If a cup of water spills on the floor, the water can’t unspill—that is, it’s inconceivable that each water molecule would exactly reverse its course to slip back into the cup. To do so would be to turn back time—something that, as far as we know, can’t be done. The water either spills or it doesn’t, but if it does, it’ll stay that way.
In that way, time as we experience it is asymmetric. We have memories of the past rather than the future, and spilled water doesn’t flow back to its cup, just as an arrow that has been let fly doesn’t return to its bow. In our everyday lives, the “arrow of time” goes only in one direction: forward.
“We know [this] is something that’s part of our common experience,” says Andrea Rocco, a theoretical physicist at the University of Surrey in England. But how exactly time’s arrow arises is less clear to physicists, in part because the math they use to describe most of the world makes no distinction between time that moves forward and time that moves backward; either direction is perfectly viable, as far as their equations are concerned.
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