Nothing in the laws of physics states that time cannot run backwards. The equations that determine how the earth revolves around the sun or how an apple falls from a tree work perfectly well whether time is running backwards or forwards. Still, in our own lives, we experience time as flowing from the past to the future. We remember the past. We grow older. We are born before we die.
Like us, the universe lives a uni-directional life, at least in some respects. From the Big Bang onward, the universe moves from a very orderly state to one that is very messy. It is this increase in disorder (in addition to our own personal experiences) that gives the appearance of time moving forward like an arrow headed in a single direction.
In a new paper, published last October in Physical Review Letters, researchers used gravity to show that this kind of unidirectional time can arise naturally, even when using laws of physics that work well in both temporal directions.
In a computer simulation, the researchers placed 1,000 particles in an infinite space, with one condition—they could only interact through gravity. Quantum physics was not allowed. Snapshots of these simulations showed that at some point the particles would always clump together in a ball with the smallest possible complexity. After that, complexity increased along both directions of time.
This simulation mirrors the increase of complexity seen in the universe from the point of the Big Bang onward. Based on this, say the researchers, it also means that the universe has one past with two possible futures, like two temporal arrows moving in opposite directions. To an observer in each universe, time would flow in a single direction toward a more disordered universe.
Their simulation also hints at an eternal universe, one leading in two directions from the same point of minimum complexity—with gravity propelling the single-headed arrow of time. The researchers have more work to do in order to show that their simulation is a good approximation of the actual universe, but they are confident in their approach.
“It’s the only clear, simple idea that’s been put forward to explain the basis of the arrow of time,” physicist Julian Barbour, a coauthor of the study, told Science News.