Researchersworking at the Australian National University (ANU) have conducted anexperiment that helps bolster the ever-growing evidence surrounding the weird causal properties inherent in quantumtheory. In short, they have shown that reality does not actually exist until it is measured – at atomic scales, at least.

Associate Professor Andrew Truscott and his PhD student, RomanKhakimov, of ANU's Research School of Physics and Engineering conducted a version of John Archibald Wheeler's delayed-choicethought experiment – a variation of the classic double-slit experiment, where light is shown to display characteristics of both waves andparticles – where anobject moving through open space is provided the opportunity (some would say "achoice") to behave like a particle or a wave.

In thisinstance, however, the ANU team replicated Wheeler's experiment using multipleatoms, which was much more difficult to do than a test using photons. This extra difficulty is due to the fact that, as they have mass, atoms tendto interfere with each other, which can theoretically influencethe results.

"Anatom is a much more classical particle," Associate Professor Truscottsaid. "For the theory to hold with a single atom is significant because itproves that it works for particles with mass."

To carryout the experiment, the ANU team initially trapped a collection of helium atomsin a Bose-Einstein condensate (a medium in which a dilute gas is cooled to temperatures very close to absolute zero),and then forcibly ejected them from their containment until there was only asingle atom left behind.

This remainingatom was then released to pass through a pair of counter-propagating laserbeams (that is, beams moving in opposite directions), which created a pattern toact as a crossroads for the atom in the same way that a solid diffusion gratingwould act to scatter light.

Afterthis, another laser-generated grating was randomly added and used to recombinethe routes offered to the atom. This second grating then indiscriminately producedeither constructive or destructive interference as if the atom had journeyed onboth paths. Conversely, when the second light grating was not randomly added,no interference would be introduced, and the atom would behave as if it had followed only one path.

However, andthis is the really weird part, the arbitrary number generated to determine if thegrating was added or not was only generated after the atom had passed throughthe crossroads. But, when the atom was measured at the end of its path – beforethe random number was generated – it already displayed the wave or particle characteristics applied by the grating after it had completed its journey.

Accordingto Truscott, this means that if one chooses to believe that the atom really didtake a particular path or paths, then one also has to accept that a futuremeasurement is affecting the atom's past.

"Theatoms did not travel from A to B. It was only when they were measured at theend of the journey that their wave-like or particle-like behavior was broughtinto existence," said Truscott. "It proves that measurement iseverything. At the quantum level, reality does not exist if you are not lookingat it.”

Eventhough the findings of the experiment add to the perceived weirdness of quantumtheory, the results also validate it. But, even without regard to the weirdaspects, quantum physics almost certainly governs the world at the atomic level,and this existence has enabled the development of quantum technologies ranging fromcryptography to solar cells.

From aneveryday point of view, our minds perceive that an object should behave like awave or a particle, quite independently of how it is measured. However, as thisexperiment supports, quantum physics predicts that it doesn’t seem to matter ifa particle or object should show wave-like behavior or particle-like behavior;it all depends on how it is actually measured at the end of its journey.

"Quantumphysics' predictions about interference seem odd enough when applied to light,which seems more like a wave, but to have done the experiment with atoms, whichare complicated things that have mass and interact with electric fields and soon, adds to the weirdness," said Roman Khakimov.

The firsttime ever that Wheeler's delayed-choice experiment has been conducted using asingle atom, the quantum weirdness represented by this experimentmuch more closely approaches the macro world in which humans perceive reality,which adds to the significance of the findings.

Theresults of this research were recently published in the journal Nature Physics

Source:ANU