A new technique for quantum computing could bust open our whole model of how time moves in the universe.
Here’s what’s long seemed to be true: Time works in one direction. The other direction? Not so much.
That’s true in life. (Tuesday rolls into Wednesday, 2018 into 2019, youth into old age.) And it’s true in a classical computer. What does that mean? It’s much easier for a bit of software running on your laptop to predict how a complex system will move and develop in the future than it is to recreate its past. A property of the universe that theorists call “causal asymmetry” demands that it takes much more information — and much more complex calculations — to move in one direction through time than it does to move in the other. (Practically speaking, going forward in time is easier.)
This has real-life consequences. Meteorologists can do a reasonably good job of predicting whether it will rain in five days based on today’s weather radar data. But ask the same meteorologists to figure out whether it rained five days ago using today’s radar images? That’s a much more challenging task, requiring a lot more data and much bigger computers.
Information theorists suspected for a long time that causal asymmetry might be a fundamental feature of the universe. As long ago as 1927, the physicist Arthur Eddington argued that this asymmetry is the reason we only move forward through time, and never backward. If you understand the universe as a giant computer constantly calculating its way through time, it’s always easier — less resource-intensive — for things to flow forward (cause, then effect) than backward (effect, then cause). This idea is called the “arrow of time.”
But a new paper, published July 18 in the journal Physical Review X, opens the door to the possibility that that arrow is an artifact of classical-style computation — something that’s only appeared to us to be the case because of our limited tools.