Time travel has been a human dream even before "Bill and Ted's Excellent Adventure," and researchers may have found a way to bring it closer to reality.
Oxford quantum computing researcher David Deutsch came up with a "simplified model of time travel to deal with the paradoxes that would occur if one could travel back in time," Louisiana State University news release reported.
The researcher tackled the "grandfather principle; which discusses a scenario in which one was to go back in time and kill their own grandfather.
"The question is, how would you have existed in the first place to go back in time and kill your grandfather?" Mark Wilde, an LSU assistant professor with a joint appointment in the Department of Physics and Astronomy and with the Center for Computation and Technology, or CCT, said in the news release.
Deutsch used a "slight change to quantum theory" in order to solve the paradox. He concluded one could change the past as long as it was done in a "self-constant manner."
"Meaning that, if you kill your grandfather, you do it with only probability one-half," Wilde said. "Then, he's dead with probability one-half, and you are not born with probability one-half, but the opposite is a fair chance. You could have existed with probability one-half to go back and kill your grandfather."
Another complication with time travel is the "no-cloning theorem, or the no "subatomic Xerox-machine" theorem," which was theorized in 1982.
"This theorem, which is related to the fact that one cannot copy quantum data at will, is a consequence of Heisenberg's famous Uncertainty Principle, by which one can measure either the position of a particle or its momentum, but not both with unlimited accuracy," the news release reported. "According to the Uncertainty Principle, it is thus impossible to have a subatomic Xerox-machine that would take one particle and spit out two particles with the same position and momentum - because then you would know too much about both particles at once."
Wilde explained quantum data cannot be copied onto the way words from on a piece of paper can be replicated. One could only copy quantum data if it took the form of this type of classical data.
In his 1991 paper Deutsch challenged the no-cloning theorem; Wilde and his team worked to advance that idea. The theory would allow a particle (or time explorer) to make multiple "loops" back in time.
"That is, at certain locations in spacetime, 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 this "time spiral," a particle would have to stay the same whenever it passed a certain point in time, thus being "self-consistent."
"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 team worked to outline a model-consistent way a "loop" could exist along with a copying of data from a time-traveling particle that did not manipulate 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 idea is not without controversy, and could pinpoint problem's in Deutsch's orginal time travel model.
"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."
If this theory were true, it would also come with significant security risks.
"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."
Researchers are working to encrypt online passwords (such as on Facebook and Gmail) using quantum information, which would be almost unbreakable; unless ill-meaning hackers gained access to the "looping closed timelike curves" described in the recent paper.
"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."
