New Theory of Quantum Time Travel --"Could We Clone the Past?"
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December 07, 2013

New Theory of Quantum Time Travel --"Could We Clone the Past?"




David Deutsch, a pioneer of quantum computing and a physicist at Oxford, came up with a simplified model of time travel to deal with the paradoxes that would occur if one could travel back in time. For example, would it be possible to travel back in time to kill one’s grandfather? In the Grandfather paradox, a time traveler faces the problem that if he kills his grandfather back in time, then he himself is never born, and consequently is unable to travel through time to kill his grandfather, and so on. Some theorists have used this paradox to argue that it is actually impossible to change the past.

“The question is, how would you have existed in the first place to go back in time and kill your grandfather?” said 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.

Deutsch solved the Grandfather paradox originally using a slight change to quantum theory, proposing that you could change the past as long as you did so in a self-consistent 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.”

But the Grandfather paradox is not the only complication with time travel. Another problem is the no-cloning theorem, or the no “subatomic Xerox-machine” theorem, known since 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. 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.

“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.”

But what if a Deutschian closed timelike curve did allow for copying of quantum data to many different points in space? According to Wilde, Deutsch suggested in his late 20th century paper that it should be possible to violate the fundamental no-cloning theorem of quantum mechanics.

Now, Wilde and collaborators at the University of Southern California and the Autonomous University of Barcelona have advanced Deutsch’s 1991 work with a recent paper in Physical Review Letters (DOI: 10.1103/PhysRevLett.111.190401). The new approach allows for a particle, or a time traveler, to make multiple loops back in time – something like Bruce Willis’ travels in the Hollywood film “Looper.”

“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.”

A single looping path back in time, a time spiral of sorts, behaving according to Deutsch’s model, for example, would have to allow for a particle entering the loop to remain the same each time it passed through a particular point in time. In other words, the particle would need to maintain self-consistency as it looped back in time.

“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.”

To be consistent with Deutsch’s model, which holds that you can only change the past as long as you can do it in a self-consistent manner, Wilde and colleagues had to come up with a solution that would allow for a looping curve back in time, and copying of quantum data based on 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.”

However, there is still some controversy over interpretations of the new approach, Wilde said. In one instance, the new approach may actually point to problems in Deutsch’s original closed timelike curve 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.”

Other researchers argue that Wilde’s approach wouldn’t actually allow for copying quantum data from an unknown particle state entering the time loop because nature would already “know” what the particle looked like, as it had traveled back in time many times before.

But whether or not the no-cloning theorem can truly be violated as Wilde’s new approach suggests, the consequences of being able to copy quantum data from the past are significant. Systems for secure Internet communications, for example, will likely soon rely on quantum security protocols that could be broken or “hacked” if Wilde’s looping time travel methods were correct.

“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.”

Today, when you log into your Gmail or Facebook, your password and information encryption is not based on physical principles of quantum mechanical security, but rather on the computational assumption that it is very difficult for “hackers” to factor mathematical products of prime numbers, for example. But physicists and computer scientists are working on securing critical and sensitive communications using the principles of quantum mechanics. Such encryption is believed to be unbreakable – that is, as long as hackers don’t have access to Wilde’s looping closed timelike curves.

“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.”

The Daily Galaxy via LSU 



"That was the major breakthrough"... No. It's just a tweaking of established sifi musings with mathematical jargon. 'Killed with a probability of one half.' Gotta love it.

Actually there is no such thing as a grandfather paradox. Taking one belief, that the future is not written; it does not exist, yet (indeterminism), the only possibility is that, if you go into the past, you are writing yourself into that past and the future you came from is only a probability, because, again, the future is not written. Sorry not scientific I know, but then again, neither is this so-called theory of quantum time travel. IMHO

I have a headache!

Doesn't the concept of going back in time prior break the laws of conservation of energy?

Adding new energy from an additional human changes the total sum energy in the universe. Thus cannot happen, if the law is correct or if all "times" exists simultaneously.

David Deutsch obviously forgets Albert Einstein and his famous E=mc^2.
Read this article to understand David's mistake:
E=mc^2: or how to visit Marilyn Monroe

If there would be only one elementary particle in the universe, this particle would have absolutely no state. It has no state because it has no relation...

Therefore, a particle's states are determined entirely by all other particles of the universe in causal relations in space and time.

