Where does a quantum experiment *begin*?

[zonde]

QFT, as it is formulated and used on the Standard Model, realizes the "no faster than light signalling" by the (probably) stronger assumption of local interactions

1. A model that is using only local interactions can not model correct statistics for entangled particle measurements.
2. A model that can model correct statistics for entangled particle measurements has to involve non-local interactions.
This follows from Bell theorem and other similar theorems or counter examples.

I have impression that QFT corresponds to the second case. I don't know why you believe there is nothing non-local in QFT but I suppose that you are imagining that it's possible to arrive at correct statistics for entangled particle measurements using only passive transformations of any non-local element of QFT (state in Fock space) while it actually requires active transformations.

 

[vanhees71]

This is nonsense. QED is modelling all the quantum optical experiments with entangled photon pairs correctly, and that's the paradigmatic example of a local relativistic QFT! You must not mix up local interactions (a basic assumption of relativstic QFT) and long-ranged correlations described by entangled states. The latter are of course possible in QFT. What all this has to do with active vs. passive (Poincare?) transformations, I don't know.

 

[atyy]

This is nonsense. QED is modelling all the quantum optical experiments with entangled photon pairs correctly, and that's the paradigmatic example of a local relativistic QFT! You must not mix up local interactions (a basic assumption of relativstic QFT) and long-ranged correlations described by entangled states. The latter are of course possible in QFT. What all this has to do with active vs. passive (Poincare?) transformations, I don't know.

Are the local interactions "physical" or "real"?

 

[zonde]

QED is modelling all the quantum optical experiments with entangled photon pairs correctly, and that's the paradigmatic example of a local relativistic QFT!

Can you give reference to example of QED model for polarization entangled photon measurements at different measurement angles?

 

[atyy]

I think vanhees is right about the incompatibility of the collapse with relativity. However, it's not because of locality, but rather because of the incompatibility with the Poincare group. There is an interplay between time evolution, translations, rotations and boosts, which is encoded in the Poincare group relations and its Lie algebra. In quantum theory, compatibility with relativity is guaranteed by the use of unitary representations of the (centrally extended universal cover of the) Poincare group. If you claim that collapse is compatible with relativity, you must explain in what sense it is supposed to satisfy the Poincare group relations (representation theory won't work, since it is a non-linear operation) and then show that it actually satisfies them. I don't think anyone has done this and I don't see how it is supposed to work. For example, in the Poincare group, two time evolutions always commute, but projectors of non-commuting observables don't commute in general. How do you resolve this issue? Another example: There is a Poincare group element corresponding to a time translation followed by a Lorentz boost. What is the Poincare group element corresponding to a collapse followed by a boost?

Well, why don't you show that collapse is not compatible with relativity? While I know of no proof that it is, all particular cases I know of show that it relativity is not violated, eg. http://arxiv.org/abs/0706.1232

 

[Demystifier]

Ok, those other worlds, do they have things that could be observed (tables, chairs, galaxies, space, time)? It seems that the answer must be yes.

Yes.

So, do they come with their own space and time? Or are the space and time shared? Since there is just one wave function it must be that space and time are unique, they are the domain of the wave function. But in my world, where I can observe things, I have access to all of space and I observe things only in some places (say a particle whent up). The other worlds will have observers that observe other things elsewhere (a particle went down). Yet, it there is just one space where all this happens.

Ah, by space and time you mean a 4-dimensional manifold, right? Well, that space and time is not shared and not in the domain D. The shared space and time in D has much more dimensions. In the simplest description this number of dimensions is 3N+1, where N is at least of the order of $10^{80}$.

 

[Demystifier]

Well, why don't you show that collapse is not compatible with relativity? While I know of know proof that it is, all particular cases I know of show that it relativity is not violated, eg. http://arxiv.org/abs/0706.1232

It should also be pointed out that motion faster than light is not in contradiction with relativity (unless some additional assumptions are taken). See e.g.
http://lanl.arxiv.org/abs/1205.1992

 

[Demystifier]

No, I only assume that at one given moment of time(when detected) the particle can exists at one place only.

Then there must be some other assumption you are tacitly taking. I am not sure what it is, so please explain to me in your own words: What exactly do you mean by "particle" and how exactly is it related to the "wave function"? From that I will probably know why MWI does not make sense to you and which of your assumptions should be questioned in order to make sense of MWI. (Unless you already decided that MWI does not make sense, in which case there is no point in bothering.)

 

[atyy]

In fact, the big message of Bell's theorem is that surprisingly - nonlocality is compatible with relativity, operationally defined.

 

[rubi]

Well, why don't you show that collapse is not compatible with relativity?

You are the one who claims that collapse is compatible with relativity, so you are obliged to prove it. What are your answers to the questions I askes in my previous post? How can collapse be compatible with relativity despite non-commutativity of projections?

