The subject of time travel has intrigued both scientists and science-fiction writers alike for centuries, but now scientists are suggesting that the concept is theoretically sound.

Back in September of this year, UK physicist, Professor Brian Cox, declared that time travel was certainly possible, but only to the future and not to the past.

"The central question is, can you build a time machine? The answer is yes, you can go into the future," the University of Manchester professor told the audience during a speech given at the British Science Festival. "You’ve got almost total freedom of movement in the future."

Professor Cox explained that time travel into the future should be achievable using Albert Einstein’s general theory of relativity, where the time traveler would need to be traveling close to the speed of light in order to jump forward in time. Cox, a particle physicist, suggested that it was much less likely to achieve time travel to the past, however, and it would require a wormhole in order to facilitate the leap backwards in time. Even if a wormhole was discovered or created, there is no way to tell whether humans would be able to use it to time travel. Wormholes appear to be certain "locations" in space-time, such that, if you jump in, you’ll emerge at some point in the past, and they seem to comply with our current comprehension of the laws of physics.

Even if it proved to be a possibility, traveling back to the past could be risky due to the potential to disturb future events, and there are two well-known paradoxes that detail the implications of dabbling with history:

1) The first and most famous is the "grandfather paradox," in which the time traveler goes back in time and kills her grandfather. If she is successful, then how was she born?
2) Secondly, there is the "Shakespeare paradox." That is, the time traveler reads the works of Shakespeare, writes them down and brings them back in time. Shakespeare then finds the information and writes everything down. Who then wrote the works of Shakespeare?

In 1991, David Deutsch discovered a model for time travel that attempted to resolve the paradox issues. Deutsch, a theoretical physicist at Oxford University, found that it would be possible for a time traveler to change the past as long as they were self-consistent, meaning that the subsequent effects followed the same probability. For example, a traveler murders her own grandfather with probability of one half, then the probability of her either not being born or being born would each be one half.

Further to this principle, however, the ‘no-cloning’ theorem, or "no quantum Xerox machine" idea, which is a fundamental principle of quantum mechanics known about since 1982, states that it is impossible to reproduce a perfect copy of the state of an unknown quantum particle.

This forms the basis of our knowledge of quantum physics, though it appears to be at odds with our known ability to copy classical information, but it is one of the concepts which defines quantum reasoning and explains why the basis of quantum communication is so solid. The "no-cloning" theorem is a consequence of Heisenberg’s famous Uncertainty Principle, whereby the position or the momentum of a particle may be measured but not both with unlimited accuracy. The Uncertainty Principle states therefore that it is thus impossible to have a subatomic Xerox-machine that would take one particle then generate two particles with the same position and momentum – because this would result in too much knowledge of both particles at once.

Using the principles of Deutsch’s model, researchers Todd Brun, Andreas Winter and Mark M. Wilde believe they have shown that risk-free time travel to the past is possible, but in a manner that also breaches the "no-cloning" concept. They suggest that a time traveler can, in fact, copy quantum data via a method that involves looping a quantum particle back many times in the past and then reading out many copies of it, in a way that made it possible to leave the past undisturbed.

“We can always look at a paper, and then copy the words on it. That’s what we call copying classical data,” Wilde said. “But you can’t arbitrarily copy quantum data, unless it takes the special form of classical data. This no-cloning theorem is a fundamental part of quantum mechanics – it helps us reason how to process quantum data. If you can’t copy data, then you have to think of everything in a very different way.”

Wilde reports that Deutsch suggested back in the late 20th century that it should be possible to contravene the "no-cloning" theorem, and Wilde’s team, from the University of Southern California and the Autonomous University of Barcelona, have attempted to build on Deutsch’s earlier work in order to prove that his theory was correct.
As Deutsch proposed, new research claims that a particle – or at some point, a time traveler – can be sent to make multiple loops back in time as long as it remained self-consistent, that is, to remain the same each time it passed through a particular point in time.

“That is, at certain locations in space-time, there are wormholes such that, if you jump in, you’ll emerge at some point in the past,” Wilde said. “To the best of our knowledge, these time loops are not ruled out by the laws of physics. But there are strange consequences for quantum information processing if their behavior is dictated by Deutsch’s model.”

“In some sense, this already allows for copying of the particle’s data at many different points in space,” Wilde said, “because you are sending the particle back many times. It’s like you have multiple versions of the particle available at the same time. You can then attempt to read out more copies of the particle, but the thing is, if you try to do so as the particle loops back in time, then you change the past.”

The most significant leap forward for the team was to discover the key that allowed for loops back in time, and the copying of a time-traveling particle without disturbing the past.

“That was the major breakthrough, to figure out what could happen at the beginning of this time loop to enable us to effectively read out many copies of the data without disturbing the past,” Wilde said. “It just worked.”

The new method may throw up discrepancies in Deutsch’s original model, however, though there are many different interpretations.

“If quantum mechanics gets modified in such a way that we’ve never observed should happen, it may be evidence that we should question Deutsch’s model,” Wilde said. “We really believe that quantum mechanics is true, at this point. And most people believe in a principle called Unitarity in quantum mechanics. But with our new model, we’ve shown that you can essentially violate something that is a direct consequence of Unitarity. To me, this is an indication that something weird is going on with Deutsch’s model. However, there might be some way of modifying the model in such a way that we don’t violate the no-cloning theorem.”

The ground-breaking new research was bound to prompt some controversy, and some of Wilde’s peers have postulated that it would not allow quantum copying to occur as the universe would acquire some knowledge of the particle every time it looped back in time.

Irrespective of the time-traveling potential, the concept of quantum copying has potential significance for the security of quantum communications such as Quantum key distribution (QKD) uses quantum mechanics to guarantee secure communication. Quantum copying would compromise this and make the systems vulnerable to hackers.

“If an adversary, if a malicious person, were to have access to these time loops, then they could break the security of quantum key distribution,” Wilde said. “That’s one way of interpreting it. But it’s a very strong practical implication because the big push of quantum communication is this secure way of communicating. We believe that this is the strongest form of encryption that is out there because it’s based on physical principles.”

These forms of quantum communication are not yet embedded into our everyday life, such as online password encryption software, but in critical and sensitive communications that use the principles of quantum mechanics to encrypt the information. This type of encryption was previously believed to be unbreakable, but this could change if Wilde’s theories are correct.

“This ability to copy quantum information freely would turn quantum theory into an effectively classical theory in which, for example, classical data thought to be secured by quantum cryptography would no longer be safe,” Wilde said. “It seems like there should be a revision to Deutsch’s model which would simultaneously resolve the various time travel paradoxes but not lead to such striking consequences for quantum information processing. However, no one yet has offered a model that meets these two requirements. This is the subject of open research.”

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