A new physics experiment has verified the concept of quantum indeterminacy, where a particle exists as both a particle and a wave until observed. As well, the experiment also shows that the "decision" made by the particle can travel backward in time.

The theory of quantum indeterminacy was first proposed to explain the outcome of what is called the "double-slit" experiment, where a single sub-atomic particle, such as a photon or electron, appears to pass through two openings simultaneously. The idea is that, at the quantum level, particles exist as both particles and waves simultaneously, and don’t resolve themselves into one or the other until observed. Now, physicists at The Australian National University’s Research School of Physics and Engineering have performed an experiment originally proposed by quantum physicist John Wheeler in 1978, which would address the idea of when the object being measured actually decides to act as either a particle or a wave.

However, instead of using sub-atomic scale objects that are traditionally used in the double-slit experiment, the team used helium atoms instead. "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," explains Roman Khakimov, a PhD student working on Truscott’s team.

A singled-out helium atom was dropped through a pair of counter-propagating laser beams, to simulate a grating, much like how a solid grating would scatter light in the previous double-slit experiment. A second grating was added at random to recombine the atom, to allow for it to either produce an interference pattern if the grating wasn’t there, indicating that it had traveled via two paths at once. The experiment succeeded, showing that the atom had traveled through two paths at once when the second grating was omitted, and via only one path when the second grating was added.

However, the random number used to determine whether the second grating was activated or not wasn’t added until after the atom had passed through the first grating, indicating that the atom decided what state it would be after it was supposed to be affected by the apparatus. "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", explains team leader Professor Andrew Truscott. "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."

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