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+classical physics

Is the macroscopic ‘classical’ world also governed by the laws of quantum mechanics? If so, how is the classical situation to be explained starting from quantum mechanics? If not, what modifications of quantum mechanics are expected on the road from microscopic to macroscopic physics?

+decoherence

Does decoherence help us understand and resolve the difficulties of quantum foundations, and if so how?

+Everett/many worlds

Does this resolve the fundamental problems of quantum mechanics, or does it simply create more problems?

+hidden variables

As in Bell inequalities or Bohmian mechanics. Do they help us understand quantum mechanics, or are they a dead end?

+information

How does one understand information in quantum mechanical terms? Does an information theoretical approach help us understand quantum mechanics?

+locality/nonlocality

Are there such things as instantaneous nonlocal influences? If so, how does this fit with special relativity? If not, how shall we understand quantum correlations as in EPR-Bohm?

+measurement problem

Is measurement essential to any formulation of quantum mechanics, or can measurements be described using fundamental quantum principles that make no reference to measurements? If the latter, what are those principles, and how can they be used to describe measurements?

What shall we do with superpositions of different measurement outcomes (Schrodinger cat states)?

What do measurement outcomes (pointer positions) tell us about the (microscopic) situation that existed before the measurement took place?

+probabilities in quantum mechanics

What is the right way to introduce and discuss probabilities in the framework of quantum mechanics? (This is connected with lots of other things:

decoherence, information, measurement, time development.)

+relativity and quantum mechanics

1. Can quantum mechanics and special relativity be combined into a single coherent theory?

2. Can quantum mechanics and general relativity be combined, possibly by modifying one or the other or both?

+spontaneous collapse

The proposals of Ghirardi, Pearle, etc. for modifying the Schrodinger equation by adding a stochastic term. Does this help us understand quantum mechanics or is it simply another dead end? What are its chances of being confirmed by experiments?

+time development

Is the time development of quantum systems governed by Schrodinger’s

(deterministic) equation, or is it, in whole or in part, stochastic (probabilistic)?

+wavefunction meaning

Does the quantum wavefunction represent reality (ontic), or only our knowledge of, or information about, reality (epistemic), or perhaps something else?

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My position on these:

+classical physics

I believe the classical physics is a very useful approximation to a more fundamental quantum physics which applies at every length scale. The connection has been worked out in part by Gell-Mann and Hartle, and by Omnes, but much more could be done.

+decoherence

I don’t think decoherence by itself resolves the quantum measurement problem. On the other hand, I consider it valuable for understanding how classical physics arises out of quantum mechanics, and hope that further research will provide a more precise understanding of decoherence.

+Everett/many worlds

In my opinion consistent histories provides a much more satisfactory solution than Everett (and many worlds) to the quantum mysteries.

+hidden variables

I consider this a dead end. Experiments have not been kind to Bell inequalities, and consistent histories have put an end to the (supposed) nonlocality that pervades Bohmian mechanics.

+information

I have published various papers in the technical quantum information literature, and at least one paper in the quantum foundations literature, on what I believe to be the proper quantum mechanical way (using consistent

histories) to extend Shannon’s information theory to the quantum domain.

The earlier enthusiasm in the foundations community for unraveling quantum paradoxes using information theory seems to have run into trouble and I am not surprised: they had no answer to Bell’s question: “Information ABOUT WHAT?”

+locality

I think the quantum world is local, and have published a long paper, which has not yet been challenged (perhaps because no one has read it!), demonstrating this. I think Bell was wrong.

+measurement problem

The fundamental formulation of quantum mechanics given by consistent histories makes no mention of measurement, and applying it to the standard von Neumann measurement model disposes (in my opinion) of Schrodinger’s cat, and tells one that the experimentalists are right when they believe their apparatus measures what it was designed to do. So I regard this problem as solved, even though no doubt more could be said.

+probabilities in quantum mechanics

The consistent histories approach was initiated in order to deal with probabilities in the quantum context, and I think it has solved the problem–while I allow that there may be alternative or better approaches.

+relativity and quantum mechanics

1. Special relativity. If Gell-Mann and Hartle had thought there was any conflict between special relativity and decoherent histories (their version of consistent histories) they would probably never have published their work.

I have confirmed the compatibility of the two using my own approach. There is no difficulty once one has a consistent way of introducing probabilities in quantum theory.

2. General relativity. This is outside my domain of competence, so I am hoping that Jim Hartle will come up with a solution …

+spontaneous collapse

The problems which motivated the development of GRW and related have, in my opinion, been solved by consistent histories, so I am no longer concerned about them. Of course, any genuine and confirmed deviation from standard quantum mechanics is worth a trip to Stockholm. but I doubt if this one is going to materialize.

+time development

My answer is that it is in general stochastic, though in particular circumstances one can think of it as deterministic.

+wavefunction meaning

As indicated at a previous workshop, the most common use of the quantum wavefunction is to calculate probabilties so a ‘pre-probability’ in my terminology, though in some circumstances it can indicate a quantum state (which is best regarded as the corresponding ray in Hilbert space).

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