Wednesday, March 28, 2018

Discussions of quantum mechanics

In a sure sign that I'm getting old, I find myself tempted to read some of the many articles, books, and discussions about interpretations of quantum mechanics that seem to be flaring up in number these days.  (Older physicists seem to return to this topic, I think because there tends to be a lingering feeling of dissatisfaction with just about every way of thinking about the issue.)

To be clear, the reason people refer to interpretations of quantum mechanics is that, in general, there is no disagreement about the results of well-defined calculations, and no observed disagreement between such calculations and experiments.   

There are deep ontological questions here about what physicists mean by something (say the wavefunction) being "real".  There are also fascinating history-of-science stories that capture the imagination, with characters like Einstein criticizing Bohr about whether God plays dice, Schroedinger and his cat, Wigner and his friend, Hugh Everett and his many worlds, etc.  Three of the central physics questions are:
  • Quantum systems can be in superpositions.  We don't see macroscopic quantum superpositions, even though "measuring" devices should also be described using quantum mechanics.  Is there some kind physical process at work that collapses superpositions that is not described by the ordinary Schroedinger equation?   
  • What picks out the classical states that we see?  
  • Is the Born rule a consequence of some underlying principle, or is that just the way things are?
Unfortunately real-life is very busy right now, but I wanted to collect some recent links and some relevant papers in one place, if people are interested.

From Peter Woit's blog, I gleaned these links:
Going down the google scholar rabbit hole, I also found these:
  • This paper has a clean explication of the challenge in whether decoherence due to interactions with large numbers of degrees of freedom really solves the outstanding issues.
  • This is a great review by Zurek about decoherence.
  • This is a subsequent review looking at these issues.
  • And this is a review of "collapse theories", attempts to modify quantum mechanics beyond Schroedinger time evolution to kill superpositions.
No time to read all of these, unfortunately.


Peter said...

Although it's not ready for prime time, I've found other experimental physicists respond fairly positively to an interpretation of QFT as a signal analysis formalism.

On the experimental side, modern experiments typically operate by engineering some part of a circuit to couple to whatever local "conditions" may be, very often using exotic materials (usually with the rest of the circuit from and to the computer that analyzes and creates records of the experiment as shielded as possible; fiber optics may be used as well as electrically conducting wires). The signal analysis, in hardware and software, to distinguish when discrete signal events happen in a noisy analogue signal and the electronic feedback to control, for example, dead times after a discrete event may have to be extremely elaborate.

On the theory side, one finds, in particular, that we can construct states in the Hilbert space of the quantized EM field (for the simplest physically relevant example) using a random field on Minkowski space instead of using the customary quantum field on Minkowski space. Measurement incompatibility is well known to classical signal analysis in the context of time-frequency distributions, and entanglement is also natural for classical random fields when they are presented in a Hilbert space formalism (what is usually referred to as a Koopman-von Neumann approach to classical physics).

One can therefore interpret quantized EM to say that local "conditions" are classical, it's just the measurements that are quantum mechanical. When you're ready to spend a little time, you might add, "Classical states, quantum field measurement" to your list of ways to understand QFT.

David Brown said...

"Is the Born rule a consequence of some underlying principle, or is that just the way things are?" My guess is that the preceding question cannot be answered correctly without a correct explanation of Milgrom's MOND. I say that Milgrom is the Kepler of contemporary cosmology — my guess is that the correct explanation of MOND requires a new paradigm in the foundations of physics. Google "kroupa milgrom", "mcgaugh milgrom", "sanders milgrom", and "scarpa milgrom".
Modified Newtonian dynamics, Wikiquote

Peter Morgan said...

Because of your post here,, where you linked to this post, I came back, confirming my slight memory that I had commented.
"Classical states, quantum field measurement" was published in Physica Scripta in 2019, with quite a few changes,, and part of that was expanded into a paper that was published in Annals of Physics 2020, "An algebraic approach to Koopman classical mechanics",
The ideas are still not ready for prime time, but they might be getting a little closer. It's still largely about signal analysis, however, and how we can understand the relationship between classical mechanics and quantum mechanics as signal analysis formalisms (because what comes out of an experiment can always, I suppose, be thought of as a signal, which a computer records in a compressed form at some number of Megabits per second; there's something of a throwback to Bohr's way of thinking that detailed descriptions of an experiment must be classical in that). There seems to have been slowly increasing interest, but my sense is that people can't see how to use the ideas, so I still have a few years to go.
I'll try out the argument in a new paper, "The collapse of a quantum state as a signal analytic sleight of hand",, without much expectation that anyone here will read even this, still less the whole article: the idea there, in paraphrase, is that we can construct both/either a noncommutativity+collapse picture of measurement and/or a commutativity+no-collapse picture of measurement, and/or other pictures in between. I personally find the mathematics of that helps to understand more clearly the relationships between quite a diverse range of interpretations of quantum mechanics as well as the relationship between classical mechanics and quantum mechanics. Annals of Physics is currently considering it for publication.
I very rarely have anything to say about nanoscale views posts because my focus is so much on foundations, but I follow it closely in the occasionally realized hope that I will absorb something about condensed matter physics from your always interesting perspective. I'll take this opportunity to thank you for them.