momentum of a billiard ball all work just as well with time flowing backward as forward. Yet unlike Merlyn, we remember the past but not the future. We get older but never younger. There is a distinct arrow of time pointing in one direction.

For nearly 140 years, scientists have tried to rule out the backward flow of time by way of nature’s preference for disorder. Left alone, nature transforms the neat into the messy, a one-way progression that many physicists have used to define time’s direction. But if nature prefers disorder now, it always has. The challenge is figuring out why the universe started out so orderly — thereby allowing disorder to grow and time to march forward — when the early universe should have been messy. Despite many proposals, physicists have not been able to agree on a satisfying explanation. [Continue reading…]

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