Does time have a direction?
You’d think so. When you pour cream into coffee, it swirls around and paints your cup o’ joe a lighter brown—the cream doesn’t tend to funnel itself out of your coffee and back into your milk carton. That’s because the second law of thermodynamics states that the entropy (or disordered-ness) of a system always increases until it reaches thermal equilibrium, when all available energy has been dispersed. In other words, there’s a temperature disparity between the coffee and the cream, and their collision brings chaos to the cup. The process—the implied passing of time from start to finish—gives all those molecules a chance to sort out their differences, to distribute, and come to a more relaxed state of disarray. Entropy requires time to have one direction.
Although this “arrow of time” governs the universe as a whole, physicists suspect that the rules are different on the quantum mechanical level, particularly when it comes to a phenomenon known as entanglement. Entangled particles start off close together, and when separated, they’re able to communicate with each other and share information at a rate that’s seemingly instantaneous—faster than the speed of light. Whatever one particle does, the other follows suit in a consistent way. But according to the theory of relativity, nothing can travel that quickly.
So some physicists and philosophers, including Huw Price from the University of Cambridge, are now proposing a solution, something they call “retrocausality.” What if, they posit, entangled particles are traveling not just in space, but in time? Here’s George Musser, writing for Nautilus:
Suppose it is not the case that the particles (or dice) communicate instantaneously with each other, and it is also not the case that their values were fixed in advance. There seem to be no options remaining. But here Price asks us to consider the impossible: that doing something to either of the entangled particles causes effects which travel backward in time to the point in the past when the two particles were close together and interacting strongly. At that point, information from the future is exchanged, each particle alters the behavior of its partner, and these effects then carry forward into the future again. There is no need for instantaneous communication, and no violation of relativity.
Basically, Price is saying that if we can’t have instantaneous information exchange over a physical distance, then maybe entangled particles whisper information in each others’ ears across the vastness of time. Then it, in turn, brings that information with it to the future. It’s like sending a letter back in time to a friend—you send it, but even before you’ve gotten to the post office, she already knows the secret message inside because, at one point, you were roommates.
In our non-quantum lives, we can’t see these things happening. We’re locked into our perception of time and causality. Time is still a forward arrow, and action comes before reaction.
At the particle level, though, some physicists believe this logic could be sound, and they’re beginning to use it to explain existing results. (Musser’s article has more.) But there’s still some work to do. Scientists need to come up with a more fleshed-out theory of retrocausality that can compete with the way we’ve traditionally thought about quantum mechanics.