Can psi be epistemic while still treating particles as real?

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Nugatory said:
The paper says no such thing, and we've reached a point in the discussion where pointing to that Nature.com article is no substitute for reading and understanding the paper itself. The paper says that any correspondence between an underlying physical state and the wave functions of pure states must be one-to-one. That's a reasonable basis for claiming that the wave function is ontic, but it doesn't take you to "physically real".
Oh ok.
But, you said that wave function only exists in Hilbert space(I know I might be repeating myself), so is it real and not a physical object in Hilbert space, or is it a real physical object in the Hilbert space?
 
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atyy said:
I usually consider the EM wave real and a physical object, so I don't understand the distinction.
What I mean is that, can one interpret (in Hilbert space) that the wave function is real but is not an actual physical object?
 
Nugatory said:
The paper says no such thing, and we've reached a point in the discussion where pointing to that Nature.com article is no substitute for reading and understanding the paper itself. The paper says that any correspondence between an underlying physical state and the wave functions of pure states must be one-to-one. That's a reasonable basis for claiming that the wave function is ontic, but it doesn't take you to "physically real".
And so your saying that the PBR theorem/paper never mentions the wave function as physically real?
 
Nugatory said:
The paper says no such thing, and we've reached a point in the discussion where pointing to that Nature.com article is no substitute for reading and understanding the paper itself. The paper says that any correspondence between an underlying physical state and the wave functions of pure states must be one-to-one. That's a reasonable basis for claiming that the wave function is ontic, but it doesn't take you to "physically real".
And isn't a Ψ ontic view of wave function mean that its real? Physically real?
 
Nick V said:
What I mean is that, can one interpret (in Hilbert space) that the wave function is real but is not an actual physical object?

Why is the EM wave not a physical object?
 
atyy said:
Why is the EM wave not a physical object?
I have never really heard that an EM wave or sound wave or any wave is classified as a physical object. But can wave function be real but not a physical object (in Hilbert space)?
 
Nick V said:
I have never really heard that an EM wave or sound wave or any wave is classified as a physical object. But can wave function be real but not a physical object (in Hilbert space)?

I don't know. I do think of an EM wave or a sound wave as a physical object, which is what I mean by real.
 
atyy said:
I don't know. I do think of an EM wave or a sound wave as a physical object, which is what I mean by real.
But I don't think that they are considered physical objects because they are not matter, so they are real but just not physical.
 
Nick V said:
But I don't think that they are considered physical objects because they are not matter, so they are real but just not physical.

You mean they are not solid objects, like a table or a cat?
 
Nick V said:
But I don't think that they are considered physical objects because they are not matter, so they are real but just not physical.
I don't understand this part. What is your criteria for considering something as being a "physical" or "material" object? And let's assume you suggest some criteria. Why should one accept it? After all, what is considered "physical" or "material" is based on our best theories/models in physics and this changes as our physics evolves. It's not as if we have some definite and fixed conception of what physical must be, independent of our best theories in physics.
 
Nugatory said:
The paper says no such thing, and we've reached a point in the discussion where pointing to that Nature.com article is no substitute for reading and understanding the paper itself. The paper says that any correspondence between an underlying physical state and the wave functions of pure states must be one-to-one. That's a reasonable basis for claiming that the wave function is ontic, but it doesn't take you to "physically real".
This is not even true for minimally interpreted quantum theory (and that's the only interpretation that does not lead to contradictions with basic principles like causality): A pure state is represented by a ray in Hilbert space.

I still don't understand don't understand what "real" means in this thread, but if anything is "real" concerning quantum states, it's rays in Hilbert space. I would define an object in a physical theory "real" if it can be measured somehow in the real world. In this sense the quantum states are not real, because their physical content is probabilistic and thus can be checked in the lab only by preparing many independent realizations of the corresponding system and measure some observables on them. At the same time the quantum states are objective, because interpreted in this way they are operationally defined by (an equivalence class of) preparation procedures, e.g., how to prepare electrons with a certain momentum within given limits of accuracy (an electron never can have a sharp momentum as in classical physics however, due to the uncertainty relation, but in principle it can be determined with any accuracy you like). The "preparation procedure" is, e.g., an accelerator with all kinds of tricks to get electrons with a "well-defined" momentum. These are for sure real objects in the real worlds, and in this sense the states are "real" in the sense of preparation procedures.

