First posted 21 Sep 2004. Reading back through it now, this seems a descent summary of about as far as I got to in understanding the debates on the interpretation of quantum mechanics. I'm not at all sure things have progressed in the last 13 years. I know that my understanding hasn't!
I find it hard to ascertain exactly what the current status of the quantum mechanics (QM) interpretation debate is.
I know that Einstein believed that QM was incomplete as a theory because it suggested an irreducibly random atomic world with 'spooky action at a distance'. He believed that God would do better by us than that, that the world must be causally deterministic and local. The seeming randomness behind QM, according to him, really came down to our lack of understanding. Future theoretical and experimental work would discover currently hidden variables that would put QM back onto causal, ontological grounds.
Niels Bohr won the day, though, with his interpretation, rooted in logical positivism. QM, he said, does not describe a quantum world but rather only our observations and measurements of subatomic activity. We cannot know and should not be concerned with what, if any, metaphysical entities underlie those observations. In bolder moments, he seemed to suggest the stronger claim that no such entities did exist independent of observation.
J.S. Bell, hoping to swing the argument back in the direction of Einstein's thinking, developed a test for whether any hidden variables approach might conform with experimental results. Alain Aspect devised the actual experiment to carry these tests out, and the resulting data supported Bohr's view, which has since come to be called the Copenhagen Interpretation (CI). Many tests, some similar and others quite different, have confirmed those initial findings.
But let's look more closely at just what those findings were. Bell's theoretical test and the experiments based on it were meant to decide whether any LOCAL and DETERMINISTIC hidden variables theory could fit our observations. David Bohm came up with a wholly deterministic, ontological interpretation of QM that passes Bell's test. Bohm's theory did not wholly satisfy Einstein, though, because it was still non-local (allowed action at a distance) - intentionally and explicitly so.
[Non-locality is demonstrated by the entanglement of particles that have interacted with one another, where the choice of measurement on one impacts the other (via conservation laws) instantaneously, even at a great distance. I need to understand this better, as it seems to me that the measurement of the one particle only affects WHAT WE CAN KNOW about the second particle, which is very different from having a causal impact on the particle itself. Anyway, many people say that such instantaneous 'action' is not a problem, as long as it doesn't involve the transmission of information at above light speed (which it doesn't)].
The scientific community has never liked Bohm's approach. As far as I can tell, this is for two reasons. First, it gives a privileged place to position vis-a-vis momentum, two characteristics of any particle that the CI gives equal weight. This asymmetry lacks elegance. Second, it imparts a physical reality to the 'wave function,' which in CI is simply a mathematical tool for describing the evolution of an unobserved system. CI itself is unable to explain the collapse of this wave function at the time of measurement, but Bohm's theory runs afoul of Ockham's Razor, which says that we should not call into existence any more entities than absolutely necessary in explaining a phenomenon. Still, Bohm's interpretation is perfectly consistent with the mathematical formalism of QM and with observed behaviour of sub-atomic systems.
In 1995, Tim Palmer pointed out that, strictly speaking, J.S. Bell's test and the results to date do NOT preclude there being a local, deterministic hidden variables theory behind QM. Chaos theory has introduced new and powerful concepts to the stage. There exist wholly deterministic non-linear dynamic systems that are non-computable - that is, there is no algorithmic way to 'solve' for their results. These systems have interlaced riddled basins (a basin is the 'region' of initial conditions that 'flows' to a given attractor). Palmer showed that just such a system could fit perfectly with the seemingly random elements of QM and the 'collapse' to concrete states upon measurement. Palmer updated and amended his approach in a later paper, but with the same conclusion - that a local deterministic interpretation of QM is NOT impossible.
Of course, neither does that mean that QM necessarily IS local or deterministic. It just means we cannot, as so many orthodox QM theorists have wanted to, close the door on the possibility. I have so far not found any responses to Palmer's papers, so I don't know whether the 'community' sees the debate as an open one or not.
Anton Zeilinger noted in his survey of the interpretations of QM that it did indeed seem to be lacking some fundamental, unifying principle. He went on to nominate just such a principle, that 'An elementary system carries one bit of information.' Here, an elementary system is a fundamental building block of the material world - like the spin of an electron. What does this principle do? Well, it explains why our experience of the microscopic world is quantised - because information, the only access we have to that world, is itself quantised. We interrogate the sub-atomic world with yes-no questions, and the answers cannot be broken down into anything simpler (e.g. smoother) than the 0s and 1s that our computers use. Zeilinger's principle also provides a more intuitive explanation of the uncertainty principle and entanglement. Zeilinger himself subscribes to Bohr's CI and believes that his information-based view gives additional support to that interpretation.
Of course I realise that I understand very little of the guts of information theory and quantum theory. I still can't help pointing out that the implications drawn from Zeilinger's principle concern what we can know about the world rather than what the world is actually like. It is epistemological rather than ontological in nature. This may be entirely appropriate, since what is beyond our knowing is beyond our knowing - full stop. Yet I still feel that we should make some commitment to what our best inference therefore points to as regards what the world IS like. David Bohm took such an ontological approach. He also died believing that information stood alongside matter and energy as a fundamental component of nature. I am anxious to reconcile (at my surface level of understanding) his views with the emerging insights from Zeilinger's work.