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"Time Travel Simulation Resolves “Grandfather Paradox”" - from Scientific American


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Time Travel Simulation Resolves “Grandfather Paradox”




What would happen to you if you went back in time and killed your grandfather? A model using photons reveals that quantum mechanics can solve the quandary—and even foil quantum cryptography


Experimenting with a curve


Recently Ralph and his PhD student Martin Ringbauer led a team that experimentally simulated Deutsch's model of CTCs for the very first time, testing and confirming many aspects of the two-decades-old theory. Theirfindings are published in Nature Communications. Much of their simulation revolved around investigating how Deutsch's model deals with the “grandfather paradox,” a hypothetical scenario in which someone uses a CTC to travel back through time to murder her own grandfather, thus preventing her own later birth. (Scientific American is part of Nature Publishing Group.)


Deutsch's quantum solution to the grandfather paradox works something like this:


Instead of a human being traversing a CTC to kill her ancestor, imagine that a fundamental particle goes back in time to flip a switch on the particle-generating machine that created it. If the particle flips the switch, the machine emits a particle—the particle—back into the CTC; if the switch isn't flipped, the machine emits nothing. In this scenario there is no a priorideterministic certainty to the particle's emission, only a distribution of probabilities. Deutsch's insight was to postulate self-consistency in the quantum realm, to insist that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to flip the switch with a probability of one half, imbuing itself at birth with a probability of one half of going back to flip the switch.If the particle were a person, she would be born with a one-half probability of killing her grandfather, giving her grandfather a one-half probability of escaping death at her hands—good enough in probabilistic terms to close the causative loop and escape the paradox.Strange though it may be, this solution is in keeping with the known laws of quantum mechanics.


In their new simulation Ralph, Ringbauer and their colleagues studied Deutsch's model using interactions between pairs of polarized photons within a quantum system that they argue is mathematically equivalent to a single photon traversing a CTC. "We encode their polarization so that the second one acts as kind of a past incarnation of the first,” Ringbauer says. So instead of sending a person through a time loop, they created a stunt double of the person and ran him through a time-loop simulator to see if the doppelganger emerging from a CTC exactly resembled the original person as he was in that moment in the past.



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The scientific paper:




Experimental simulation of closed timelike curves


Martin Ringbauer,


Matthew A. Broome,


Casey R. Myers,


Andrew G. White


& Timothy C. Ralph




Closed timelike curves are among the most controversial features of modern physics. As legitimate solutions to Einstein’s field equations, they allow for time travel, which instinctively seems paradoxical. However, in the quantum regime these paradoxes can be resolved, leaving closed timelike curves consistent with relativity. The study of these systems therefore provides valuable insight into nonlinearities and the emergence of causal structures in quantum mechanics—essential for any formulation of a quantum theory of gravity. Here we experimentally simulate the nonlinear behaviour of a qubit interacting unitarily with an older version of itself, addressing some of the fascinating effects that arise in systems traversing a closed timelike curve. These include perfect discrimination of non-orthogonal states and, most intriguingly, the ability to distinguish nominally equivalent ways of preparing pure quantum states. Finally, we examine the dependence of these effects on the initial qubit state, the form of the unitary interaction and the influence of decoherence.



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Would you mind elaborate a bit more about which part do you disagree with and the reason why do you disagree?

Real time has flows and eddies to it.In some ways the flow of real time acts as if its mollasis or mucus from up someone's nose.So predictive outcomes from the kill grandfather complex equations are not always adherent.


An example was the repeated but false reporting of a politician who was reported dead while he was alive, only to have him die not too much later.There was also a prominent action that was also reported as a casualty and then to have him come back to life.


The phenomenon you had reported is not always as a knee jerk reaction.There is a phenomenon from overtime engineering known as lateral slide of timeline failure.This is when all events in time become jumbled together in one mass, or T mass locale, due to time line over engineering or attempted engineering of that time line by time changing devices at all. Pinter



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Interesting article and thanks for posting your synopsis and explanation. while i am no scientist, though, it always seems odd to me, to apply quantum physics as a metaphor for larger systems. Perhaps this probability model would apply on a greater scale, though.

Well, if CTC is proven to be working in quantum level, then I assume the next step is putting physical objects into quantum state and bringing it back to physical world.


What I say may sounds a bit like "Beam me up, Scotty" in Star Trek. :) Quantum teleportation is explained in this video.



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