The wave packet description

Careful:"QFT has no realistic, local single event interpretation, period (and mind MWI is not realistic)."

I agree. I claim that non-relativistic QM is complete. I don't claim that QFT is complete. And this is not a question. We discuss the description of the statistical ensembles in terms of wave packets.

(and mind MWI is not realistic).

... for sufficiently naive versions of "realistic"

(meaning: where events really, and uniquely, happen)

You did not answer my question: Do you agree that lossless beamsplitter is real life realization of the "wall"?

Although, for non-relativistic QM, Bohmian mechanics is ontologically clearer, its "clearness" is sometimes overstated, because Bohmian mechanics needs TWO ontological parts:
- the particles, and that's what everybody stresses, and what looks like Newtonian mechanics with an added potential, so this seems at first sight to be very clear
- but there is ALSO, as an independent entity, the wavefunction, which does NOT live in spacetime, but which lives over configuration space all together. It is NOT a classical field, and it contains also all the "ghosts" of MWI.

You still have an interpretational problem, in the following sense: somehow, an "observer", although that observer has TWO ontological parts (its "particle" part, and its "wavefunction" part), can only be "aware" of its "particle part", and not of its "wavefunction" part, because if he were so, there wouldn't be any probabilistic part to it, and hence the predictions of BM wouldn't coincide with QM.

The other problem with BM of course is the fact that it is not compatible with a relativistic spacetime: there is no geometric formulation of BM possible (which would come down to being able to write BM in a lorentz-invariant way, which is not possible).

Not at all. QM is defined in hilbert space, which is the functional space over configuration space. This only coincides with "normal space" in the case of a single point particle.

There are a lot of misunderstandings about MWI. MWI is simply defined, as in "normal" QM, over Hilbert space. This Hilbert space has a basis which can be "indexed" using a configuration space of a classical system, and that classical system can be a field over spacetime, or a set of particles in Euclidean space, or... whatever.

A given "observer" in MWI corresponds to certain subspaces of Hilbert space which correspond to a certain "history of observations" (just like a given observer state in classical phase space corresponds to certain patches in phase space corresponding to a certain "record of observation"). Now, in classical phase space, we usually consider only ONE point which is wandering around (the "state of the universe") in phase space, following a Hamiltonian flow. This point will enter and leave certain "observer patches" and this will correspond to "experienced observations". Given that these patches, classically, are disjoint (you do not have a patch that corresponds at the same time to "the light bulb was on" and "the light bulb was off"), there is no ambiguity to any observation.

In Hilbert space, the patches of "states of observers" are subspaces of hilbert space. And the "state of the universe" is a vector in Hilbert space that follows the unitary evolution of the Hamiltonian flow. However, the difference is now that this state of the universe can have components in DIFFERENT observer subspaces at the same time.
In MWI, we simply say that these different and incompatible observations are then taking place in different "worlds", and that you, as an observer, are just experiencing one of these subspaces and not all of them, simply because we can only experience one subspace. The other subspaces then correspond to experiences of "copies". What matters, for a specific subjective observer, is, what is the probability that he will be one of the copies. It is the specific structure of the subspaces which makes us have the illusion of a "theatre that is like spacetime".
You could compare this situation with a classical phase space where there are different points corresponding to different "worlds" wandering around on the Hamiltonian flow. These different points can then be in different "observer patches" at the same time, but you will only experience "one of these patches".

This is not postulated a priori. It is because it follows naturally out of the Schroedinger evolution equation that this consideration is taken. The reason for postulating "many worlds" is not a crazy idea that is imported, it is because it follows from the formalism. One has introduced a specific EXTRA mechanism in quantum theory to GET RID OF IT, which is projection, but that extra mechanism is the core of all difficulties in QM: it is explicitly non-local and not Lorentz-invariant, irreversible, dynamically ill-defined (when exactly does it happen) etc... It is because of all these difficulties *introduced by the patch that is projection* that Everett first considered to get rid of it, and to keep the one and only dynamical law that is well-defined in QM: hamiltonian unitary evolution. But IF you keep that as a universal dynamics, well then you end up *naturally* with a state of the universe where observers occur in superpositions of "macroscopically different observations". It is just because of this natural appearance of different classical observation states in the "state of the universe" that the idea was then to see this as "parallel worlds". There's no more or no less to it. MWI is simply: let us take the unitary dynamics of quantum theory as fundamental and universal, without introducing a patch to make it fit "classical outcomes" which introduces a lot of difficulties.

