Could time travel soon become a reality? Physicists simulate sending quantum light particles into the past

University of Queensland scientists simulate photons moving through time



They showed how two wormhole-travelling photons might behave



Time-travel in the quantum world seems to avoid famous paradoxes

The experiment shows bizarre behaviour of such quantum particles

But on larger scales time travel still remains implausible, say researchers



If a time traveller went back in time and stopped their own grandparents from meeting, would they prevent their own birth?



That’s the crux of an infamous theory known as the 'grandfather paradox', which is often said to mean time travel is impossible - but some researchers think otherwise.



A group of scientists have simulated how time-travelling photons might behave, suggesting that, at the quantum level, the grandfather paradox could be resolved

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Researchers at the University of Queensland in Australia have discovered that two photons travelling through time can interact. In the simulation a photon stuck in a closed timelike curve (illustrated) through a wormhole was found to be capable of interacting with one travelling through regular space-time

The research was carried out by a team of researchers at the University of Queensland in Australia and their results are published in the journal Nature Communications.



WHAT IS A WORMHOLE?

Space-time can be warped and distorted. It takes an enormous amount of matter or energy to create such distortions, but theoretically, distortions are possible. In the case of a wormhole, a shortcut is made by warping the fabric of space-time. Imagine folding a piece of paper with two pencil marks drawn on it to represent two points in space-time.

The line between them shows the distance from one point to the other in normal space-time.

If the paper is now bent and folded over almost double - the equivalent to warping space-time - then poking the pencil through the paper provides a much shorter way of linking the two points, in the same way a wormhole would create a shortcut.

The problem with using wormholes to travel in space or time is that they are inherently unstable. When a particle enters a wormhole, it also creates fluctuations that cause the structure to collapse in on it.

A recent study suggests there are unusual-shaped wormholes than may be able to stay open longer than normal.



The study used photons - single particles of light - to simulate quantum particles travelling back through time.



By studying their behaviour, the scientists revealed possible bizarre aspects of modern physics.

In the simulation, the researchers examined two possible outcomes for a time-travelling photon.



In the simulation, the researchers examined the behaviour of a photon traveling through time and interacting with its older self .

In their experiment they made use of the closely related, fictitious, case where the photon travels through normal space-time and interacts with another photon that is stuck in a time-travelling loop through a wormhole, known as a closed timelike curve (CTC).



Simulating the behaviour of this second photon, they were able to study the behaviour of the first - and the results show that consistent evolutions can be achieved when preparing the second photon in just the right way.



By definition ‘quantum’ refers to the smallest possible particles that can independently exist - such as photons.



However, for macroscopic systems time-travel still faces problematic paradoxes.



In 1991 it was first predicted that time travel would be possible in the ‘quantum world’ because quantum particles behave almost outside the realms of physics.



Wormholes are theoretical tunnels that create shortcuts in space-time. A study in May from Dr Luke Butcher at Cambridge University argued that if a thin wormhole stayed open long enough, people could send messages through time using pulses of light, or photons

'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 professor Timothy Ralph, one of the researchers on the latest study.

The results also give a better understand to how two theories in physics, on the biggest and smallest scales, are able to relate to one another.



'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,' said PhD student Martin Ringbauer from the University of Queensland.



'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.'



Einstein's theory suggests the possibility of travelling backwards in time by following a space-time path that returns to the starting point in space but at an earlier time - a closed timelike curve (CTC).



This possibility has puzzled physicists and philosophers alike since it was discovered by Austrian-American scientist Kurt Gödel in 1949, as it seems to cause paradoxes in the classical world.

These include the 'grandparents paradox', where a time traveller could stop their grandparents from meeting, thus preventing the time traveller's birth.



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

