I QM: Interesting View - Get the Inside Scoop

[Interested_observer]

There is a distinction that can be drawn between the following questions:
1) Is QM a complete theory?
2) Is QM the most complete theory possible?
3) Does QM give us a complete description of physical reality?

It's possible to answer 'Yes' to questions 1 & 2 and 'No' to question 3.

EPR were asking question 3 and they set out the following as a necessary requirement for a theory to be considered 'complete':
Every element of the physical reality must have a counterpart in the physical theory.

If the mathematics of QM only makes predictions about the outcomes of ensembles of experiments (and possibly even individual experiments) or it only describes the properties of the system upon interaction with a measurement device, then, by definition, it does not give a complete description of reality, since it doesn't describe the system prior to measurement.

I suggest that until we have a theory that predicts exactly where an individual photon will hit the screen in a 2-slit experiment, and the path of an individual gas atom in a confined space, and why one atom of uranium decays and not the one next to it, etc., we will not have a complete physical theory of reality. My guess is that this will not happen. But we should keep looking, for we will undoubtedly learn much from the search.

 

[AlexCaledin]

But why any "reality" has to be "physical"? It might well only use some physics to be better ordered to a certain extent and no more.

 

[vanhees71]

We are back at the problem that the word "reality" is useless, because nobody knows the concise scientific meaning of it anymore. You have to define, what you mean by the word "reality" applied in the context of physics.

By chance, I just stumble over a nice AJP paper by Fuchs. If expressed in this way, I'd even could soon become a qubist ;-)):

https://arxiv.org/abs/1311.5253
https://doi.org/10.1119/1.4874855

 

[A. Neumaier]

But why any "reality" has to be "physical"? It might well only use some physics to be better ordered to a certain extent and no more.

Because physical reality is what we actually observe, with or without measurements.

 

[AlexCaledin]

physical reality is what we actually observe

- then physical = actual? why then use the two words with identical meaning?

 

[gentzen]

- then physical = actual? why then use the two words with identical meaning?

He also wrote "observe". However, I liked your "original" comment, but others interpreted "physical" different from how I read it. I read it in the sense of our physical theories with their natural laws.

 

[A. Neumaier]

- then physical = actual? why then use the two words with identical meaning?

They are not identical. Because physical refers to substance, i.e., what actually exists, while actual is the contrast to imagined.

 

[Interested_observer]

We are back at the problem that the word "reality" is useless, because nobody knows the concise scientific meaning of it anymore. You have to define, what you mean by the word "reality" applied in the context of physics.

By chance, I just stumble over a nice AJP paper by Fuchs. If expressed in this way, I'd even could soon become a qubist ;-)):

https://arxiv.org/abs/1311.5253
https://doi.org/10.1119/1.4874855

From the 1st paper cited above:

"Why, then, do many people wrongly claim that quantum mechanics is nonlocal? They do so by denying at least one of three fundamental precepts of QBism: (1) A measurement outcome does not preexist the measurement. An outcome is created for the agent who takes the measurement action only when it enters the experience of that agent. The outcome of the measurement is that experience. Experiences do not exist prior to being experienced."

This "fundamental precept" seems to me to be more "hand-waving" (as A. Neumaier fondly puts it) in order to sweep the "measurement problem" under the rug.

 

[vanhees71]

Well, but I think it's the essence to cure the confusion on locality (in the sense of relativistic QFT, where it means microcausality and excludes faster-than-light causal effects/interactions by construction) and what's often called "nonlocality" but what's really meant is "inseparability" in Einstein's sense, and this doesn't give rise to violations of causality, because it describes strong correlations between far-distant observations on entangled parts of a quantum system. The correlations are, when interpreted in the sense of Fuchs's "qbism", due to the preparation in the entangled state.

I've another quibble with Fuchs's presentation of qbism: My problem is the claim that the probabilities and the quantum state are purely subjective. That's also not true, because operationally a quantum state is not defined by some agent's knowledge but by a "preparation procedure". If you have a complete preparation procedure, i.e., if you determine a complete set of compatible observables the state is uniquely determined to be the corresponding pure state.

It's of course true that there is a general problem how to choose the probabilities of a probabilistic theory given incomplete information. There are different attempts. The one that seems most successful in physics is the principle of maximum entropy with von Neumann-Shannon-Jaynes entropy in quantum theory.

