Hindsight, as they say, is 20/20, but sometimes it would be nice to have known the outcomes before making a choice. This is as true in day-to-day life as it is in quantum mechanics. But it seems that the quantum world has something we do not have: a way to alter yesterday’s choices today, before they become tomorrow’s mistakes.
None of this is real time-travel. Physicists remain skeptical about that possibility. However, it is possible to simulate a closed time-loop with quantum mechanics, thanks to the property of entanglement. When two particles are entangled, they are in a single state even if they are separated by huge distances. A change to one is a change to the other, and this happens instantaneously.
So a particle can be prepared for an experiment, entangled, and sent to the experiment. Then scientists can modify its entangled companion, changing the way the particle in the experiment behaves.
“In our proposal, an experimentalist entangles two particles,” co-author Nicole Yunger Halpern, researcher at the National Institute of Standards and Technology (NIST) and the University of Maryland, said in a statement. “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.”
“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,” said lead author David Arvidsson-Shukur, from the Hitachi Cambridge Laboratory. “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.”
You might be thinking, if they are changing the outcome of the experiment, could they truly be time-traveling? Unfortunately not. The setup is a simulation because it produces this effect probabilistically. So, there are a certain number of times when it looks like you have time-traveled, but not every time.
“The effect is remarkable, but it happens only one time out of four!” said Arvidsson-Shukur. “In other words, the simulation has a 75% chance of failure. But the good news is that you know if you have failed. If we stay with our gift analogy, one out of four times, the gift will be the desired one (for example a pair of trousers), another time it will be a pair of trousers but in the wrong size, or the wrong colour, or it will be a jacket.”
So you can refine an experiment in this way after the fact by sending a lot of entangled photons – particles of light – with some of them carrying the correct information. You would use a filter to actually see the photons that would appear to have time-traveled.
“That we need to use a filter to make our experiment work is actually pretty reassuring,” said Arvidsson-Shukur. “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.”
The study is published in Physical Review Letters.
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