Researchers at the University of Queensland, Australia claim to have simulated the behavior of a single photon traveling back in time and interacting with an older version of itself, in an effort to investigate how such a particle would behave. Their results suggest that, under such circumstances, the laws of quantum mechanics would stretch to become even more bizarre than they already are.

"General relativity seems to allow for so-called closed timelike curves (CTCs), paths in space-time that return to same point in space at an earlier time," PhD student and lead author of the study Martin Ringbauer told Gizmag. "No CTC has been observed so far, but they appear in many solutions of Einstein's field equations, which makes them an interesting object of study because traversing such a CTC would imply traveling backwards in time."


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The possibility of traveling back in time would open the door to inconsistencies in the classical world, such as the grandfather paradox: namely, if a time traveler were to prevent his own grandparents from meeting, he would also be preventing his own birth, which means he couldn't have traveled back in time in the first place.

However, British physicist David Deutsch showed back in 1991 that while the grandfather paradox may be inescapable for macroscopic objects, the uncertainty principle that governs quantum particles such as photons leaves enough "wiggle room" to avoid such inconsistencies.

"An important aspect of classical objects is that they can only exist in a well defined state," Ringbauer explained. "For the time traveler this means they either exist or don’t exist, which is at the heart of the grandfather paradox."

"For quantum systems this is different, since they can exist in superpositions and mixtures of states," he continued. "For the grandfather paradox, the corresponding quantum state of the time-traveler (now a photon) would be a mixture of existing and non-existing, which resolves the paradox and leads to a consistent evolution."

The Australian researchers set out to study the consequences that Deutsch's theory would have on the way quantum particles behave in a CTC. Specifically, the team studied how single photons would behave as they traversed a simulated CTC, traveled back in time, and then interacted with their older self. (The time-travel was simulated by using a second photon to play the part of the past incarnation of the time traveling photon.)

Such a system doesn't give rise to time-traveling paradoxes. But the researchers did conclude that, in the presence of a closed time-like curve, the laws of quantum mechanics might change, giving rise to peculiar behaviors that are different to what standard quantum mechanics would predict.

In particular, such a quantum system might violate Heisenberg’s uncertainty principle, as it would be possible to perfectly distinguish the different states of a quantum system (which are usually only partially detectable).

This would make it possible to break quantum cryptography and perfectly clone quantum states. This, in turn, would lead to very dramatic speed increases in quantum computations – even beyond what they already promise compared to a classical computer.

The results do not have any implications for time-travel in the macroscopic case, and don't answer the question of whether, how or why time travel might be possible in the quantum regime. However, they could help us understand the consequences of the existence of CTCs and provide insight into where and how nature might behave differently from what our theories currently predict.

The study appears in a recent issue of the journal Nature Communications.

Source: University of Queensland