Thus it is impossible to make an exact copy of a particle, because two particles cannot coexist in the same space and time. Even photons can't. They can in 3D but not in 2D.

The indeterminism of quantum mechanics comes from the fact that to predict the future states of a particle with 100% certainty, you must know the states of the entire universe, which is clearly impossible because we are a part of it.

It is impossible to go backward in time without having any influences on the states of the universe, therefore it is impossible to go backward in time.

hmmm, sounds like this relies on Relativity being a proof of 'time' and not just 'rates of change' > suggests it 'may' be otherwise :)
M.M. (timelessness)

The present universe is the proof of it...

How to define non causal laws?

Science is the study of causality. Without causality, there is no science. Open your eyes and see. Where is Dr. Who?

Relativity and Quantum Mechanics are time reversal symmetric laws. But there is the laws of entropy which show that time is non reversal symmetric. There is no unified laws of GR, QM and Entropy. Entropy is still a separate part of the puzzle, too often forgotten.

Assumption: If I said such a story, to my grandmother, she would answer to me. My little child, stop telling me such nonsense. Question: Did she was right?

Backward timetravel is Nonsense....lets instead focus on a time machine to the future!

Sorry, correct link is just

For what it is worth, I agree that there is a possibility that the future doesn't exist. It would be based on our actions at present and is formed from the results. However, how do we know the past exists? My view is that we live for the moment. Our "time" exists in the momet we are in and then its gone. The past is only in history books and there is no "physical" past to travel to.

as per Einstein's theory of relativity both past and future have to exist at same time to different observers across the universe because some observers are coming nearer to each other and some are traveling further apart due to expansion of the universe.

The flim "Looper" did not star Bruce Willis. His 'time jump' film was "12 Monkeys".

My error, I was thinking of the film "Jumper"... Bruce Willis was in the film "Looper".

yeah that's y math is self referencing approximations.

The grandfather paradox is simplistic bullshit. You can't step into the same river twice. The water flux is constantly flowing, so the river is never exactly the same. The path is the same, but the time isn't. You aren't the same from one to another moment, no matter in which direction you go.

Not an easy concept. Having read the comments here I would say none of you have grasped it.

While time travel will always remain within the realm of our imagination, it is a good logic puzzle. If you erase your ability to exist, then you never really were around to time travel. Consequently, the argument is flawed.

The Time Travel concepts in general purports that you continue to age irrespective of what time you reside in.

The concept of time travel cloning is time dependent on how far you travel into the past and shortens your lifetime between birth and death, but not your actual life span. If you travel back into the past, then both of you are living to the point where initiated your time travel - Then only the older version of you exists.

The concept of time travel into the future extends your time between birth and death, but not your actual life span. Depending on how far you jump into the future, you may be restarting your life with only what you can bring with you. You can basically end up without any legal identification and become an illegal immigrant with no rights or past employment history.

Why time travel into the past is impossible

Never mind all the blather about closed time-like curves (CTC’s), wormholes, quantum tunneling and the like. What is so obvious no one cares to mention it is that time travel into the past is impossible because the past no longer exists. Sure there is evidence of the past all around us, from commonplace objects in a home, to trees, library archives, crumbling stone buildings built in centuries gone by, dinosaur fossils, and the background radiation left over from the Big Bang. But records of the past are actually a part of the present, and not the same as THE PAST as a complete unit. For example , 1:32 p.m. on April 3, 1931 is as ephemeral as any other moment in time, and the universe has moved on in an almost infinite number of changes large and small since then. Time is a process, the universe is a process and time is a continuum of instants marking the progression of that process. You cannot invoke a “freeze-frame” of a fragment of that process so that anyone or anything can just inject itself into that fragment. And you don’t even need to invoke the grandfather paradox to realize that believing time travel to the past is possible is anything other than fantasy or idiocy.

How anyone can think that a human being or any other non-quantum object can project himself or itself into some arbitrary date or moment in the past is just preposterous. Those that argue otherwise talk about time as “the frozen river” where every moment that has taken place still exists somehow, or that every instant in time, the universe divides into an infinite number of parallel universes, all of which exist and all of which are equally possible. These ideas are mere phantasms, fictional creations of the human mind which is so good a concocting fiction. While the idea of time travel to the past has resulted in some entertaining fictional stories, this only proves that humans are way better at concocting fiction than at studying the way things really are.

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