Reality does not exist until it is measured

Physicists at The Australian National University (ANU) have performed a famous experiment in quantum theory, created by the late theoretical physicist John Wheeler, which suggests that reality does not exist until it is measured. Their study has been published in the journal Nature Physics.

Originally proposed in 1978, Wheeler’s delayed-choice thought experiment involves a moving object that is given the choice to act like a particle or a wave. The experiment then asks - at which point does the object decide?

Common sense says the object is either wave- or particle-like, independent of how we measure it. But quantum physics predicts that whether you observe wave-like behaviour (interference) or particle behaviour (no interference) depends only on how it is actually measured at the end of its journey.

The ANU team carried out the experiment utilising atoms scattered by laser light, as opposed to Wheeler’s original concept of light beams being bounced by mirrors. Study co-author Roman Khakimov noted, “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.”

The team first trapped a collection of helium atoms in a suspended state known as a Bose-Einstein condensate and then ejected them until there was only a single atom left. The single atom was then dropped through a pair of counter-propagating laser beams, which formed a grating pattern that acted as crossroads in the same way a solid grating would scatter light.

A second light grating to recombine the paths was randomly added, which led to constructive or destructive interference as if the atom had travelled both paths. When the second light grating was not added, no interference was observed as if the atom chose only one path. However, the random number determining whether the grating was added was only generated after the atom had passed through the crossroads.

According to lead author Associate Professor Andrew Truscott, if one chooses to believe that the atom really did take a particular path or paths, then one has to accept that a future measurement is affecting the atom’s past.

“The atoms did not travel from A to B,” he said. “It was only when they were measured at the end of the journey that their wave-like or particle-like behaviour was brought into existence.”

The results confirm the validity of quantum theory and prove that “measurement is everything”, according to Associate Professor Truscott.

“At the quantum level, reality does not exist if you are not looking at it.”

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