New Scientist
Physics may not have allowed a sci-fi mailbox to exist yesterday, but quantum theory is testing something almost as strange: information moving backwards through time.
We may have figured out a way to send messages backwards in time
Author: Karmela Padavic-Callaghan
WHAT if you could send a message into the past? The laws of physics don’t forbid it – and in fact, in some cases, communicating backwards in time might actually be easier than the usual direction.
The possibility of sending a message to the past emerges from a particular kind of solution to the equations of general relativity, our best theory of how space-time, the fabric of reality, behaves. Every object in the universe follows a path through space-time, and one such path allowed by general relativity is called a closed time-like curve (CTC), which sees an object travel into the future before returning to the past and ending up in the present, forming a time loop.
There is just one catch: at cosmic scales, building a CTC would mean bending space-time until it closes in on itself, which would require an impossibly large amount of energy. But quantum entanglement might offer a solution.
When two particles are entangled, the state of one is always sensitive to the state of the other, even when they are extremely distant. Instead of interpreting this as the two being part of some very spread-out quantum state, some physicists posit that one particle’s sensitivity to what is happening to the other comes from that second particle sending messages backwards in time to the first, alerting it how to react later.
This possibility emerges from a particular kind of solution to the equations of general relativity
Not everyone agrees with this interpretation, but in 2010, Seth Lloyd at the Massachusetts Institute of Technology and his colleagues used entangled particles of light, or photons, to mimic a quantum CTC. “It was the equivalent of sending a photon a few nanoseconds backwards in time, and having it try to kill its former self,” says Lloyd.
Now, Lloyd and his colleagues have imagined a new version of the experiment in which something goes wrong and the CTC becomes crackly and noisy, like a faulty phone line. Evaluating the messaging capacity of a noisy channel is a common problem in information theory, and using this framework, the team found that not only is communication with the past still possible, but it works better than an equivalently noisy phone line operating in time’s conventional direction (Physical Review Letters, doi.org/q424).
While sending messages backwards in time isn’t a practical issue, better communication strategies for noisy devices are, says Lloyd. In fact, he says the new result ought to be straightforward to turn into an experiment similar to the 2010 quantum CTC made with photons. This could then let the team investigate real-life noisy channels and possibly uncover new ways of using them even for conventional communication.
Andreas Winter at the University of Cologne in Germany says the new work illuminates how different kinds of feedback, such as a sender in the future using their memory to help them encode a message, can enhance communication protocols, but practical applications are unlikely. “As far as we know, time travel or signalling back in time is not possible in our world,” he says.
Credits: TCA, LLC.