How Time-Traveling Could Affect Quantum Computing
November 20, 2008 By Lisa Zyga(PhysOrg.com) -- If space-time were constructed in such a way that you could travel back in time, it would create some pretty strange effects. One of these oddities, as many people know, is the “grandfather paradox.” Here, a person travels back in time to kill their grandfather before the person’s father is born, thus preventing their own birth.
The type of space-time that enables time traveling involves “closed time-like curves” (CTCs), and, besides personal fates, CTCs can also provide insights into quantum information and computing. In a recent study, computer scientists Scott Aaronson of MIT and John Watrous of the University of Waterloo have discovered that, if closed time-like curves exist, then quantum computers would be no more powerful than classical computers.
But researchers shouldn’t stop working on quantum computing technology just yet, as no one has any evidence that closed time-like curves actually exist. Closed time-like curves are strange: sometimes physicists describe them as a piece of paper folded over on itself, so that opposite ends touch and create a shortcut. A person standing at the front end could then easily step onto the back end, thereby easily stepping into the past.
CTCs provide interesting but complex insights into computation. At first it may seem that, if CTCs existed, researchers could perform computations of unlimited length in an instant, by simply computing the answer, and then sending it back in time to before they started. However, this proposal, like the grandfather paradox, breaks the rules of causality, since the input could be changed, affecting the future output. Further, the computation may have actually taken 100 years, so Aaronson and Watrous don’t consider this an honest computation method.
Instead, the scientists attempt to overcome causality breaking and its paradoxical consequences. One way to do this is to simply argue that nature must somehow enforce causality. For example, in 1991, physicist David Deutsch proposed a resolution to the grandfather paradox that relies on the parallel universes theory in quantum mechanics: everyone is born into a universe with a certain probability, so if you go back in time to kill your grandfather, there’s a probability that you won’t be born in that universe, but another. Not all physicists agree with such interpretations, but Aaronson and Watrous adopt Deutsch’s ideas for their demonstration.
The scientists envision a scenario in which classical and quantum computers contain CTCs, and these CTCs contain bits or qubits. Computations start at a fixed point, which nature somehow decides on, and which ensures causal consistency (avoiding the grandfather paradox).
Aaronson and Watrous show that classical and quantum computers with polynomial-size CTCs both have the same amount of computing power. They can both solve the set of problems in an abstract space called 'PSPACE,' which is all the problems that a classical computer can solve using a polynomial amount of memory. This computing power is extremely large, and both types of computers are very efficient when using CTCs.
“The way you use CTCs to compute, in one sentence, is by forcing nature to solve an exponentially hard computational problem just in order to make the universe causally consistent,” Aaronson told PhysOrg.com. “You can set up the computation inside the CTC to take as input a proposed solution to the problem, and then do the following:
“IF the input solution is correct, THEN output that same solution.
IF the input solution is incorrect, THEN output the next solution in some standard ordering (looping around to the first solution after you've reached the last one).
Go back in time, and feed in the output of the computer as its input.”
The key, Aaronson explained, is determining what the input to this computation needs to be, in order for everything to be causally consistent.
“Assuming there are any correct solutions at all, the input must itself be a correct solution!” he said. “For otherwise we'd have an inconsistency: the output of the computation would not match the input.”
He added that, if there are no correct solutions, then the output and input both have to be completely random; that's the only way to ensure causal consistency in that case.
“Admittedly, how nature manages to actually do the work of finding the fixed point (and thereby making the universe causally consistent) remains a great mystery,” he said. “But the point is, this is the sort of thing nature would presumably need to do if CTCs existed.”
In their study, Aaronson and Watrous described CTC computing using a rough analogy of Shakespeare’s plays being written by someone from the present going back in time and dictating the plays to him. As long as the person from the present dictates the plays correctly, there is no paradox, and causal consistency is maintained.
“In the Shakespeare example, there's no actual logical inconsistency,” Aaronson explained. “You go back in time and dictate the plays to him, therefore he writes the plays, therefore the plays come down to you, therefore you're able to go back in time and dictate the plays to him, etc. Everything is consistent – there's no paradox! Or rather, the only ‘paradox’ is one of computational complexity: a difficult task seems to have been performed (namely, Shakespeare's plays being written), but without anyone ever doing the actual creative work of writing those plays.”
By pinning down the exact computational power of CTCs, the computer scientists hope that their findings may lead to applications in quantum information.
“In order to solve this problem, Watrous and I had to give an algorithm for computing fixed points of quantum operations using a polynomial amount of memory,” Aaronson said. “And that's something that one could easily imagine finding other uses for in quantum computing theory. In fact, I'm working on a paper right now about using bounded-memory quantum computers to extract the bias of a classical coin. And in order to solve the problem in that paper, I'm using many of the same techniques as in the closed time-like curves paper.