 

[Lynch101]

I suggest that until we have a theory that predicts exactly where an individual photon will hit the screen in a 2-slit experiment, and the path of an individual gas atom in a confined space, and why one atom of uranium decays and not the one next to it, etc., we will not have a complete physical theory of reality. My guess is that this will not happen. But we should keep looking, for we will undoubtedly learn much from the search.

It is possible that we cannot ever have a complete description of physical reality. Some of the no-go theorems of QM actually point to this, don't they?

 

[Lynch101]

We are back at the problem that the word "reality" is useless, because nobody knows the concise scientific meaning of it anymore. You have to define, what you mean by the word "reality" applied in the context of physics.

By chance, I just stumble over a nice AJP paper by Fuchs. If expressed in this way, I'd even could soon become a qubist ;-)):

https://arxiv.org/abs/1311.5253
https://doi.org/10.1119/1.4874855

If we're talking about a complete description of [physical] reality we are essentially just talking about a complete description of the universe, or the parts of the universe under consideration.

Some interpretations might propose complete descriptions of the universe but which cannot be verified by observation as there may be a limit to how far we can probe nature. However, some interpretations rule themselves out of being complete, by definition.

We can make certain deductions beyond the measurement results by applying if this-then that reasoning.

 

[Lynch101]

Because physical reality is what we actually observe, with or without measurements.

Does the term 'observe', here, imply measurement?

 

[A. Neumaier]

Does the term 'observe', here, imply measurement?

No, but it includes it. We observed long before we were able to measure.

 

[Lynch101]

No, but it includes it. We observed long before we were able to measure.

Does it imply then that physical reality is limited to what we can observe and/or measure?

 

[A. Neumaier]

Does it imply then that physical reality is limited to what we can observe and/or measure?

No. It must include what we can observe and/or measure, but to count as scientific it must also be defined in an objective, observer-independent way.

 

[Lynch101]

No. It must include what we can observe and/or measure, but to count as scientific it must also be defined in an objective, observer-independent way.

Thanks for the clarification.

 

[A. Neumaier]

By chance, I just stumble over a nice AJP paper by Fuchs. If expressed in this way, I'd even could soon become a qubist ;-)):

https://arxiv.org/abs/1311.5253
https://doi.org/10.1119/1.4874855

? What is nice about this paper?
It is essentially devoid of physics, replacing it by a set of should's for the personal beliefs of individual agents, without telling how the agents come to a mutual, objective understanding of the physical world.

 

[facenian]

Well, but I think it's the essence to cure the confusion on locality (in the sense of relativistic QFT, where it means microcausality and excludes faster-than-light causal effects/interactions by construction) and what's often called "nonlocality" but what's really meant is "inseparability" in Einstein's sense, and this doesn't give rise to violations of causality, because it describes strong correlations between far-distant observations on entangled parts of a quantum system. The correlations are, when interpreted in the sense of Fuchs's "qbism", due to the preparation in the entangled state.

I am very suspicious that a theory based on quantum mechanics could be local by construction. It seems more a wishful declaration of how we want the theory to be. On the other hand, inseparability would be another word for nonlocality.
However, I believe there is an unambiguous way of considering the theory local by a more careful definition of locality.

 

[A. Neumaier]

there is an unambiguous way of considering the theory local by a more careful definition of locality.

See the book 'Local quantum physics' by Rudolf Haag. This is the most far reaching notion of locality, and holds without known exception.

Bell nonlocality is not a statement that the dynamics of microsystems is nonlocal. It is only a statement that if modeled by a classical hidden variable theory, quantum mechanics would be nonlocal. Indeed, Bell's assumption are completely independent of quantum mechanics!

 

[vanhees71]

I am very suspicious that a theory based on quantum mechanics could be local by construction. It seems more a wishful declaration of how we want the theory to be. On the other hand, inseparability would be another word for nonlocality.
However, I believe there is an unambiguous way of considering the theory local by a more careful definition of locality.

In relativistic QFT a very important defining ingredient is locality aka microcausality of local observables, i.e., the Hamiltonian density must commute with all local observables at spacelike separation of the arguments of the corresponding operators. That's the only solid definition of locality I know of, and it's sufficient to exclude spooky actions at a distance.

 

[vanhees71]

? What is nice about this paper?
It is essentially devoid of physics, replacing it by a set of should's for the personal beliefs of individual agents, without telling how the agents come to a mutual, objective understanding of the physical world.

