Researchers working at the Australian National University (ANU) have conducted an experiment that helps bolster the ever-growing evidence surrounding the weird causal properties inherent in quantum theory. 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, Roman Khakimov, of ANU's Research School of Physics and Engineering conducted a version of John Archibald Wheeler's delayed-choice thought experiment – a variation of the classic double-slit experiment, where light is shown to display characteristics of both waves and particles – where an object moving through open space is provided the opportunity (some would say "a choice") to behave like a particle or a wave.
In this instance, however, the ANU team replicated Wheeler's experiment using multiple atoms, 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 tend to interfere with each other, which can theoretically influence the results.
"An atom is a much more classical particle," Associate Professor Truscott said. "For the theory to hold with a single atom is significant because it proves that it works for particles with mass."
To carry out the experiment, the ANU team initially trapped a collection of helium atoms in 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 a single atom left behind.
This remaining atom was then released to pass through a pair of counter-propagating laser beams (that is, beams moving in opposite directions), which created a pattern to act as a crossroads for the atom in the same way that a solid diffusion grating would act to scatter light.
After this, another laser-generated grating was randomly added and used to recombine the routes offered to the atom. This second grating then indiscriminately produced either constructive or destructive interference as if the atom had journeyed on both 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, and this is the really weird part, the arbitrary number generated to determine if the grating was added or not was only generated after the atom had passed through the crossroads. But, when the atom was measured at the end of its path – before the random number was generated – it already displayed the wave or particle characteristics applied by the grating after it had completed its journey.
According to Truscott, this means that if one chooses to believe that the atom really did take a particular path or paths, then one also has to accept that a future measurement is affecting the atom's past.
"The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behavior was brought into existence," said Truscott. "It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it.”
Even though the findings of the experiment add to the perceived weirdness of quantum theory, the results also validate it. But, even without regard to the weird aspects, quantum physics almost certainly governs the world at the atomic level, and this existence has enabled the development of quantum technologies ranging from cryptography to solar cells.
From an everyday point of view, our minds perceive that an object should behave like a wave or a particle, quite independently of how it is measured. However, as this experiment supports, quantum physics predicts that it doesn’t seem to matter if a 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.
"Quantum physics' predictions about interference seem odd enough when applied to light, which seems more like a wave, but to have done the experiment with atoms, which are complicated things that have mass and interact with electric fields and so on, adds to the weirdness," said Roman Khakimov.
The first time ever that Wheeler's delayed-choice experiment has been conducted using a single atom, the quantum weirdness represented by this experiment much more closely approaches the macro world in which humans perceive reality, which adds to the significance of the findings.
The results of this research were recently published in the journal Nature Physics