Quantum Model Drift
7 September 2012
Some time ago in The Pleasures of Model Drift I discussed how contemporary cosmology is challenged by the accelerating expansion of the universe, and that there are no really good explanations of this yet in terms of the the received cosmological models. The resulting state of cosmological theories, then, is called model drift. This is a Kuhnian term. Almost exactly a year ago, when it was reported that some neutrinos may have traveled faster than light, it looked like we might also have had to face model drift in particle physics. Since these results haven’t been replicated, the standard model not only continues to stand, but has recently been fortified by the announcement of the discovery (sort of discovery) of the Higgs Boson.
But theoretical physics isn’t over yet. Some time ago in The limits of my language are the limits of my world I took up Wittgenstein’s famous aphorism from the perspective of recent work in particle physics that had “bent” the rules of quantum theory. Further work by at least of of the same scientific team at Centre for Quantum Information and Quantum Control and Institute for Optical Sciences, Department of Physics, University of Toronto, Aephraim M. Steinberg, has continued this line of research, which has been reported by the same BBC Science and technology reporter, Jason Palmer, who wrote up the earlier results (cf. Quantum test pricks uncertainty). This story covers research reported in Physical Review Letters, “Violation of Heisenberg’s Measurement-Disturbance Relationship by Weak Measurements.”
The abstract of this most recent research reads as follows:
While there is a rigorously proven relationship about uncertainties intrinsic to any quantum system, often referred to as “Heisenberg’s uncertainty principle,” Heisenberg originally formulated his ideas in terms of a relationship between the precision of a measurement and the disturbance it must create. Although this latter relationship is not rigorously proven, it is commonly believed (and taught) as an aspect of the broader uncertainty principle. Here, we experimentally observe a violation of Heisenberg’s “measurement-disturbance relationship”, using weak measurements to characterize a quantum system before and after it interacts with a measurement apparatus. Our experiment implements a 2010 proposal of Lund and Wiseman to confirm a revised measurement-disturbance relationship derived by Ozawa in 2003. Its results have broad implications for the foundations of quantum mechanics and for practical issues in quantum measurement.
Experimentalists are chipping away at Heisenberg’s Uncertainty Principle. They aren’t presenting their research as something especially radical — one might even think of this recent work as an instantiation of radical theories, modest formulations — but this is theoretically and even philosophically quite important.
We recall that despite himself making crucial early contributions to quantum theory, Einstein eventually came reject quantum theory, offering searching and subtle critiques of the theory. In his time Einstein was isolated among physicists for coming to reject quantum theory at the very time of its greatest triumphs. Quantum theory has gone on to become one of the most verified theories — and verified to the most exacting standards — in the history of physics, notwithstanding Einstein’s criticisms. Einstein primarily fell out with quantum theory over the notion of quantum entanglement, though Einstein, himself a staunch determinism, was also greatly troubled by Heisenberg’s uncertainly principle. Many, perhaps including Einstein himself, conflated physical determinism with scientific realism, so that a denial of determinism came to be associated with a rejection of realism. Heisenberg’s uncertainly principle is Exhibit “A” when it comes to the denial of determinism. So I think that if Einstein had lived to see this most recent work, he would have been both fascinated and intrigued by its implications for the uncertainty principle, and indeed its philosophical implications for physics.
Einstein was a uniquely philosophical physicist — the very antithesis of what recent physics has become, and which I have called Fashionable Anti-Philosophy (and which I elaborated in Further Fasionable Anti-Philosophy). From his earliest years, Einstein carefully studied philosophical works. He is said to have read Kant’s three critiques in his early teens. And Einstein’s rejection of quantum theory, which he modestly and humorously characterized as saying that something in his little finger told him that it couldn’t be right, was a philosophical rejection of quantum theory.
The recent research into Heisenberg’s uncertainly principle is not couched in philosophical terms, but it is philosophically significant. The very fact that this research is going on suggests that others, not only Einstein, have been dissatisfied with the uncertainly principle as it is usually interpreted, and that scientists have continued to think critically about it even as the uncertainty principle has been taught for decades as orthodox physics. This is a perfect example of what I have called Science Behind the Scenes.
The uncertainty of quantum theory, given formal expression in Heisenberg’s uncertainty principle, came to be interpreted not only epistemically, as placing limits on what we can know, but it was also interpreted ontologically, as placing limits on the constituents of the world. In so far as Heisenberg encouraged an ontological interpretation of the uncertainty principle, which I believe to be the case, he was advancing an underdetermined theory, i.e., an ontological interpretation of the uncertainty principle goes beyond — I think it goes far beyond — the epistemic uncertainty that we must posit in order to do quantum theory.
It seems to me that it is pretty easy to interpret the recent research cited above as questioning the ontological interpretation of the uncertainty principle while leaving an epistemic interpretation untouched. The limits of human knowledge are often poignantly brought home to us in our daily lives in a thousand ways, but we need not make the unnecessary leap from limitations on human knowledge to limitations on the world. On the other hand, we also need not make any connection between realism and determinism. It is entirely consistent (even if it seems odd to some of us) that there should be an objective world existing apart from human experience of and knowledge of that world, and that that objective world should not be deterministic. It may well be that it is essentially random and only stochastically defined, when a given radioactive substance decays, but the radioactive substance and the event of decay are as real as Einstein’s little finger. If I could have a conversation with Einstein, I would try to convince him of precisely this.
Indeterminate realism is also an underdetermined theory, and it is to be expected that there are non-realistic theories that are empirically equivalent to indeterminate realism. It is for this reason that I believe there are other arguments, distinct from those above, that favor realism over anti-realism, or even realism over some of the more extravagant interpretations of quantum theory. But I won’t go into that now.
We aren’t about to return to classical theories and their assumptions of continuity such as we had prior to quantum theory, any more than we are about to give up relativistic physics and return to strictly Newtonian physics. That much is clear. Nevertheless, it is important to remember that we are not saddled with any one interpretation of relativity or quantum theory, and we are especially not limited to the philosophical theories formulated by those scientists who originally formulated these physical theories, even if the philosophical theories were part of the “original intent” (if you will forgive me) of the physical theory. Another way to put this is that we are not limited to the original model of a theory, hence model drift.
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