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Quantum Theory Looks at Time Travel

Daniel M. Greenberger, Karl Svozil

Published Jan 1, 2005• 15 citations
OpenAlex

Abstract

We introduce a quantum mechanical model of time travel which includes two figurative beam splitters in order to induce feedback to earlier times. This leads to a unique solution to the paradox where one could kill one's grandfather in that once the future has unfolded, it cannot change the past, and so the past becomes deterministic. On the other hand, looking forwards towards the future is completely probabilistic. This resolves the classical paradox in a philosophically satisfying manner.

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Comment on 'Quantum Theory Looks at Time Travel' by D.Greenberger & K.Svozil

J.S. Wykes

It is claimed in the above paper that, if time travel were possible, quantum propagation would prevent classic time travel paradoxes by establishing consistent loops; an example circuit is used to demonstrate such a loop. It is argued here that established loops are not the framework in which the classic paradoxes arise; rather they arise via the establishment of a concrete initial history in which no disturbing time travel is allowed and then disturbing that history via the launch of the time traveller. It is shown that, operated in this two-pass fashion, if the first forward evolution of their circuit produces a definite triggering of a backwards time travelling state, the re-evolution thereby engendered may be organised so as to prohibit the triggering of this state, thereby creating a classic time travel paradox.

arXiv:quant-ph/0507052•Jul 6, 2005•0 citations
We introduce a quantum mechanical model of time travel which includes two figurative beam splitters in order to induce feedback to earlier times. This leads to a unique solution to the paradox where one could kill one's grandfather in that once the future has unfolded, it cannot change the past, and so the past becomes deterministic. On the other hand, looking forwards towards the future is completely probabilistic. This resolves the classical paradox in a philosophically satisfying manner.
A time machine that sends information back to the past may, in principle, be built using closed time-like curves. However, the realization of a time machine must be congruent with apparent paradoxes that arise from traveling back in time. Using a simple model to analyze the consequences of time travel, we show that several paradoxes, including the grandfather paradox and Deutsch's unproven theorem paradox, are precluded by basic axioms of probability. However, our model does not prohibit traveling back in time to affect past events in a self-consistent manner.
This paper introduces a non-deterministic model of time as a three-dimensional structure comprising past, present, and future. The present is described as a superposition of all causal futures, with weights evolving deterministically, while the collapse into a single future remains non-deterministic. This model naturally resolves the EPR paradox by introducing a shared present for entangled particles, eliminating the need for non-locality or hidden variables. The framework is consistent with quantum mechanics, thermodynamics, and relativity.
The retrocausality inherent in the retrodictive description of a quantum optical device called the quantum scissors allows this device to be used as a non-deterministic time machine. We show how the device can be used to model the time travel paradox of retrospective suicide. Here a man goes back in time and interferes with his own birth, weakening his infant self in such a way that he is killed by the time machine and is unable to interfere with his own birth. We find that the paradox is avoided by interfering quantum mechanical amplitudes making the time travel event impossible in this case. A slight modification that limits the interference with the birth so that the infant is only injured by the time machine to give a self-consistent cycle does, however, render the time travel event possible. This lends support to the idea that time travel is only possible if it yields a self-consistent cycle that avoids a paradox.
It is claimed in the above paper that, if time travel were possible, quantum propagation would prevent classic time travel paradoxes by establishing consistent loops; an example circuit is used to demonstrate such a loop. It is argued here that established loops are not the framework in which the classic paradoxes arise; rather they arise via the establishment of a concrete initial history in which no disturbing time travel is allowed and then disturbing that history via the launch of the time traveller. It is shown that, operated in this two-pass fashion, if the first forward evolution of their circuit produces a definite triggering of a backwards time travelling state, the re-evolution thereby engendered may be organised so as to prohibit the triggering of this state, thereby creating a classic time travel paradox.