“We call it the two-headed arrow of time.”
All of us experience time moving forward — we age, and have memories from the past, but no idea what will happen in the future. However, nothing in the laws of physics asserts that time must move forward.
Is it possible that there exists a place where time actually moves backwards? In theory, yes, in what are called mirror universes.
To understand why time seems to only move in one direction, many physicists believe it comes down to entropy, the level of molecular disorder in a system, which is always increasing. However, two separate groups of physicists are working on models that look at how the initial conditions, such as the Big Bang, might have created the arrow of time, and remarkably, both show time moving in two different directions.
When the Big Bang created our universe, it may have also created a mirror universe where time moves in the opposite direction. So from our perspective, time in this parallel universe moves backwards. However, anyone in the parallel universe would perceive our universe’s time as moving backwards. Crazy!
The first model, published just over a year ago in Physical Review Letters, states that one of the implications of Newton’s theory of gravity creates the conditions for time to move in a certain direction.
Julian Barbour from the University of Oxford, Tim Koslowski from the University of New Brunswick, and Flavio Mercati from the Perimeter Institute for Theoretical Physics argue that for any confined system of particles — for example, a self-contained universe, not affected by anything outside of it, like ours — gravity will create a point when the distance between particles is minimal.
And when the particles begin to expand outwards, they will do so in two different directions. Barbour and his colleagues created a simple 1,000 particle point model of the universe showing this dual expansion, with gravity creating structure in both directions.
The moment before the expansion is called the Janus point. “Time is not something that pre-exists,” Barbour told Quartz. “The direction and flow of time we have to deduce from what’s happening in the universe. When we look at it that way, it’s natural to say that time begins at that central point and flows away in opposite directions.”
Unfortunately, even though time could be flowing in different directions, we will probably never be able to experience the reverse. “We’re on one side of the Janus point,” Mercati told Quartz. “On one side you get your arrow of time and can never experience the other one. It’s in your past.”
According to New Scientist, two other physicists, Sean Carroll from California Institute of Technology in Pasadena and Alan Guth from Massachusetts Institute of Technology, created a similar model that shows time moving in two different directions, in two parallel universes, from the Big Bang.
It is a more simplified model than Barbour’s, since it does not rely on gravity or particles being in a confined system — rather it uses entropy alone and particles in an infinite space. Their results show half of the particles expanding outward, increasing entropy, and the other half converging and becoming highly compact, decreasing entropy until they pass through the system’s central point and create entropy in the opposite direction.
“We call it the two-headed arrow of time,” Guth told New Scientist. “Because the laws of physics are invariant, we see exactly the same thing in the other direction.”
This theory is still far from widely accepted, but it is based on classical physics. Once quantum physics enters the equation, “all bets are still off,” according to Barbour. Time could be “more like a fountain where you have lots of pairs of springs. Or just a whole host of springs flowing out of a fountain in different directions.”
Maybe there is more to time than our linear, one-directional experience.