The Science of Time Travel
Time machines may seem more like a concept from science fiction than something that could exist in a physics lab. However, experts believe that this futuristic technology might one day become a reality. Researchers have uncovered how time travel could actually function using the principles of quantum physics.
Although their method won’t allow you to travel back to the time of the dinosaurs, scientists suggest it could be possible to send messages into the past. This mind-bending technique is similar to what was depicted in Christopher Nolan’s sci-fi film, Interstellar. In the movie, an astronaut played by Matthew McConaughey sends a message to his daughter in the past by adjusting the hands on her watch.
While the reality may not be as cinematic, the researchers argue that this “causal loop” mirrors how real time travel would operate. Dr Kaiyuan Ji, a researcher at Cornell University, explained: “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.”
It might seem surprising, but there is nothing in the laws of physics as we understand them that makes time travel impossible. According to general relativity, which is our best description of the universe, everything moves through space and time on a set path. One such path is known as a closed time-like curve (CTC). Something traveling on a CTC moves into the future before looping back on itself via the past to end up exactly where it started.
The laws of physics allow for these loops to form, but creating one on a large scale requires twisting spacetime with an infinite amount of energy. However, on the very small scale, structures like CTCs might naturally form through the laws of quantum physics.
On the quantum level, two particles can become ‘entangled’, meaning that what happens to one particle affects the other, even if they are light-years apart. One possible explanation for this effect, which Einstein called “spooky action at a distance,” is that one particle is sending messages backwards in time to the other.
How to Send a Message Back in Time
To send a message back in time, you need to create a quantum system with two entangled particles. These particles form a closed time-like curve. Altering one particle creates a change in the other, sending information backwards through time.
If you remember how the message was decoded in the past, you can use this to change how you encode the message in the future. That allows you to send legible messages, no matter how noisy the connection is.
Rather than assuming that they are part of one massive system or that they are sending information faster than light speed, the particles’ “sensitivity” is explained by their receiving messages in the past that tell them how to react later.
This might sound mad, but in 2010, scientists came up with a way of mimicking closed time-like curves using entangled particles. Professor Seth Lloyd, a quantum physicist from MIT, said: “It was the equivalent of sending a photon a few nanoseconds backwards in time, and having it try to kill its former self.”
What this creates is like a telephone with a direct connection to another device a few moments earlier. In theory, you could use something like this to pass messages back to yourself in the past.
Just like a real phone line, the connection on a CTC isn’t always going to be perfect, and noise or disruption will make it hard to pass information with 100% accuracy. Professor Lloyd says: “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.”
This is where an insight from Interstellar comes in handy. In their new paper, accepted for publication in Physical Review Letters, Professor Lloyd and his co-authors write: “The father, who is in the future, may retrieve his memory of past events he has witnessed, even including the daughter’s decoding of the message which he is about to send! It would thus not be surprising that he will consult his memory of the daughter’s decoding when encoding his message, so as to maximize the efficiency of the communication.”
Essentially, if you’ve already watched someone struggle to piece together your garbled message, you should know how to send it so that it’s easier for them to decode. Even if the connection is very noisy, a backwards time-travelling message would still be legible.
The slightly weird conclusion of this is that sending messages backwards in time is likely to be clearer than sending a message in normal time. Although no one has built a real closed time-like curve, Professor Lloyd says it should be fairly easy to turn this new idea into an experiment on the quantum level. That could let scientists investigate how information is transmitted through “noisy channels” and even improve real-life communication methods.
