Closed time-like curves offer a route to the past
Roman Budnikov/Alamy
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, which is 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 that is 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. That seems to rule out sending a message back in time – 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.
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 surprisingly, not only is communication with the past still possible, but it actually works better than an equivalently noisy phone line operating in the conventional direction of time.
Team member Kaiyuan Ji, also at MIT, says the team was inspired by the climax of Interstellar. At the end of the film (spoiler alert), an astronaut played by Matthew McConaughey sends a message to his daughter in the past by manipulating the hands on her watch, using something that appears to operate like a CTC. The researchers treated this as a noisy quantum channel and calculated that, unlike with conventional messaging, a backwards time-travelling message would still be legible, because the sender can use their memories of the past. “The father remembers how the daughter decodes his future message, so he can instruct himself on what is the best way to encode the message,” says Ji.
While sending messages backwards in time isn’t a practical issue, better communication strategies for noisy devices are, says Lloyd. “Nobody’s built an actual physical, closed time-like curve, and there are reasons to think it’s very hard to make one. But all channels are noisy,” he says. In fact, Lloyd 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, can enhance communication protocols, but practical applications are rather unlikely. “As far as we know, time travel or signalling back in time is not possible in our world. We don’t know of any mechanism that would make it possible,” he says.
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