> According to Englert [...] the Bohm trajectories exist as mathematical objects but “lack physical meaning.”
That sounds strange coming from a defender of orthodoxy. By my (layman's) understanding the rallying cry of the Copenhagen school could be paraphrased as "Just do the maths. Everything else is just metaphysics" or to put it another way - questions about what the equations 'mean' are unscientific and Occam's Razor supports Copenhagen because it is the simplest interpretation that doesn't contradict observation.
>While it is easy to understand and agree with this on the epistemological level, the answer that I and many others would give is that we expect a physical theory to do more than merely predict experimental results in the manner of an empirical equation; we want to come down to Einstein's ontological level and understand what is happening when an atom emits light, when a spin enters a Stern-Gerlach magnet, etc. The Copenhagen theory, having no answer to any question of the form: What is really happening when - - - ?", forbids us to ask such questions and tries to persuade us that it is philosophically naive to want to know what is happening. But I do want to know, and I do not think this is naive; and so for me QM is not a physical theory at all, only an empty mathematical shell in which a future theory may, perhaps, be built.
Of course, all such objections rest on the implicitly defined concept of "what is REALLY happening". But trying to strictly define this - which is necessary before we can meaningfully talk about it - is very tricky, and is firmly in the metaphysical / philosophical realm, and not science.
I disagree. You seem to be asserting that ontologies don't drive empirical or theoretical science, but this is obviously false. Einstein himself devised relativity via thought experiments surrounding ontological properties he believed made sense (such as the light speed barrier). Formulating theories and testing them is exactly what science is all about.
I'm not disputing that intuition can be a driver for empirical science, but that's different from asserting that intuition overrides observable results. If we do see multiple experiments showing that the world is non-deterministic, trying to shoehorn it into a deterministic box because it "makes sense" to us (which is really just a roundabout way to say that our ape brains are wired to think that way because the portion of the physical world that we deal with has that nature), and rejecting theories and interpretations on the grounds that they are "troublesome", is not really science.
I also liked chapter 10 of Jaynes' "Probability Theory: the Logic of Science"...
"Biologists have a mechanistic picture of the world because, being trained to believe in causes, they continue to search for them and find them. Quantum physicists have only probability laws because for two generations we have been indoctrinated not to believe in causes - and so we have stopped looking for them. Indeed, any attempt to search for the causes of microphenomena is met with scorn and a charge of professional incompetence and "obsolete mechanistic materialism." Therefore, to explain the indeterminacy in current quantum theory we need not suppose there is any indeterminacy in Nature; the mental attitude of quantum physicists is already sufficient to guarantee it."
On the contrary, immediately accepting non-determinism simply because that's what you seem to have measured is also unscientific. Or do you believe dipping pencils in water actually breaks them, and then reconstitutes them when you pull them back out? Don't mistake an illusion for reality.
We shouldn't simply naively accept what our experiment seem to be telling us, we should always seek alternative avenues to explain the evidence. It can yield compelling insights.
Sure. But if we stick to the scientific method, then alternative avenues are more experiments. With the pencil, for example, assuming that water actually breaks them is a reasonable premise of the initial experiment, but then you see that it's not actually broken - that's the second observation, disproving the theory.
But if all your experiments say that it is non-deterministic, again and again and again, Occam's razor approach is to say "yes, it is non-deterministic". Creating an elaborate framework which basically lets you say "it's ACTUALLY deterministic, except for all these other things that make it LOOK non-deterministic for all practical purposes" is not really science, it's just placating a preconceived (and possibly hardwired) notion.
> But trying to strictly define this - which is necessary before we can meaningfully talk about it - ...
It is difficult to formally define, but it is not necessary to have such a definition to meaningfully talk about it. This is quite general - many concepts do not have formal definition and certainly not all can have one - formal definition only works well when its expanded form has root in concepts that require no formal definition. "really happening", like "matter" are intuitive concepts that refer to experience, do not have formal definition and so cannot be further reduced.
Jaynes's argument applies equally well to Newtonian mechanics, even if SR/GR/QM didn't exist in the universe.
There is always a yearning for "what is really happening when", and a desire for results that feel real but that is metaphysics and psychology, not physics. It isn't so different from wanting to "feel God's presence", which has been discovered to be a neurologically mechanism.
Funnily enough, a central piece of the Copenhagen Interpretation is the collapse of the wave function. But there is no mathematics to describe the actual process of the collapse, only the outcome. It even happens instantaneously and discontinuously, which natural processes usually don't.
I mean, it does it's job, it gives correct observable results. And everyone knows when and where to apply the "and then it just collapses" rule to get the results. But the rule is defined in English, not mathematically.
I do not think this is a fair description of the Copenhagen Interpretation in practice. I would consider working with density matrices, Lindblad equations, and more general Master equations to still be covered under the Copenhagen interpretation, and in this case there is not much "collapse of the wave function".
Maybe I am just misunderstanding what people mean when they say "Copenhagen Interpretation", but it seem unproductive to argue about it if we do not first include system-environment interactions (e.g. with a Master equation).
When you bring in Linblad and master equations, aren't we then already talking about quantum decoherence? At least Wikipedia puts decoherence as an alternative to, not as a refined form of, Copenhagen Interpretation.
Alternatives to the Copenhagen Interpretation include the many-worlds interpretation, the De Broglie-Bohm (pilot-wave) interpretation, and quantum decoherence theories.
> I would consider working with density matrices, Lindblad equations, and more general Master equations to still be covered under the Copenhagen interpretation
Yes, but neither of those solved the problem of how to describe macroscopic measurements that result in definite outcome (eigenvalue of some operator). The collapse is still needed to continue the description after the measurement, only instead of psi function that should collapse, it is the density matrix that should collapse.
Since when is that the Copenhagen school? I think there are people like that in every school, but the specific claims of the Copenhagen interpretation (like wavefunction collapse) are not the simplest.
That sounds strange coming from a defender of orthodoxy. By my (layman's) understanding the rallying cry of the Copenhagen school could be paraphrased as "Just do the maths. Everything else is just metaphysics" or to put it another way - questions about what the equations 'mean' are unscientific and Occam's Razor supports Copenhagen because it is the simplest interpretation that doesn't contradict observation.