While I know of know proof that it is, all particular cases I know of show that it relativity is not violated, eg. http://arxiv.org/abs/0706.1232

Your paper addresses some paradoxes, but it doesn't prove the compatibility. Compatibility with relativity means that full Poincare symmetry is somehow implemented. If this can be done, then you should be able to answer the two questions that I askes in my previous post. Moreover, I'd like to see the mathematical implementation of the Poincare group.

1. A model that is using only local interactions can not model correct statistics for entangled particle measurements.
2. A model that can model correct statistics for entangled particle measurements has to involve non-local interactions.
This follows from Bell theorem and other similar theorems or counter examples.

I have impression that QFT corresponds to the second case. I don't know why you believe there is nothing non-local in QFT but I suppose that you are imagining that it's possible to arrive at correct statistics for entangled particle measurements using only passive transformations of any non-local element of QFT (state in Fock space) while it actually requires active transformations.

You keep repeating your personal theories that aren't in accordance with accepted science. If PF were serious about its rules, you should have been banned by now. QM is fully compatible with locality. One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

 

[ddd123]

Can you give reference to example of QED model for polarization entangled photon measurements at different measurement angles?

I asked about this earlier. I'd like to know as well.

 

[zonde]

You keep repeating your personal theories that aren't in accordance with accepted science. If PF were serious about its rules, you should have been banned by now.

There is "Report" link under every post. Simply push it and write what rule this post violates as you see it. All moderators will see your message and will take action if necessary.

QM is fully compatible with locality. One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

I like this counter example to locality of QM. It does not use any probabilities.

 

[vanhees71]

Are the local interactions "physical" or "real"?

They are clearly defined in the mathematical formalism, using the equal-time (anti-)commutation relations, local realizations of the unitary irreps. of the covering group of the Poincare group with $m^2 \geq 0$ and polynomial Lagrangians of the fields and their first derivatives, which leads to a Poincare covariant S-matrix and microcausality.

 

[vanhees71]

Can you give reference to example of QED model for polarization entangled photon measurements at different measurement angles?

Take any textbook on quantum optics, e.g., Scully&Zubairy for the foundations. The classical paper on the parametric-downconversion process is

C. K. Hong and L. Mandel, Phys. Rev. A 31, 2409 (1985)
http://dx.doi.org/10.1103/PhysRevA.31.2409

 

[rubi]

I like this counter example to locality of QM. It does not use any probabilities.

There is of course no counterexample to a proven theorem. Using frequencies instead of probabilities doesn't change that. The existence of a local probabilistic model that predicts the QM probabilities proves beyond doubt that these probabilities are compatible with locality. What's wrong with your counterexample? Frequency proofs of Bell's theorem make the same assumptions, they are just less obvious, because nobody is used to the frequency formulation. The choice of subsequences in the frequency formulation of probability is dual to the choice of a conditional probabilities in the measure formulation.

 

[ShayanJ]

One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

That seems to be an important paper. Can we call it an interpretation of QM?

 

[David Lewis]

...is it true that we can have a situation of two "equal probability paths" without having a situation that is modeled by a superposition of states?

Yes. If we send particles through one at a time, and place detectors that tell us which path was taken then there can be no superposition. The presence of the detectors force the particle to either take one path or the other.

 

[atyy]

You are the one who claims that collapse is compatible with relativity, so you are obliged to prove it. What are your answers to the questions I askes in my previous post? How can collapse be compatible with relativity despite non-commutativity of projections?

Your paper addresses some paradoxes, but it doesn't prove the compatibility. Compatibility with relativity means that full Poincare symmetry is somehow implemented. If this can be done, then you should be able to answer the two questions that I askes in my previous post. Moreover, I'd like to see the mathematical implementation of the Poincare group.

My claim is at the physics level of rigour - in the same way that QED is said to be compatible with relativity. In fact, my claim is found in the standard textbooks.

You keep repeating your personal theories that aren't in accordance with accepted science. If PF were serious about its rules, you should have been banned by now. QM is fully compatible with locality. One can embedd the QM probabilities in a local classical probabilistic model (https://arxiv.org/abs/1412.6987), so the compatibility with locality is established at full rigor.

It's you that it is not accepting standard science.

 

[atyy]

They are clearly defined in the mathematical formalism, using the equal-time (anti-)commutation relations, local realizations of the unitary irreps. of the covering group of the Poincare group with $m^2 \geq 0$ and polynomial Lagrangians of the fields and their first derivatives, which leads to a Poincare covariant S-matrix and microcausality.

Sure, but if you use that language, then conjugate position and momentum can also exist simultaneously. In the Heisenberg picture, after a preparation, the time evolution of position and momentum observables are governed by the Hamiltonian, and they remain canonically conjugate.

