A group of physicists at the University of Queensland in Australia has used photons to simulate quantum particles traveling through a wormhole back in time.

“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. 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,” said Martin Ringbauer, the lead author of a paper that reports the results in the journal Nature Communications.

Einstein’s theory suggests the possibility of traveling backwards in time by following a space-time path that returns to the starting point in space, but at an earlier time – a closed time-like curve.

This possibility has puzzled physicists and philosophers alike since it was discovered by Kurt Gödel in 1949, as it seems to cause paradoxes in the classical world, such as the Grandparents paradox, where a time traveler could prevent their grandparents from meeting, thus preventing the time traveler’s birth.

This would make it impossible for the time traveler to have set out in the first place.

“It was predicted in 1991 that time travel 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,” said senior author Prof Timothy Ralph.

There was no evidence that nature behaved in ways other than standard quantum mechanics predicted, but this had not been tested in regimes where extreme effects of general relativity played a role, such as near a black hole.

The new study provides insights into where and how nature might behave differently from what theories predict. Examples of the intriguing possibilities in the presence of closed time-like curves include the violation of Heisenberg’s uncertainty principle, cracking of quantum cryptography and perfect cloning of quantum states.

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Martin Ringbauer et al. 2014. Experimental simulation of closed timelike curves. Nature Communications 5, article number: 4145; doi: 10.1038/ncomms5145