Is Quantum Theory About Reality or What We Know?

One just says "we fundamentally can't know, lulz random" and the other says "we could know, if we knew everything". PWT just has the local probabilistic behavior because we can't know the little fiddly bits of the rest of the universe, even if we know the dominant local forces. Copenhagen just says that it does random shit, no reason.

I'm not sure why you think the other interpretation of QM gives predictive power PWT doesn't.

Ed: To specifically address the stats point --

PWT says that if we had total knowledge of the universe, we could determine that the coin would always be, eg, heads under specific conditions, but due to measurement limitations we can only say that it will come up 50/50, and that the discrepancy between the theoretical ability to perfectly predict and our need to statistically approximate is related to our ability to measure (and compute).

By contrast, Copenhagen says there is no coin between when it leaves your hand and when it lands, so clearly we can't do any better to predict than statistics of how often we see the coin heads on the table after it leaves our hand.

You seem to fault PWT for making the same predictions as other models, just because it doesn't declare the problem of doing better fundamentally impossible.

How does "hyper-determinism" differ from "determinism"?

I find that when people use adjectives like "hyper" or "ultra" for some theoretical view (like people who talk about 'ultra-darwinism') they are trying to make something appear "extreme" without making any real argument for that position.

> You end up going into hyper-determinism, which is almost all about negating free will.

That phrasing implies that people come up with, or want, "hyper-deterministic" views because they want to negate the idea of free will. Is that what you mean? If not, what are you trying to say?

This isn't too surprising, because it's a QM interpretation that's designed to be indistinguishable from the others. The MWI is unfalsifiable (you can't see the other worlds) and Copenhagen is too (you can't​ see wave function collapse actually happen).

You seem to think this is a bad thing. Why?

Clearly this is useless if that is all it provided. But so was classical mechanics. For example, to compute the gravitational force on my finger, technically, one needs in Newtonian theory to know the position and masses of all particles in the whole universe. Equally clearly, that's neither practical nor relevant.

Instead, we use the well-defined dynamics to do a statistical analysis of the theory. What happens when we do that is that we get a quantum thermodynamics equivalent which is what the Copenhagen interpretation is. But Copenhagen suffers from being vague and undefined (what the heck is a macroscopic measurement?) It is very imprecise and not a theory at all. And yet, when it appears as a statistical framework from an underlying well-define theory, the measurement problem completely recedes.

And it is important to understand that it is the theory that ought to tell us what a measurement actually means.

The next point to raise is, what is the point if we just derive Copenhagen? Well, the point is that it allows to both understand the setup and thus extend it. If all questions were known and answered, it would be largely philosophical. But we have not resolved the problems (QM + relativity) in standard quantum theory after many decades of research.

What are a few things that PWT explain?

1) Identical particles. By having configurations of particles, one can investigate what a configuration is. And, after a moment's thought, identical particles are not labelled by nature. Choosing a configuration space of unlabelled particles, we end up with the identical particle choices of symmetric or antisymmetric wave functions. It gets more complicated with spin, but it turns out spin is best represented as a value of the wave function located at a spatial point, not an intrinsic part of the particle. And then the problems just disappear from that.

2) Arrival times. There is a time of arrival naturally enough in a theory with particles actually moving about. In most theories, the collapse or splitting of the universe, etc., does not really fit in with experiments of time arrival. That doesn't stop physicists from coming up with answers, of course, but a theoretical framework is conceptually trivial based on this.

3) Collapse of the wave function. This collapse is central to the whole business. There is no collapse in the fundamental theory. But in suitable situations (measurements), the environment can be conditioned on to get a local wave function for the system in question. When a measurement happens with the environment registering it, the environmental conditioning changes that local wave function and that appears as a collapse.

This also helps deal with the murkiness of a position measurement which is never some precise state to collapse into. It is all a bit wishy-washy which is a problem for a theory such as Copenhagen in which that is all there is. It is trivial shrug in PWT.

4) Creation and annihilation of particles. There are particles that exist and they can be born or destroyed. There are formulas that give a probabilistic form for the creation of particles. This makes the theory no longer deterministic, but that was never the point of PWT. The point was to be an actual theory, one which was well-defined. There is nothing wrong with randomness. So the creation and annihilation of particles is perfectly consistent with PWT. It requires wave functions defined on a disjoint union of configuration spaces of different number of particles. This is exactly what quantum field theory does.

5) Well-defined quantum field theory. It is legendary that QFT is not well-defined mathematically. Recent work, inspired by PWT, looks promising in finding a well-defined path in QFT. Essentially, the perturbation from a free evolution cannot work. Instead, one has to work with functions in which the different number of particles spaces are linked in such a way as to preserve the probability. Once that is done, divergences fade away. The mathematical work is still quite difficult and it is early days, but it has had successes in toy theories that were not workable before.

6) Relativity. This is the big one. By having a clear theory to work with, one can understand the nonlocality that is present in reality (Bell's theorem) [unless you deny the results of experiments when they happen as in MWI]. There are explorations of natural foliations of space-time which provides the kind of nonlocality needed. In terms of something that works mathematically, that exists. A theory that is philosophically satisfactory is still being pursued. Essentially, the foliation is needed but is not detectable which is kind of what is needed, but it is unsatisfactory.

PWT is not a return to determinism. It is a return to clearly defined theories, whether they are deterministic or not. PWT is a theory that one could envision a computer being fed data and then computing out without further intervention. Copenhagen is not like that. One needs to know what experiments and measurements are to be done which, of course, can't really be specified in advance since what experiments are done may rely on what happened in the past.

Hypothesis 2: Schroedinger's cat is simultaneously both alive and dead. Our taking a look will "collapse the wave function" which could then result in the cat dying.

Is there any concrete evidence to believe in hypothesis 2 over hypothesis 1? If not, I'd rather go with the hypothesis that doesn't sound absurd.

In fact, it'd make Einstein laugh. His late life was devoted to unify the theories and asking quantum physicists to demonstrate the quantum theory in the real world or through though experiments. He'd probably still be upset today.

I, for one, am still waiting for the 'unified relative quantum theory' as well.

Are you sure ?

> The field of quantum lithography is in its infancy, and although experimental proofs of principal have been carried out using the Hong–Ou–Mandel effect,[3] it is still a long way from commercial application.

From https://en.wikipedia.org/wiki/Quantum_lithography

Could you give a bit more detail on how the observatsions, plus nuclear physics, plus general relativity, led to that conclusion?

"Is there a science of the wavefunction beyond QM?"

Can you ask more questions about it, break it down into constituent processes, maybe re-engineer it?

Or is it opaque and closed and there is no possible way to do further science on it?

My money is on option 1, because it's hard to imagine an organising principle you can't possibly do any science on.

Wavefunction science may be weird and like nothing ever seen before - especially given non-locality and possibly non-realism. But it's a good rule of thumb in science that when nature becomes counterintuitive you're on the right track. If the answer was obvious and familiar you wouldn't need original and creative thinking to find it.

Option 2 seems like an admission of total defeat. It would be the first time in science where we hit a wall and didn't even try to understand what it was made of.

That doesn't mean it can't be correct. But it does mean we shouldn't give up without trying for quite a while longer. And if we do give up, it should only be after discovering some kind of meta-Bell theorem which proves there's no way to go further.