Prospect magazine has an excellent new article by Philip Ball on recent developments in the fundamental problem of the interpretation of quantum mechanics: why don’t we see superpositions? Most popular discussions of this seem to me to be stuck back in debates from the 1930s, and ignore the main question. QM is a simple, beautiful mathematical structure that works perfectly experimentally, the confusing question is that of how classical behavior emerges during a measurement process (typically involving huge numbers of degrees of freedom, making analysis difficult).

At the end of the article, Ball mentions one particularly intriguing set of ideas, due to Wojciech Zurek, about “quantum Darwinism”. For more about this, see Zurek’s survey here.

There’s a new preprint out by Steven Weinberg on Collapse of the State Vector. Weinberg claims that “There is now no entirely satisfactory interpretation of quantum mechanics”, and refers for more detail about this claim to Section 3.7 of his *Lectures on Quantum Mechanics*, a manuscript that is “to be published”. I’m definitely looking forward to this book when it comes out. The sort of thing that Weinberg examines in the preprint though, modifying QM to take into account wave-function collapse, is the kind of idea I’ve never found promising. Why modify QM, it works perfectly and is mathematically extremely aesthetically compelling? Better to keep QM as is, and closely examine one’s understanding of what the problem really is.

Peter,

I agree that modifying QM does not look promising. As Zurek notes in the paper to which you link, his mathematical approach looks a lot like Everett — which just takes the equations as they come — without making any ontological judgements, which Zurek calls a “virtue”. Not sure I agree on that point, but I do agree with you that we probably shouldn’t go around changing a very successful theory just to find an “acceptable” foundational interpretation — if that’s the point you were making.

The Ball article did not mention Feynman’s approach in ‘QED: the strange theory…’, which seems quite a bit less spooky than the Schrodinger wave approach – though the two are supposed to be equivalent. The possible paths go thru both slits, but nothing like a photon somehow going through both in some spooky fashion.

Joel Rice,

The Feynman path integral formalism is supposed to be completely equivalent to the Schrodinger equation formalism for QM, and has nothing to say about the measurement problem that Ball was writing about. You can’t invoke it to solve the problem.

A “measurement” (at least in human terms) always involves, even if implicitly, space position and time. So it’s not unfathomable if the final answer would be given by unified quantum theory of space-time as suggested by R.Penrose. It just may be that classical notions of space-time are logically incompatible with the quantum world that is supposed to live in it. That’s why asking these questions isn’t useless and may lead, in the eventuality, to a valuable insights.

Anon,

Quantizing space and time variables brings in a much more difficult set of problems. They’re important and interesting, but I’ve never seen much evidence for the idea that they have something to do with the measurement problem. It seems to me that what Zurek is doing is much more promising, concentrating on trying to understand the (difficult enough) problem at hand (QM in a fixed space-time geometry). One should be wary of trying to solve a hard problem by embedding it in an insanely hard one, especially if one hasn’t first completely understood what the hard problem itself actually is.

Could someone comment on whether the Couder experiment (Single-particle interference observed for macroscopic objects) is, in any way, relevant to quantum theory?

Somewhat embarrassingly, I learned of both this experiment and the de Broglie–Bohm theory / Pilot wave theory via an episode of Through the Wormhole (section begins at 16:30).

A request to all: please limit comments to the topics of this post (articles by Ball, Zurek, Weinberg). I’ve no more time for moderating a general discussion of quantum mechanics.

Derek,

I hadn’t heard of the Couder experiment before. From a quick look, it seems like it doesn’t actually have much to do with QM, but rather is an experimental set-up that is supposed to behave deterministically according to the pilot-wave equations. It’s typical that this and de Broglie-Bohm are the sort of thing you “learn” about from “Through the Wormhole”, which completely mixes up real physics issues with hogwash. “Pilot wave theory” may or may not succeed as an awkward way to rewrite single-particle QM, but mathematically it is much more shallow than QM. Furthermore it doesn’t generalize well to a true fundamental theory, which needs to be a relativistic quantum theory of fields and incorporate the symmetries and structures of the Standard Model.

I agree with Mike that what Zurek writes sounds a lot like the many-worlds interpretation. Except that for some reason he doesn’t want to go there, and so he writes gibberish like the paragraph at the end of section III.

For example, he says there that “Quantum states acquire objective existence when reproduced in many copies.” What on earth does that mean? Would he apply this criterion of existence to the Andromeda galaxy as well? You know, there only seems to be one Andromeda galaxy, and yet we don’t consider it any less real for that reason.

Mitchell,

The difference is that Zurek is trying to actually understand how classical behavior emerges from QM, this is not part of the standard Many Worlds Interpretation as far as I know.

I question Ball’s assertion that decoherence can be suppressed with an “isolated” Schrodinger cat experiment. If he means an actual cat, there is no way it can be isolated. If he means a laboratory buckyball as a metaphoric cat, then the essence of the paradox (quantum uncertainty applying to a familiar macroscopic entity) is lost.

Peter,

Zurek article is interesting, but I think it goes into the category “philosophically interesting”. I´m not sure how can one go forward from Zurek´s ideas. Can one suggest new experiments, new tests that will shed new light on what we already know about QM? Otherwise I´m afraid the debate will end up going to where you don ´t want it to go, .i.e., a general debate about QM interpretation, that will be appealing to some but not others with no way of deciding who is right.

For example concerning how classical behavior emerges during a measurement process, how do decide if quantum darwinism is any better then what we learn at undergraduate school if not by philosophical taste?

Off topic, I know, but congratulations for successfully predicting the Nobel this year. (Comments appear to have been turned off in the relevant post.)

I find it refreshing to see a science journalist suggesting that we address interpretational questions experimentally. Can anyone point me to any reviews of the real experiments that explore the classical-quantum boundary (or whatever it is)? I thought Haroche and Raimond did a pretty good job of this in Exploring the Quantum (2006).

Hi Peter,

I enjoyed your book and I’m a fan of your blog. Regarding the Ball article, many discussions of the “measurement problem” neglect the fact that classical and quantum-mechanical measurement theories have much in common. See, for example, A.E. Glassgold and D. Holliday, Phys. Rev. 155, 1431-1437 (1967). In particular, note the discussion of the limit hbar –> 0.

Keep up the good work!

With due respect, as best I can tell, Weinberg’s paper is contentless.

I agree with Peter. Modifying a theory that has never been wrong seems like a bad idea. There are some things worth changing. Throw out those things in the Copenhagen Interpretation that aren’t actually part of QM–for example the idea that a measurement must disturb A, or must disturb it by a certain amount, which is disproved by Aharonov’s weak measurement.