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The arrow of time

The universe may have one past (the Big Bang) and two futures

2:23pm, July 10, 2015
time illustration

LOOKING TO THE FUTURE(S)  In this illustration, the dual faces of the Roman god Janus gaze at the universe in opposing directions of time. A new simulation suggests that gravity sends an orderly universe in two temporal directions. 

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

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