“I don't want to oversell this,” he added. “It's not going to revolutionize quantum information; it's just (even at the purely technical level, and ignoring the closed time-like curve part) a useful trick.”
More information: Aaronson, Scott and John Watrous. “Closed timelike curves make quantum and classical computing equivalent.” Proc. R. Soc. A. doi:10.1098/rspa.2008.0350.
Copyright 2008 PhysOrg.com.
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Shadowram, time travel is crazy to think about and such, makes for good sci-fi, but its important to realize that our perception of reality necessarily involves mental ordering, and so its not always easy to pick out the reality from the mental constructs. For example they said,...
"Admittedly, how nature manages to actually do the work of finding the fixed point (and thereby making the universe causally consistent) remains a great mystery,"
Well, there is no analytic link between cause and effect. Causality is a constant conjunction of individual events, its not a thing 'nature manages' of itself,... it's a means of mental synthesis of sense impressions given our mental design, and therefore is not a self consistent aspect of reality itself, such that talk of 'CTCs' are relevant, apart from intellectual masturbation.
We are 'compelled' in believing that there is a necessary connection between cause and effect, when we observe a regularity in our perceptions, ...this is due to a means of mental ordering of reality, not to the reality itself. A rational order of reality can not be accounted for merely by the chance accumulation of sense perceptions, ...the synthesis occurs in our functioning bio-mechanical mind, and so the conceptual form in which our understanding of reality takes is a product of a-prior mental paradigms, ...causality being one.
Dam these thoughts cause a subconscious brain freeze like they were ice cream.
Kudos to Physorg
We presume there is a global thing called "time" when we watch two non-interacting clocks ticking at apparently the same rate. But if we look closely at the quantum level, we see that those two clocks are exchanging many virtual photons. They are parts of the same system. When we construct extremely tiny clocks, remove them from each other, and cool them down to the point that they are exchanging far fewer particles, their time rates are far less synchronized and are more independent.
Our presumption that classical time is an intrinsic part of quantum interactions, has served only to muddle our understanding of quantum mechanics.
-fleem
(almost like gravity). Some folks like to make what I believe is a ludricrous leap from quantum behavior to something tangible traveling back in time. I would wager everything I own that its no more possible to transmit even just useful information back in time than its possible to make something from nothing (in the rigorous sense). To me it would be literally that, causing something to be that wasnt 'before'. This is how it also violates conservation of matter. And infinite parallel realms being a workaround is just as absurd. I think its all popular mythical hooey, no matter what solutions are included in the math. I know open thinking is a vital part of discovery, but I think ones energy could be more productively applied not quite so far out in the fringes of speculation. Time travel in the 'controversial' sense is strictly about going backwards, not speeding up, slowing down, or other aspects. Those are more practical topics. I for one would love it if human hibernation were possible, as myself and others could use the benefits future medicine will provide, not to mention the compound interest :).
Time is the same thing. Time is just a measurement of regular occurrences within our universe. Also, if could not remember the past, then there would be no possibility of 'Time Travel'. If you truly think about it, the 'Past' is nothing more than memory's stored within our individual minds.
Another problem you have with %u2018Time Travel%u2019 is the failure to recognize %u2018Location Travel%u2019 as well. Let%u2019s say you wanted to start at location X and wanted to %u2018Travel%u2019 back one minute. Location X must therefore move back to its original position as of one minute ago. Where does that leave you? You must not only calculate the rotation of the Earth, but the rotation of the Earth around the Sun, the Sun around our Galaxy, Our Galaxy through Space, etc.
So, to %u2018Travel back in Time%u2019, unless you continue to be part of that space-time, you may have a bit of a problem. If you stay part of that space-time, then %u2018Time Travel%u2019 into the past would be just as fast as time travel into the future; i.e. one minute equals one minute.
You have a better chance of cross dimensional travel than you do of %u2018Time Travel%u2019.
However, time would presumably not stand still while one was in the past there would be departure to "now" time where the traveler would not exist in the "now" therefore he/she could not return to that period as it was without him in the "now". In fact he would have to return to the exact micro second which we all know as "now" but the problem is the future becomes the past without any perception of the "now" instance this leads one to assume that he could not return to hid departure point in time or the "now" or into the future. which ultimately means that we would not know if anyone has travelled in time as they could not have returned to our "present".
Isn't it about time that physicists stop talking about time travel? There is no time travel at all, forward or backward. There is only the changing NOW, the present. And no, we are certainly not moving toward the future at 1 second per second. Please PhysOrg, give this crackpottery a rest. It's annoying. And it does not matter if Stephen Hawking or some other famous scientist like Carl Sagan believes in the possibility of time travel. It is still crackpottery.
