Despite common sense telling us that an object behaves like either a wave or a particle independent of our actions (or lack thereof), quantum theory states its behaviour actually hinges on the way we measure it – a quirk that once prompted Richard Feynman to describe it as a “mystery that cannot go away”.
In order to put this peculiar hypothesis through yet another test, a group of physicists from the Australian National University (ANU) have conducted John Wheeler’s canonical “delayed-choice” thought experiment, which involves a moving object that is given the “choice” to act like a particle or a wave, and is designed to determine at which point does it “decide” what course to take.
Even though Wheeler’s experiment, proposed way back in 1978, was thought at the time to be impossible, the ANU team managed not just to build, but also to reverse its original concept (realised back in 2007), based on photons bouncing against mirrors, by using atoms scattered by light.
“A photon is in a sense quite simple,” said project leader Associate Professor Andrew Truscott from the ANU Research of Physics and Engineering. “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.”
First, the team trapped around a hundred helium atoms in a suspended state known as the Bose-Einstein Condensate, and then – to rule out the possibility that the atoms were affecting each other – 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. These were shown to cause atoms to either pass through one arm, like a particle, or both, like a wave.
To recombine the paths, a second light grating was randomly added, which led to constructive or destructive interference as if the atom had travelled both paths. In the absence of the second light grating, however, no interference was observed as if the atom chose only one path.
Crucially, the random number determining whether the grating was added was only generated after the atom had passed through the crossroads.
Truscott claims that 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. 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,” he said. “It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it.”
Details of the experiment were published in the current issue of Nature Physics.