I've the impression PBR's ontology is demanding more than quantum theory in the minimal interpretation, namely that all observables of a quantum object should "in reality" have determined values. That means they take the state description as "real" only if it determines all possible observables, but this contradicts quantum theory, and so far nobody has found an alternative theory which is as successful. Also it seems to me that PBR don't give any description of how the alternatives of an ontic vs. an epistemic interpretation of QT states can be distinguished experimentally in the "real world". That makes their ideas a bit limited in the sense of natural science.

An example of this kind is Bell's work. First of all he gives a clear mathematical description to the "reality" in the sense of EPR and then proves a theorem (the famous "Bell inequality") that can be tested in the "real world" by doing experiments, and this has been done with high accuracy (starting with Aspect's work in the 80ies). The outcome is very clear: If there is a deterministic theory reproducing the quantum probabilities it must be a non-local one, and so far nobody has been able to formulate such a theory. At the same time the predictions of QT have been confirmed.
 
vanhees71 said:
I've the impression PBR's ontology is demanding more than quantum theory in the minimal interpretation, namely that all observables of a quantum object should "in reality" have determined values. That means they take the state description as "real" only if it determines all possible observables, but this contradicts quantum theory, and so far nobody has found an alternative theory which is as successful. Also it seems to me that PBR don't give any description of how the alternatives of an ontic vs. an epistemic interpretation of QT states can be distinguished experimentally in the "real world". That makes their ideas a bit limited in the sense of natural science.

Yes, the real motivation for PBR should be seen as the measurement problem, which requires additional variables to be introduced (unless MWI works). One can roughly think of the PBR hidden variables in the Bohmian sense. Bohmian Mechanics is not a full solution of the measurement problem, but a restatement of quantum mechanics in a form that looks like classical kinetic theory. The uncertainty is traced back to a "quantum equilibrium" distribution of initial positions. However, by analogy to classical kinetic theory, there should be an H-theorem that shows how equilibrium is attained from nonequilibrium. Valentini argued that there is an H-theorem for de Broglie dynamics in the Bohmian framework. Bohmian Mechanics in nonequilibrium should produce deviations from quantum mechanics, and so is in principle testable. Similarly, all other hidden variable theories considered by PBR that do reproduce quantum mechanics should do so only in some regime, and should deviate from quantum mechanics beyond that regime, In that sense, the theories considered by PBR are all testable in principle.

vanhees71 said:
An example of this kind is Bell's work. First of all he gives a clear mathematical description to the "reality" in the sense of EPR and then proves a theorem (the famous "Bell inequality") that can be tested in the "real world" by doing experiments, and this has been done with high accuracy (starting with Aspect's work in the 80ies). The outcome is very clear: If there is a deterministic theory reproducing the quantum probabilities it must be a non-local one, and so far nobody has been able to formulate such a theory. At the same time the predictions of QT have been confirmed.

Indeed. However, the main problem lies in the same place as a lattice formulation of the standard model: chiral fermions interacting with non-abelian gauge fields.
 
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My problem is PBR then is, why they rule out that quantum theory is valid as it is now (in the minimal interpretation), if you want to call the state ontic? Why can't it be true in an ontic interpretation of the quantum state that only those observables have a sharp value (or determined values at high accuracy if it's not possible for the observable to have a sharp value, which is always the case with observables that have only continuous values as position and momentum) that have them because of the preparation of the system in a state such that this is the case? Then quantum theory simply implies that not all observables can have determined values at once. If nature is like this (and I think a lot of very accurate findings hint that this might be actually the case), it's like this, and then quantum theory in the minimal interpretation is "realistic". So what?
 
vanhees71 said:
My problem is PBR then is, why they rule out that quantum theory is valid as it is now (in the minimal interpretation), if you want to call the state ontic? Why can't it be true in an ontic interpretation of the quantum state that only those observables have a sharp value (or determined values at high accuracy if it's not possible for the observable to have a sharp value, which is always the case with observables that have only continuous values as position and momentum) that have them because of the preparation of the system in a state such that this is the case? Then quantum theory simply implies that not all observables can have determined values at once. If nature is like this (and I think a lot of very accurate findings hint that this might be actually the case), it's like this, and then quantum theory in the minimal interpretation is "realistic". So what?

That sense of ontic is not ruled out by PBR, they simply don't address it. I don't think PBR is interesting unless one is also asking what potential solutions of the measurement problem can look like.

