Australian researchers simulate a time-traveling photon


June 24, 2014

Researchers at the University of Queensland have simulated the behavior of a photon traveling back in time through a wormhole (Image: University of Queensland)

Researchers at the University of Queensland have simulated the behavior of a photon traveling back in time through a wormhole (Image: University of Queensland)

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."

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

About the Author
Dario Borghino Dario studied software engineering at the Polytechnic University of Turin. When he isn't writing for Gizmag he is usually traveling the world on a whim, working on an AI-guided automated trading system, or chasing his dream to become the next European thumbwrestling champion. All articles by Dario Borghino

The trouble with time-travel schemes is that the Earth is orbiting the Sun at about 70,000 mph, or 19 miles per second. In turn the sun is orbiting the centre of the galaxy at about 500,000 mph, or 140 miles per second. So time travel of even a single second would be a catastrophe for anybody...


Anyone tinkering about with this kind of technology should be forced to watch "Primer" before they get their selves in a great big pickle.


I second Grunchy.

As time travel relies on fixed points and we are moving through the universe at such a rate, you would not be able to use this kind of technology practically, nor would you have much luck to keep track of a cluster of photons through time under an experiment.

Almost as if the expansion and rotation of our universe has made it such that it is harder for civilisations to discover this stuff. Classic Murphy's laws right there applied to time travel tech. :)


The article explicitly states that it doesn't apply to the macro scale. It's not even primarily about time travel. "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."

James Hallam

The grandfather paradox assumes that once you have arrived in the past how you got there is still relevant.

@ Grunchy Your assumption is that the time machine stays at one place in spacetime instead of being tied to the matter around it which is no more valid than the idea that you will move with the point on earth that you started at when you move through time until you have the theoretical basis for building your time machine.


I get it, if they don't/didn't follow protocol they just simply never existed. Given the momentum everything has is it that much of a reach that time itself would fall into place?


Dr. Alice Sheldon (AKA James Tiptree Jr.) wrote a short story "The Man who Walked Home" about a time traveler sent into the future in 1989 and spent over 500 years walking back to his starting point, only intersecting with his starting point once a year.

She didn't account for the motion of the solar system around the galaxy, the motion of the galaxy etc... poor fellow would never ever touch Earth again.

Gregg Eshelman

I enjoy this model and would love to experience this great journey. However, I don't believe my senses would allow it. Perhaps they have... deja vu!


@ Gregg Eshelman

The author did account for stellar motion. It is all in the trajectory.


Photons travel in oceans of smaller particles and these particles mimick the bigger photon particles. These smaller particles (gravitons and sub gravitons) can carry the motion of bigger particles over a long period of time. Shadows of moving objects like humans and animals inprinted in these gravitons might once be monitored and reveal history in a new fashion

Theo Prinse

Perhaps another solution or opportunity may present itself; Perhaps we may not be able to go back in time but a message might.

Frank Lyon

perhaps the physics of sending ones' physical form through time may be daunting,but what of ones' thoughts?

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