An experiment conducted recently at the University of Queensland in Brisbane has lent further credence to the weirdness underlying the world of quantum mechanics, demonstrating that many of the reassuring epistemological interpretations of the quantum wave function must be invalid in a universe where an objective reality exists.
The very fact that writing about the subject requires such a bizarre qualification as “where an objective reality exists” goes some way toward illustrating the strangeness of the world of quantum mechanics, as do casual references in the paper describing the experiment to “retro-causality” and “many worlds.”
Physicists have engaged in an uncomfortable game of Twister to bend their theories around the observable outcomes of quantum experimentation since the early 20th century. Having to choose between unintuitive concepts, such as superposition of states or the ability to change the past, can make anyone of a logical bent a little cranky. Einstein’s ideas on the subject have been fighting a rear-guard action, and this latest paper represents another defeat for the epistemological view of quantum theory.
Schrodinger’s cat may be the most well-known feline in physics, and can be explained thus:
The theory goes that, if you were to put a cat into a sealed box with a substance that had a 50/50 chance of decaying within an hour and releasing a vial of poisonous gas, then what we have is a cat in a box with a 50% chance of living and a 50% chance of dying. Pretty simple so far.
So, dead cats and living cats…what do they have to do with physics? Well what Schrodinger was trying to show was that, after an hour, the cat would be in a superposition where it would be both alive and dead at the same time to whoever is on the outside of the box. This is because the actual outcome wouldn’t be decided until the person checked inside the box. I mean, if we can’t see what’s happening inside the box then we don’t know the fate of the cat right? This relates to physics because quantum objects can be in a supeposition of two different states (or places) at he same time until they are observed. At this point, they collapse into just one.
So in short, due to the unintuitive nature of quantum measurement, the cat could be described as being simultaneously dead and alive… though Schrodinger intended the thought-experiment as an illustration of the absurdity of that very concept.
Since the conception of quantum field theory, there have been differing schools of thought over the reality of quantum superposition — the idea that a system may exist in two simultaneous, contradictory states prior to observation — and over the existence of an objective reality in the first place.
One camp consisted of Niels Bohr and Werner Heisenberg, who felt that the very question of a discrete state before measurement was meaningless … the so-called Copenhagen interpretation, which holds that observation itself “collapses” the wave function into the observed state, and that only the probabilities existed before that act.
Schrodinger, like Einstein, rejected the idea that no further explanations were to be had behind the probability-driven character of quantum observations. The quantum wave function was not a real artifact, in and of itself, but merely a mathematical model describing some underlying function which scientists were, as of yet, unable to fully appreciate or describe. Or, as the paper (PDF) describes it, “ … the wavefunction has to be treated as a representation of the limited knowledge about the real state of the system.” The cat must be either dead or alive, and, as common sense might dictate, and only the observer’s inability to make the determination without affecting the outcome was in question.
Therefore, if the idea that a cat could be simultaneously both dead and alive, as the ontological interpretation of the wave function apparently demanded, was ridiculous, so was the idea that such probabilistic notions undergirded particle and wave interactions on a small scale.
Unfortunately for Einstein, Schrodinger, and the cat, the Brisbane team was able to put together an experiment involving encoded photons shot through a beamsplitter and into a detector measuring the encoding. According to this New Scientist article, if the wave function was only a mathematical tool, the results should have shown that the photon was in that single encoded state all along. But in fact, the results were unable to determine what state the photons were in before they were measured.
Another explanation of the experiment, by team member Eric Cavalcanti, can be read here on Phys.org. In it, Cavalcanti likens the experiment to randomly picking cards from two different decks with potentially overlapping states and finding results which cannot be supported by the number of cards in the decks.
These uncomfortable results all but guarantee that the strangeness of the universe is bound to get increasingly strange, it’s starting to look like the very idea of a concrete, underlying reality might be the most unlikely idea of all.