Wednesday, February 20, 2013

Quantum Mechanics: As Mystifying Now as 100 Years Ago

A new survey of physicists working in the field of quantum mechanics (including a poll reported in a recent preprint on the physics arXiv server1),) discloses that the so-called experts remain as mystified as ever. The semi-serious poll of 33 key thinkers on the fundamentals of quantum theory shows that opinions on some of the most profound questions are fairly evenly split over several quite different answers.

For example, votes were roughly evenly split between those who believe that, in some cases, “physical objects have their properties well defined prior to and independent of measurement” and those who believe that they never do. Despite the famous idea that observation of quantum systems plays a key role in determining their behavior, 21% felt that “the observer should play no fundamental role whatsoever”.

Regrettably, QM is not a descriptive field, i.e. amenable to straightforward English interpretation. It is primarily a mathematical theory. If one ventures outside the bounds of the mathematical descriptions to offer English interpretations, one risks nonsense. I believe it was Feynman, in a Preface to his 'Lectures in Physics', Vol. III, who remarked that once one tries to use purely English descriptions to button hole QM he or she will "disappear down a rabbit hole, never to appear again."

For readers who want a comprehensive and understandable book that provides the basis for the Copenhagen Interpretation, I recommend Heinz Pagels `The Cosmic Code' (Bantam, 1982) - which will dispel a lot of incorrect perceptions and assumptions that have accumulated over the past 25 years. In his book, Pagels endorses the best policy in quantum mechanics as simply being a `fair witness'. That means absolutely avoiding embellishment and exaggeration of the results, including projection of personal `fantasies'. If one insists on reading more into quantum measurement results than their statistical significance allows, self delusion ensues.

Back to the poll devised by Anton Zeilinger of the University of Vienna, together with Maximilian Schlosshauer, now at the University of Portland, Oregon, and Johannes Kofler at the Max-Planck-Institute of Quantum Optics in Garching, Germany. It was then disseminated at a meeting of the Templeton Foundation where attendees were given 16 multiple-choice questions on key foundational issues in quantum theory.

Disagreements over the theory’s interpretation have existed ever since it was first developed, but Zeilinger and his colleagues believe that their poll might be the first to interrogate the full range of views held by experts. A previous poll at a 1997 quantum mechanics workshop in Baltimore asked attendees the single question of which interpretation of quantum theory they favored most.

Probably the most famous dispute about what quantum theory means was that between Albert Einstein and his peers, especially the Danish physicist Niels Bohr, on the question of whether the world was fundamentally probabilistic rather than deterministic, as quantum theory seemed to imply. One of the few issues in the new poll on which there was something like a consensus was that Einstein was wrong. Quantum theory IS probabilistic!

This is because most quantum physicists still adhere to the comprehensive, original interpretation of quantum theory developed in the 1920s: the so-called Copenhagen interpretation. This proposed that the physical world is unknowable and in some sense indeterminate, and the only meaningful reality is what we can access experimentally. The mathematical underpinning was perhaps first explicated by Max Born, who showed the quantum wave function (PSI) was a statistical artifact, not a real physical wave.

Consider: you want to find the probability that some particle will be found in a region of length a. The probability is given as an integral in terms of the wave function (PSI):  INT (-oo to + oo) PSI* (PSI) dx

where PSI =

A sin (2π x/ a) exp (- iEt/h/2π)

and PSI* = A sin (2π x/ a) exp (iEt/ h/2π)

with h/ 2π = 1.054 x 10 -34   J-s (modified Planck constant h)

Obviously, without doing the full integration, the complex functions for PSI and its complex conjugate (PSI*) cannot be identified with any real world entity. Hence (Max) Born's conclusion to treat the wave function as primarily statistical in nature is amply justified.

As the earlier Baltimore meeting, the Austrian poll found the Copenhagen interpretation to be favored over all others, including the Stochastic interpretation of David Bohm and Brian Hiley (which treats PSI as a physically real wave) and the ‘Many World’ interpretation of Hugh Everett. Even so, only 42% of the voters endorsed the Copenhagen. However, the same 42% also admitted that they had switched interpretation at least once. And whereas a few decades ago the options were very few, says Schlosshauer, “today there are more ‘sub-views’.”

The most striking implication of the poll is that, while quantum theory is one of the most successful and quantitatively accurate theories in science, interpreting it is as plagued with as many controversies as it was at the outset.  According to Schlosshauer “Nothing has really changed, even though we have seen some pretty radical new developments happening in quantum physics, from quantum information theory to experiments that demonstrate quantum phenomena for ever-larger objects. Some thought such developments would push people one way or the other in their interpretations, but I don't think there’s much evidence of that happening.

However, he says there was pretty good agreement on some questions, adding.

“More than two-thirds believed that there is no fundamental limit to quantum theory — that it should be possible for objects, no matter how big, to be prepared in quantum superpositions like Schrödinger’s cat. So the era where quantum theory was associated only with the atomic realm appears finally over.”

Wow! So we can now look at the superposition effect experienced in rockets and automobiles?

Other notable views: 42% thought that it would take 10–25 years to develop a useful quantum computer, whereas 30% placed the estimate at 25–50 years. Meanwhile in the much debated role of measurement in quantum theory — how and why measurements affect outcomes —the votes split with 24% regarding it as a severe difficulty and 27% as a “pseudoproblem”.

Zeilinger and colleagues do not claim that their poll is rigorous or necessarily representative of all quantum researchers. John Preskill, a specialist in quantum information theory at the California Institute of Technology in Pasadena, suspects that “a broader poll of physicists might have given rather different results”.

Perhaps the most amazing line of agreement from all participants is that quantum theory does its job so well yet stubbornly resists answering our deeper questions. Maybe this "contains a lesson in itself,” according to Schlosshauer. In fact, the most revealing answer may well have been that 48% believe that there will still be conferences on the foundations of quantum theory in 50 years time.

I would not dispute that at all!

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