Reply To: What are the most pressing problems? and how to solve them?

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#2212
H. Dieter ZehH. Dieter Zeh
Participant

There is indeed considerable confusion about the most pressing problems of quantum theory and, depending on them, their possible solutions. It is even more unfortunate that this confusion seems to be accompanied by a certain amount of prejudice (for or against some kinds of proposals). Since it seems that statements starting with ”I believe that …” are not very helpful in this debate, let me suggest first to categorize different existing proposals. Everybody may then decide (and tell us) to which category his own proposal belongs (in many cases that may be obvious), or, alternatively, explain why he would dismiss some of these categories for being ”not at all promising” or even inconsistent. I think that every individual proposal that does not explicitly postulate the superposition principle (that is, the origin of the wave function in configuration space and beyond) as part of its kinematics should at least indicate how it would justify the well established general applicability (or possible limits) of this most important principle of quantum mechanics. (Or is this already a prejudice?). Similarly, it should indicate its position regarding non-locality if this is not obvious.

The following is meant as a rough suggestion to begin with, and it can, of course, be subject to change or an introduction of subcategories (but I would prefer not to go into too much detail at this point):

1. Ontic wave function postulated as the exclusive kinematics (psi-complete)
a) collapse of the wave function (modified dynamics)
b) Everett (universal unitarity)
2. Classical kinematics (ontic particles and fields) under novel (non-local) dynamics
a) pilot wave (psi-ontic but not psi-complete)
b) other novel (such as stochastic) dynamics
3. New kinematics (”hidden variables”)
a) including the wave function (psi-ontic but not psi-complete again, but not just classical variables as in 2a)
b) excluding the wave function (psi-epistemic: psi to be explained by ensembles or else)
b1) nonlocal dynamics
b2) nonlocal kinematics other than wave function
b3) both nonlocal
4. Denial of any underlying microscopic reality (pragmatic approach only, that is, ”shut up and calculate”?)
5. Anything else (such as no concepts of kinematics and dynamics any more)

It may be too simple yet (or incomplete), but that question may become part of the debate. For the historians among us it may also be fun (and probably not always trivial) to classify great quantum physicists of the past (or their versions at various times) according to this or a similar scheme. However, it seems that we cannot decide between all these possibilities without any novel empirical evidence. Are they, therefore, presently no more than a matter of taste?

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