Nothing Can Move in Spacetime:
http://www.rebels...ious.htm
Nothing Can Move in Spacetime:
http://www.rebels...ious.htm
They confuse science with science fiction, Too much TV which they confuse with reality. when someone can define the time legnth of "NOW"/the present then I will beleive there is some merit to research on the subject.
I like to consider three different time concepts, first the real time that we measure with our clocks, second, the perception time which depends on our personal experience and third, aging.
The first one is constant and linear, it's just an abstract concept to reference all activities of humankind. You can't travel through it to the future or past because, simply, it does not exist. We can't change our past, only our present or future. Our future depends on what we do in our present. Basically only our present is real, our past is a reference and our future is a plan or a wish.
The second one, the perception time is within our minds. We can experience a long day when we are bored, when nothing happens, or when we are waiting anxiously for something very important, or suffering a tragedy. If we are happy and busy time runs faster. In this perception level we can travel to our past or future through our imagination only.
Aging and health are our real timers, because both will limit live. When we accelerate close to the speed of light, aging is supposed to be reduced compared to the people left behind. Still our linear time hasn't change. We know we can alter our aging factor by only changing our lifestyles.
The space-time block universe does not exclude time-travel. Time travel would simply be the case in which space-time has singular points. If we imagine space as a 2-D sheet and the third (to us) dimension as time (for the flat-worlders on the sheet), then a time machine would act like a black hole in the origin time slice, and like a white hole in the destination time slice. One sheet would be connected to another by the time machine (which, by the way, would only work in one direction in order for an object's position to be a (possibly discontinuous) function of its proper time.) So, in the block version, a universe in which time travel to the past has occurred would have singularities at the points of origin and destination of the time machine.
Not that I can think of any ways to build a time machine.
What makes spacetime a block universe is the time dimension. Contrary to intuition and common wisdom, a time dimension makes change or motion impossible. If you don't get it, don't fret. Most people don't.
If one so chooses, for the moment, ponder the "perpetual now", no past/future, then leave behind any taking-for-granted that matter is "physical matter" as we understand, then much can come in the way of creative insights.... or at least some entertaining thoughts.
Of course it could be argued against but after all it is only an argument and there is no way to prove it wrong or right.
The act of traveling doesn't imply motion it is only inferred, my guess is that one would leave space as we know it which would mean we would exist outside of space and time, as to how this would be possible we would have to leave up to people in the future, and no if it was possible we wouldn't be getting time travelers, they would be traveling to an alternate universe, so we would never get to benefit from the technology unless we were the time travelers.
Disclaimer: This is only an idea which doesn't violate this 'block universe' theory, try to refrain yourself from spitting a page full of rebelscience maths in reply.
Nothing would change, and the world-line of the particle wouldn't move, but from the particle's point of view it would make a loop through time, since it's effective direction would have changed.
The main point of my post was:
Which I don't think implies change or motion. Granted, the wording of the sentence isn't all that great, but the idea is that the block universe would have singularities in the space-time regions corresponding to the space and time in which the time machine operates. If the time machine were based on wormhole manipulation (through time-dialation one mouth to form a two-way time machine) one would likely need to include at least one more spatial dimension, with the benefit of not having singularities (at least intrinsically, or so I think.)
different than particles that accelerate in the presence of some force. We just don't realise what the forces are (nothing spiritual implied, mind you!) and probably our movements are more complicated. That's why, if we manage to build a time machine, then closed time loops exist and we found a use for them. Just as we found a use of electricity. If we don't find a way to do it, CTC either doesn't exist or more likely, we're not smart enough to find a way to use it.
I'm not saying we should, or that they exist. But the whole idea of causality is ill-defined. After all, causal relations are defined only as part of the light-cone of a particle. How this defines causality? What happens with your world line is very dependent of the space-time you're in. If you manage to meet a CTC then your world line will probably go crazy. But this isn't a property of YOU, it's a property of the metric so to say. Your psychic is not a problem of Nature! If you consider space-time freak-outs in the presence of a black hole for real, for example, where's the problem to consider CTC for real?
Nature doesn't have any obligation to protect your worldline and keep is out of troubles. I personally think that if you manage to run on a paradox, the Nature will flatten it out with the least possible tension. Like killing you off in some nasty way and radiate your perturbation back to infinity. Not such a big deal! Some people simply tend to think Universe for more fragile than it is.
Also,I would like to discuss the article. Did I understand correctly that if you feed the right answer into the quantum time pc, it will either give you the right answer or a random one? This sounds soooooooo useful :)