The Quantum Experiment That Simulates A Time MachinePhysicists have simulated a photon interacting with an older version of itself in an experiment that could help reconcile quantum mechanics and relativity

One of the curiosities of general relativity is that it seems to allow time travel. Various physicists have discovered solutions to Einstein’s field equations that contain loops that return to the same point in space and time. Physicists call them closed time-like curves.

At first glance, these kinds of time machines seem to lead to all kinds of problems, such as the grandfather paradox. This is where somebody travels back in time and kills their grandfather meaning they could never have been born and so could not have gone back to kill the grandfather.

That’s just bizarre so physicists have attempted to find ways to prevent these paradoxes. In the early 90s, for example, cosmologists showed that a billiard ball entering a wormhole that leads to a closed time-like curve must always meet its older self coming out of the wormhole. What’s more, the resulting collision always prevents the ball entering the wormhole in the first place. In other words, the billiard ball would simply bounce off the entrance to a closed time-like curve.

So much for classical objects and time travel. But what would happen if a quantum particle entered a closed time-like curve? In the early 90s, the physicist David Deutsch showed that not only is this possible but that it can only happen in a way that does not allow superluminal signalling. So quantum mechanics plays havoc with causality but in a way that is consistent with relativity and so prevents grandfather-type paradoxes.

Deutsch’s result has extraordinary implications. It implies that closed time-like curves can be used to solve NP-complete problems in polynomial time and to violate Heisenberg’s uncertainty principle.

As far as we can tell, nobody has ever created a Deutsch closed time-like curve. So it’s easy to imagine that until we do, we will never know whether Deutsch’s predictions are true. But today Martin Ringbauer and a few pals at the University of Queensland in Australia say that it’s not necessary to create a closed time -like curve to test how it behaves.

Instead, these guys have created a quantum system that reproduces the behaviour of a photon passing through a closed time-like curve and interacting with its older self. In other words, these guys have built a time machine simulator.