So MWI is nothing else but: let us take the hilbert space formalism of QM, and its unitary dynamics, for real, and see what it tells, without wanting to force any specific a-priori of what "should" reasonably, come out.

there are no particles, there are only waves

You are kidding. I am asking seriously. For example, investigations of R.J. Glauber and others established the connection between classical and quantum statistical mechanics. On the other side, the single particle approach also led to enormous progress in QT: QED, local gauge abelian and non-abelian interactions, electroweak unification, quarks, QCD, etc. However, in that game the role of “interpretations” is not clear. Looks like something stand alone.
May you present coherently what is the content of the “normal physicist” criticism of the standard approach to QM?

You cannot discuss physics without taking into account special relativity, that is like going to restaurant and eat with bare hands (which might still be a habit in some parts of the world). I will reverse the question to you, what makes you think that nonrelativistic quantum mechanics has a sensible single event interpretation (there are some ``options'' so it is simply more efficient to ask you) ?

The idea of a public forum is to write something that will be interesting to many people reading it, not just to one person. If anybody else here finds out that some of your arguments are viable, I will give a more scientific answer. If, one the other hand, you want to argue only with me, send me a private message.

Normal QM is defined on a Newtonian background

Again, the hilbert space is a derived concept. What could be the meaning of the configuration space without first defining the background? The position of a particle depends on the number of dimensions in which the particle exists so it doesn't seem to me that the "theatre that is like spacetime" follows from quantum formalism but the other way arround.

Yes, you can. One can get relativistic equations out of nonrelativistic QM.
Here is an example.
Effective theories for low energy corner of nonrelativistic quantum liquids tend
to have Lorentz invariance, among other symmetries. Moreover, one can get something like Einstein's equations for propagation of excitations with, so called, acoustic metric.
c is replaced with the speed of sound in the liquid. See works of Volovik or
his book "The Universe in a helium droplet".

In short you have an absolute frame of reference (the one of the
center of mass of the liquid) and Lorenz invariance (for low energy excitations) in one system.

Cheers!

The wavefunction describes the force acting on the particles. The particles exist in 3D space and the force acts upon them in 3d space as well. We need not to ascribe a fundamental character to the mathematical formalism needed to calculate that force.

And what do you mean by ghosts?

The wavefunction describes how the particles move. I don't understand your point about "awareness". Is this like saying that we are aware of the Moon but not of its gravitational force?

I think there is some debate about this issue.

The Hilbert space is a mathematical construct. The derivation of the wavefunction involves the assumption of point particles existing in a 3d space + 1d time background.

Normal QM is defined on a Newtonian background

Again, the hilbert space is a derived concept. What could be the meaning of the configuration space without first defining the background?

The position of a particle depends on the number of dimensions in which the particle exists so it doesn't seem to me that the "theatre that is like spacetime" follows from quantum formalism but the other way arround.

I have a hard time understanding how can you write down a wavefunction without first assuming point particles in a 3d background. And if you assume that as real, then how can the wavefunction be real and the spacetime an illusion?

As in BM, the wavefunction evolves strictly according to the Schroedinger equation, there is no projection, and hence there are exactly the same "branches" present as in MWI. That's why I sometimes say (to enerve Bohmians) that BM is MWI plus a tag

Yes, you can. One can get relativistic equations out of nonrelativistic QM.
Here is an example.
Effective theories for low energy corner of nonrelativistic quantum liquids tend
to have Lorentz invariance, among other symmetries. Moreover, one can get something like Einstein's equations for propagation of excitations with, so called, acoustic metric.
c is replaced with the speed of sound in the liquid. See works of Volovik or
his book "The Universe in a helium droplet".

In short you have an absolute frame of reference (the one of the
center of mass of the liquid) and Lorenz invariance (for low energy excitations) in one system.

1. In the introduction you notice that the object that satisfies Klein-Gordon equation is not
a wave function but a field operator. However, in the third section you call it a wave
function anyway and even worse you introduce in eq. 3 a third quantized operator
on the right hand side. Do you realize that?
The left hand side of eq. 3, \psi, is already a "second quantized" operator and to get a wave function for a Fock state |n> you should have put \psi in place of \hat \phi in the RHS!

2. But even
there how do you pick initial conditions for the Bohmian trajectories (defined correctly
i.e. not as you did it)?
Don't you have to draw them from some probability density? If the answer is
afirmative then you have your "statistical transparency".