Pieter Kuiper via Wikimedia Commons. A comparison of double slit interference patterns with different widths. Similar patterns produced by atoms have confirmed the dominant model of quantum mechanics

Physicists have succeeded in confirming one of the theoretical aspects of quantum physics: Subatomic objects switch between particle and wave states when observed, while remaining in a dual state beforehand.

In the macroscopic world, we are used to waves being waves and solid objects being particle-like. However, quantum theory holds that for the very small this distinction breaks down. Light can behave either as a wave, or as a particle. The same goes for objects with mass like electrons.

This raises the question of what determines when a photon or electron will behave like a wave or a particle. How, anthropomorphizing madly, do these things “decide” which they will be at a particular time?

The dominant model of quantum mechanics holds that it is when a measurement is taken that the “decision” takes place. Erwin Schrodinger came up with his famous thought experiment using a cat to ridicule this idea. Physicists think that quantum behavior breaks down on a large scale, so Schrödinger's cat would not really be both alive and dead—however, in the world of the very small, strange theories like this seem to be the only way to explain what we we see.

In 1978, John Wheeler proposed a series of thought experiments to make sense of what happens when a photon has to either behave in a wave-like or particle-like manner. At the time, it was considered doubtful that these could ever be implemented in practice, but in 2007 such an experiment was achieved.

Now, Dr. Andrew Truscott of the Australian National University has reported the same thing in Nature Physics, but this time using a helium atom, rather than a photon.

“A photon is in a sense quite simple,” Truscott told IFLScience. “An atom has significant mass and couples to magnetic and electric fields, so it is much more in tune with its environment. It is more of a classical particle in a sense, so this was a test of whether a more classical particle would behave in the same way.”

Trustcott's experiment involved creating a Bose-Einstein Condensate of around a hundred helium atoms. He conducted the experiment first with this condensate, but says the possibility that atoms were influencing each other made it important to repeat after ejecting all but one. The atom was passed through a “grate” made by two laser beams that can scatter an atom in a similar manner to a solid grating that can scatter light. These have been shown to cause atoms to either pass through one arm, like a particle, or both, like a wave.

A random number generator was then used to determine whether a second grating would appear further along the atom's path. Crucially, the number was only generated after the atom had passed the first grate.

The second grating, when applied, caused an interference pattern in the measurement of the atom further along the path. Without the second grating, the atom had no such pattern.

An optical version of Wheeler's delayed choice experiment (left) and an atomic version as used by Truscott (right). Credit: Manning et al.

Truscott says that there are two possible explanations for the behavior observed. Either, as most physicists think, the atom decided whether it was a wave or a particle when measured, or “a future event (the method of detection) causes the photon to decide its past.”

In the bizarre world of quantum mechanics, events rippling back in time may not seem that much stranger than things like “spooky action at a distance” or even something being a wave and a particle at the same time. However, Truscott said, “this experiment can't prove that that is the wrong interpretation, but it seems wrong, and given what we know from elsewhere, it is much more likely that only when we measure the atoms do their observable properties come into reality.”