 

[vanhees71]

There's no state where position and momentum are determined simultaneously. If a position operator exists (i.e., for all kinds of massive particles or for massless particles with spin $\leq 1/2$), then is obeys the Heisenberg algebra with momentum, which implies that for any state the uncertainty relation $\Delta x \Delta p \geq \hbar/2$ holds. Of course, the Heisenberg algebra of the operators in the Heisenberg picture is invariant under time evolution since time evolution is unitary.

 

[atyy]

There's no state where position and momentum are determined simultaneously. If a position operator exists (i.e., for all kinds of massive particles or for massless particles with spin $\leq 1/2$), then is obeys the Heisenberg algebra with momentum, which implies that for any state the uncertainty relation $\Delta x \Delta p \geq \hbar/2$ holds. Of course, the Heisenberg algebra of the operators in the Heisenberg picture is invariant under time evolution since time evolution is unitary.

The position and momentum observables exist simultaneously.

 

[vanhees71]

What do you mean by that. Of course, they exist if they exist (a photon, e.g., has no position observable, but that's another story). What has this to do with the discussion about locality of interactions, etc.?

 

[atyy]

What do you mean by that. Of course, they exist if they exist (a photon, e.g., has no position observable, but that's another story). What has this to do with the discussion about locality of interactions, etc.?

What I mean is that the locality you are talking about is locality of the Hamiltonian - these concern operators just like position and momentum. So if those "exist" for you, then in the same language position and momentum also simultaneously exist.

So the language you use is not at all the usual language. In the usual language, canonically conjugate position and momentum do not simultaneously exist.

Also in the usual language, quantum mechanics does require nonlocal interactions, assuming reality.

The "locality" that you are talking about should be called "no faster than light signalling".

Relativistic quantum mechanics in the minimal interpretation is about no faster than light signalling. It is not at all about the locality of interactions of mathematical terms representing physical objects, In fact, Bell's theorem guarantees that no such local theory can exist.

Basically, there are two notions of locality - classical relativistic causality (which is where we usually say that interactions are local) and no faster than light signalling, and you are mixing them up.

 

[vanhees71]

I think this is totally confusing. What do you mean by "exist" here? Of course, the observables exist, because you can measure them. In standard relativistic QFT the "no faster than light signalling" is realized by using local interactions. Bell's inequality is violated by QT and also by standard relativistic QFT although it's built with local interactions. So something is mathematically wrong in your argument. I only talk about the mathematical construction of the theory. We should this get clear first, before we enter the unsharp terrain of philosophy again!

 

[atyy]

I think this is totally confusing. What do you mean by "exist" here? Of course, the observables exist, because you can measure them. In standard relativistic QFT the "no faster than light signalling" is realized by using local interactions. Bell's inequality is violated by QT and also by standard relativistic QFT although it's built with local interactions. So something is mathematically wrong in your argument. I only talk about the mathematical construction of the theory. We should this get clear first, before we enter the unsharp terrain of philosophy again!

But what is the interaction between? The fields? If you mean observables exist, then the field do not necessarily exist, since the fields may not be Hermitian - eg. fermion fields.

 

[vanhees71]

Fermion fields are usually not representing observables. Observables are defined via Noether's theorem, which provides you with energy, momentum, and angular momentum (densities) and with various conserved current densities of global symmetries. Everything is expressed in terms of the field operators, of course. For the details, see a good textbook on QFT (e.g., Schwartz or for the finer details and more generality Weinberg).

 

[atyy]

Fermion fields are usually not representing observables. Observables are defined via Noether's theorem, which provides you with energy, momentum, and angular momentum (densities) and with various conserved current densities of global symmetries. Everything is expressed in terms of the field operators, of course. For the details, see a good textbook on QFT (e.g., Schwartz or for the finer details and more generality Weinberg).

So in your language fermion fields do or do not exist?

 

[Demystifier]

@atyy and @vanhees71 you will never come to an agreement because you are talking at two very different levels. While atyy discusses the ontology, vanhees71 talks at the epistemic level. Since it is impossible to make vanhees71 talk about ontology, perhaps you could both agree to talk only about the epistemic aspects? But then you would come to an agreement very soon, and there would be nothing to talk about.

 

[atyy]

@atyy and @vanhees71 you will never come to an agreement because you are talking at two very different levels. While atyy discusses the ontology, vanhees71 talks at the epistemic level. Since it is impossible to make vanhees71 talk about ontology, perhaps you could both agree to talk only about the epistemic aspects? But then you would come to an agreement very soon, and there would be nothing to talk about.

No actually, I prefer to talk at the epistemic level (ie. minimal interpretation). But at the epistemic level, collapse is compatible with relativity.

So I suspect that vanhees71 is objecting to collapse because he is working at the ontological level.

In fact, my standard position is epistemic or operational - it is vanhees71 who brings in ontology by objecting to collapse.