Actually, the epistemic interpretation of the wave function has problems in the minimal interpretation (Copenhagen!), which is why I usually say "the wave function is not necessarily real, and is a tool to calculate the probabilities of events". Here I do intend to loosely use the "epistemic" view when I say the wave function is a "tool", but by saying that it is not "necessarily real" I don't rule out that it is "ontic". It is hard to transition from this loose use of the term "epistemic" to something sharp. The traditional analogy is that collapse is like throwing a die, and getting a definite result after the die is thrown. In classical probability, the updating after the die is thrown is usually done by Bayes rule, but the analogy is very partial when one really tries to work it out. I think one can see caution in the text by Cohen-Tannoudji, Diu and Laloe, where they don't commit to a purely epistemic view, although they clearly indicate that it helps the intuition to think of collapse in this way.

Two very good attempts that only partially succeed in making the analogy between collapse and Bayesian updating are:
http://arxiv.org/abs/quant-ph/0106166
http://arxiv.org/abs/1107.5849

To get around this, one may say that quantum theory is a generalization of probability theory. But then one may question whether classical probability is really failing, and the hidden variables approaches indicate that one need not generalize classical probability theory, and that quantum mechanics is a special case of classical probability theory.
 
atyy said:
Why is the EM wave not a physical object?
Here's the bottom line of what I am asking you, In Ψ ontic view ( wave function is real), is like a wave like an EM wave or any other classical wave, or is it an real object that is waving, like a piece of string that is waving?
 
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Nick V said:
Here's the bottom line of what I am asking you, In Ψ ontic view ( wave function is real), is like a wave like an EM wave or any other classical wave, or is it an real object that is waving, like a piece of string that is waving?

In the Ψ-ontic view, the wave function is a wave like an EM wave. However, the wave function is a wave in Hilbert space, and whereas an EM wave is a wave in spacetime.
 
atyy said:
In the Ψ-ontic view, the wave function is a wave like an EM wave. However, the wave function is a wave in Hilbert space, and whereas an EM wave is a wave in spacetime.
Ok, so your saying that it's not the like a physical object that's waving like a piece of string that's waving. But, don't Ψ ontic interpretations of QM ( ie. many worlds, de broglie, penrose interpretations) require the wave function to exist in space time? Atleast that's what I read on the Wave Function wikipedia under ontology.
 
Nick V said:
Ok, so your saying that it's not the like a physical object that's waving like a piece of string that's waving. But, don't Ψ ontic interpretations of QM ( ie. many worlds, de broglie, penrose interpretations) require the wave function to exist in space time? Atleast that's what I read on the Wave Function wikipedia under ontology.

In both MWI and dBB, the wave function is not a wave in spacetime , it is a wave in Hilbert space.
 
atyy said:
In both MWI and dBB, the wave function is not a wave in spacetime , it is a wave in Hilbert space.
OK, so the Ψ ontic view of the wave function (MWI, dBB, any other Ψ ontic interpretation), the wave function is real. But the wave function is physically real only in Hilbert space, correct?
 
Nick V said:
OK, so the Ψ ontic view of the wave function (MWI, dBB, any other Ψ ontic interpretation), the wave function is real. But the wave function is physically real only in Hilbert space, correct?

The wave function exists only in Hilbert space in all interpretations of QM, so yes, it is real only in Hilbert space in Ψ-ontic proposals such as MWI and dBB.
 
atyy said:
In both MWI and dBB, the wave function is not a wave in spacetime , it is a wave in Hilbert space.
This is the reason why Einstein believed that the wave function was epistemic and not ontic. If the wave function is an ontic entity that evolves in configuration space, what then is the status of configuration space? Does that imply that configuration space must also be "real"?
 
atyy said:
The wave function exists only in Hilbert space, so yes, it is real only in Hilbert space in Ψ-ontic proposals such as MWI and dBB.
Then how come on the Wave Function wiki page under the ontology section, it says that Everett (created MWI) says that the wave function must be physically real.
 
bohm2 said:
This is the reason why Einstein believed that the wave function was epistemic and not ontic. If the wave function is an ontic entity that evolves in configuration space, what then is the status of configuration space? Does that imply that configuration space must also be "real"?

The configuration space (Hilbert space) is real in dBB. It's not much different from the extra-dimensions of string theory.
 
Nick V said:
Then how come on the Wave Function wiki page under the ontology section, it says that Everett (created MWI) says that the wave function must be physically real.

Because real and physically real mean the same thing, whereas you are making a distinction that I don't understand.
 
atyy said:
Because real and physically real mean the same thing, whereas you are making a distinction that I don't understand.
Oh ok. So, the Hilbert space is real in MWI as well, correct?
 
Nick V said:
Oh ok. So, the Hilbert space is real in MWI as well, correct?

Yes. Real just means that it exists even after you die.
 
It is time to close this thread, as it is becoming clear that the thread starter is just picking quotations out of papers that he has either not read completely or does not understand.