For most of us, the passage of time flies in just one inexorable direction.
But for theoretical quantum physicists, time's direction isn't quite so inflexible. It's possible to theoretically model, simulate, and observe the backwards flow of time in ways that are impossible to achieve in the real world.
And now, scientists have shown that simulations of backwards time travel can help solve physics problems that cannot be resolved with normal physics.
Led by physicist David Arvidsson-Shukur of Cambridge University, a team of physicists conducted an experiment in which the input state can be altered by simulating a backwards loop of time that allows them to alter the parameters after they have already been set.
These loops are purely hypothetical, of course – but they can be simulated using quantum teleportation circuits created with entangled particles, in order to mathematically solve problems.
"Imagine that you want to send a gift to someone: you need to send it on day one to make sure it arrives on day three," Arvidsson-Shukur explains. "However, you only receive that person's wish list on day two. So, in this chronology-respecting scenario, it's impossible for you to know in advance what they will want as a gift and to make sure you send the right one.
"Now imagine you can change what you send on day one with the information from the wish list received on day two. Our simulation uses quantum entanglement manipulation to show how you could retroactively change your previous actions to ensure the final outcome is the one you want."
Quantum entanglement is a state in which the properties of two particles become linked prior to being measured. Measuring the properties of one particle immediately establishes the complementary state of the other, regardless of how far apart they might be.
Scientists have even been able to influence the properties of one particle, and observe simultaneous changes in the other, across a significant distance. That's quantum teleportation.
The team's work leverages entangled particles not just to teleport information across physical space, but backwards through time as well.
"In our proposal, an experimentalist entangles two particles," says physicist Nicole Yunger Halpern of the National Institute of Standards and Technology (NIST) and the University of Maryland.
"The first particle is then sent to be used in an experiment. Upon gaining new information, the experimentalist manipulates the second particle to effectively alter the first particle's past state, changing the outcome of the experiment."
The nature of the closed loop in time also isn't the kind that would allow anybody to travel back and paradoxically kill their grandfather, relying on a condition in probability called postselection, which restricts measures based on set events.
The team does not make the argument that such loops exist. Quantum theory, they say, allows for the simulation of these loops that, as a consequence, entanglement can exploit.
Their calculations show that the time loop can be successfully exploited only 25 percent of the time; but this means that it is testable in a real experiment.
This experiment is yet to be performed, but it can be done on a large scale by entangling vast numbers of photons – quanta of light – and using time travel simulations to alter their states after they have been sent towards a special camera, with a filter designed only to detect the photons with the updated information.
The detection of these photons would mean that the simulation has worked.
"That we need to use a filter to make our experiment work is actually pretty reassuring. The world would be very strange if our time-travel simulation worked every time. Relativity and all the theories that we are building our understanding of our Universe on would be out of the window," Arvidsson-Shukur says.
"We are not proposing a time travel machine, but rather a deep dive into the fundamentals of quantum mechanics. These simulations do not allow you to go back and alter your past, but they do allow you to create a better tomorrow by fixing yesterday's problems today."
The research has been published in Physical Review Letters.