What is nice about this paper is the emphasis of locality, which is in accordance with locality in relativistic QFT. As I wrote today somewhere already in this thread, I also don't like the subjective interpretation of probabilities. The association with a (pure or mixed) state to preparation procedures is far better than just a subjective guess as all the highly accurate descriptions of real-world experiments with quanta demonstrates.

 

[A. Neumaier]

The association with a (pure or mixed) state to preparation procedures is far better than just a subjective guess as all the highly accurate descriptions of real-world experiments with quanta demonstrates.

Yes, but in your discussion of the postulates you also linked the mixed state to knowledge, which is a subjective entity. You also call the collapse subjective though it is objectively measurable for light passing a polarization filter.

qBism is at least consistent in this respect, while you mix subjectivity and objectivity to a syncretistic religion...

 

[facenian]

In relativistic QFT a very important defining ingredient is locality aka microcausality of local observables, i.e., the Hamiltonian density must commute with all local observables at spacelike separation of the arguments of the corresponding operators. That's the only solid definition of locality I know of, and it's sufficient to exclude spooky actions at a distance.

I agree that it is solid as a "definition". However, it is not clear to me that it excludes spooky action unless you again define the absence of spooky action by the commutativity of operators.
I guess It is hard to separate ideas from words.

 

[vanhees71]

Of course a mixed state also reflects some knowledge about the system as any probability distribution does. As I said, you need additional concepts to associate the "right" stat. op. to a situation of incomplete knowledge. One very succuessful use is the maximum entropy principle with the von Neumann-Shannon-Jaynes entropy. I know that you don't like this approach for whatever reason, but it works well in practice.

In which sense is collapse objectively measurable for light passing a polarization filter? For a single photon the photon gets absorbed or is let through as a whole and if the photon goes through its polarized in a direction given by the orientation of the polarization filter. There's no need for non-local interactions as claimed by a collapse but just the local interaction of the em. field with the filter.

 

[vanhees71]

I agree that it is solid as a "definition". However, it is not clear to me that it excludes spooky action unless you again define the absence of spooky action by the commutativity of operators.
I guess It is hard to separate ideas from words.

If the Hamilton density commutes with any local observable at space-like separated distances by construction there cannot be faster-than light interactions. That together with Poincare invariance of the S-matrix is why one makes this assumption to begin with.

 

[A. Neumaier]

In which sense is collapse objectively measurable for light passing a polarization filter? For a single photon the photon gets absorbed or is let through as a whole and if the photon goes through its polarized in a direction given by the orientation of the polarization filter. There's no need for non-local interactions as claimed by a collapse but just the local interaction of the em. field with the filter.

The collapse is not a nonlocal interaction; nobody claims that when they use this term.

The collapse is the fact that (due to the local interaction of the em. field with the filter) the ingoing photon can have any state but the outgoing photon (if there is one) is in the eigenstate of the projector to the polarization direction.

Thus, in the standard terminology, the state of the ingoing photon collapsed. This is the consensus of the physics community even though you don't use this terminology (and in fact misinterpret the concept in discussions).

 

[facenian]

Bell nonlocality is not a statement that the dynamics of microsystems is nonlocal. It is only a statement that if modeled by a classical hidden variable theory, quantum mechanics would be nonlocal.

I completely disagree with this view! Unfortunately, it is very widespread among working physicists. Shut up and calculate is a good strategy to make progress but should not be taken to the point of annihilating critical and logical thinking.
That statement is a mere tautology. It is not surprising that different theories give different predictions, therefore, predict different bounds for the result of a certain experiment, e.g., a Bell test experiment.

Indeed, Bell's assumption are completely independent of quantum mechanics!

Of course, they are!. It is about evaluating whether certain spooky predictions made by QM can find a local explanation by shifting to another theory. The relevant result is that we can't. Rejecting an explanation because it is purportedly classical means that QM's spooky predictions have no local explanation at all.
The only way out is to declare the absence of nonlocality by decree or reformulate our concept of locality more coherently. Fortunately, that can be done by shifting locality to no-signaling.

 

[A. Neumaier]

It is about evaluating whether certain spooky predictions made by QM can find a local explanation by shifting to another theory.

No. It is about evaluating whether certain predictions made by local quantum physics can also find a local explanation by shifting to a local deterministic (or classically probabilistic) theory.

 

[facenian]

No. It is about evaluating whether certain predictions made by local quantum physics can also find a local explanation by shifting to a local deterministic (or classically probabilistic) theory.

You said local quantum physics. Then I have to agree, you just have declared that QM is a local theory. