The most popular understanding of Quantum Mechanics (the Copenhagen Interpretation) argues that in quantum reality a particle exists in infinite number of possible states until someone (us?) observes the particle and causes the probability cloud to collapse to a unique observable state. This raises questions of exactly how the particle knows it's being observed... and why our observations should be effectively privileged. (Which is the weird part of the Schroedinger's Cat thought experiment.
A simple way to imagine this was suggested by Einstein and others years ago. Imagine a photon stream traveling toward a double slit. If the stream is a wave, we'll see an interference pattern on the other side of the slits. If it's a particle, we'll see a ballistic distribution pattern. Einstein suggested that if we could determine which slit a single photon went through, we could effectively destroy the interference pattern just by having that knowledge. (Which again raises the question of how the photon knows that we know...?)
This was mostly just a cool thought experiment until today.
An announcement was made that a team of physicists have been able to observe a single photon as it goes through a double slit. And the resulting observations appear to mean that we're going to have chuck the Copenhagen interpretation out the window in favor a little known interpretation proposed by de Broglie himself! (along with Bohm).
From a report:
""In our experiment, a new single-photon source developed at the National Institute for Standards and Technology in Colorado was used to send photons one by one into an interferometer constructed at Toronto. We then used a quartz calcite, which has an effect on light that depends on the direction the light is propagating, to measure the direction as a function of position. Our measured trajectories are consistent, as Wiseman had predicted, with the realistic but unconventional interpretation of quantum mechanics of such influential thinkers as David Bohm and Louis de Broglie," said Steinberg.
The original double-slit experiment played a central role in the early development of quantum mechanics, leading directly to Bohr's formulation of the principle of complementarity. Complementarity states that observing particle-like or wave-like behaviour in the double-slit experiment depends on the type of measurement made: the system cannot behave as both a particle and wave simultaneously. Steinberg's recent experiment suggests this doesn't have to be the case: the system can behave as both.
"By applying a modern measurement technique to the historic double-slit experiment, we were able to observe the average particle trajectories undergoing wave-like interference, which is the first observation of its kind. This result should contribute to the ongoing debate over the various interpretations of quantum theory," said Steinberg. "It shows that long-neglected questions about the different types of measurement possible in quantum mechanics can finally be addressed in the lab, and weak measurements such as the sort we use in this work may prove crucial in studying all sorts of new phenomena."
Full article here.
The Bohm-de Broglie understanding is a non-local model, but a deterministic one. It appears that the Universe is giving us back determinism as long as we're willing to accept a strong form of associated spooky action at a distance.
So, we're both deterministic and non-local at the same time. What fun getting our heads wrapped around this conundrum is going to be!