(You can see that this in my post #2, where I use collapse in the standard shut up and calculate way. It is vanhees71 who is somehow objecting to shut up and calculate.)

 

[stevendaryl]

I think this discussion has drifted off from the question asked in the thread title (the answer to that question is: a physics experiment begins with a grant application )

I like to think of the collapse hypothesis as having two parts:

1. When you measure an observable, you get an eigenvalue (with probabilities given by the Born rule).
2. Afterward, the system is in the state obtained by projecting the state onto the subspace corresponding to that eigenvalue.

(There is a sense in which rule #2 is only relevant for entangled systems. Typical measurements are destructive; when you measure a photon, the photon is gone afterward. So rule #2 comes into play when you have entangled subsystems: measuring a property of one subsystem can cause the other subsystem to "collapse" into a particular state.)

Rule #2 is definitely true, empirically, in the sense that it correctly predicts subsequent measurement results. But it's possible that it isn't necessary as an additional assumption, because you can always recast a sequence of measurements as a single, compound measurement, and so rule #1 would be sufficient. Rule #2 is more of a practical rule of thumb, because without collapse, you can't calculate probabilities for a sequence of measurements without describing the measuring devices quantum mechanically, which is infeasible. Collapse allows us to treat macroscopic measurement devices as if they were classical, having definite states at all times, and reserve QM for the description of microscopic systems (or extremely simple macroscopic systems).

To me, the weirdness of QM comes not from the collapse hypothesis, but from Rule #1. Why are observed values definite, when unobserved values are not? That seems to me to make "measurement" into a different class of interaction, but surely measurement should be explainable as quantum mechanical interactions between the system being measured and the measuring device?

 

[vanhees71]

I think this discussion has drifted off from the question asked in the thread title (the answer to that question is: a physics experiment begins with a grant application )

This made my day! It's very true!

 

[ddd123]

(You can see that this in my post #2, where I use collapse in the standard shut up and calculate way. It is vanhees71 who is somehow objecting to shut up and calculate.)

As I understood it, vanhees71 says that you can avoid using collapse altogether although it makes things more difficult in certain cases. So having collapse even as a calculational tool is not really "minimal", if you see the minimal interpretation not as the way to shut up and calculate but as the minimal core of quantum theory. And for him this also makes things more elegant with respect to relativity, since changing the inertial frame doesn't force you to change where collapse is supposed to happen.

 

[vanhees71]

@atyy and @vanhees71 you will never come to an agreement because you are talking at two very different levels. While atyy discusses the ontology, vanhees71 talks at the epistemic level. Since it is impossible to make vanhees71 talk about ontology, perhaps you could both agree to talk only about the epistemic aspects? But then you would come to an agreement very soon, and there would be nothing to talk about.

I don't care whether it's ontic or epistemic, I'm talking about physics, and theory has a mathematical level, which should be clarified first. The words "local interaction" and "long-range correlations" have a very clear and unambiguous meaning, and are well compatible as is proven by relativistic local QFT. A collapse implies non-local interactions and is thus not compatible with the relativistic space-time structure and not with the usual formulation of relativistic QFT which is compatible with this very space-time structure by construction.

If it comes to "ontology" it's very simple for a physicist: Things exist that can be observed reproducibly and objectively in nature.

 

[atyy]

As I understood it, vanhees71 says that you can avoid using collapse altogether although it makes things more difficult in certain cases. So having collapse even as a calculational tool is not really "minimal", if you see the minimal interpretation not as the way to shut up and calculate but as the minimal core of quantum theory. And for him this also makes things more elegant with respect to relativity, since changing the inertial frame doesn't force you to change where collapse is supposed to happen.

Yes, you can avoid collapse the same way you can avoid having the Bell inequalities violated at spacelike separation. But if we are talking at the level at which the Bell inequalities are violated at spacelike separation, then that is the same level at which standard quantum mechanics does contain collapse. Mind you, vanhees71 has explicitly said a standard text like Cohen-Tannoudji is not acceptable to him on the issue of collapse. So I am just using standard Cohen-Tannoudji, or Sakurai, or Weinberg, or Nielsen and Chuang which all have collapse. The big issue about interpretation is always brought up by vanhees71 because somehow the standard physics textbooks are not acceptable.

 

[atyy]

I don't care whether it's ontic or epistemic, I'm talking about physics, and theory has a mathematical level, which should be clarified first. The words "local interaction" and "long-range correlations" have a very clear and unambiguous meaning, and are well compatible as is proven by relativistic local QFT. A collapse implies non-local interactions and is thus not compatible with the relativistic space-time structure and not with the usual formulation of relativistic QFT which is compatible with this very space-time structure by construction.

If it comes to "ontology" it's very simple for a physicist: Things exist that can be observed reproducibly and objectively in nature.

Your mistake is in the phrase "a collapse implies non-local interactions". A collapse does not have any "interactions" in the way you use "local interactions".