Time travel simulation [CTC]
Space-time structure exhibiting closed paths in space (horizontal) and time (vertical). A quantum particle travels through a wormhole back in time and returns to the same location in space and time.
Scientists at University of Queensland have simulated time travel using particles of light. The researchers used photon – a particle representing a quantum of light – on a closed timelike curve (CTC) [picture above] to simulate the behavior of quantum particles traveling through time, indicating how quantum mechanics might affect the presence of CTC. The study could eventually help us set light on how time-travel could be possible in the quantum world.

In the study, the research team simulated the behavior of a single photon of light that travels through wormhole and interacts unitarily with an older version of itself. The researchers used mathematical equivalence between: 1) where the photon first travels to, and 2) where it interacts with its pseudo-self.

First, the photon 1 travels through wormhole into the past and interacts with its older self. Second, the photon 2 travels through normal space-time continuum, but interacts with another photon that’s trapped inside a CTC. “Using the (fictitious second case) and simulating the behavior of photon 2, we were able to study the more relevant case 1,” said Ringbauer, who is the author of the study.

“We used single photons to do this,” said UQ Physics Professor Tim Ralph, “but the time-travel was simulated by using a second photon to play the part of the past incarnation of the time travelling photon.”

“The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein’s general relativity and quantum mechanics,”Ringbauer said. “Einstein’s theory describes the world at the very large-scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small-scale of atoms and molecules.”

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General theory of relativity shows the possibility of traveling back in time by following a space-time continuum that returns to the starting point. The quantum model of closed timelike curves can be formulated consistently with relativity and this avoids general relativity paradoxes such as meeting relatives by going back in time. As Ringbauer said, “General relativity predicts the existence of closed timelike curves. This would allow travel back in time. In the classical world this is unlikely to be possible, since it causes paradoxes, such as the grandfather paradox. In the quantum world, however, these paradoxes are resolved and time-travel can be formulated in a self-consistent way.”

Tim Ralph said it was first predicted in 1991 that time traveling in the quantum world could avoid such paradoxes.

“The properties of quantum particles are ‘fuzzy’ or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations,” he said.

Whilst Tim said that there was no evidence that nature behaved in ways other than standard quantum mechanics predicted, it had not been tested in regimes where extreme effects of general relativity played a role, such as near a black hole – where there is extremely high gravitational field.

“Our study provides insights into where and how nature might behave differently from what our theories predict,” he said.

Simulating could result in many effects which are forbidden in standard quantum mechanics as could break quantum cryptography. This way of intriguing possibilities in presence of CTC could violate Heisenberg’s uncertainty principle or principle of indeterminacy – the quantum-mechanical principle that the momentum and position of a particle cannot both be precisely determined at the same time.

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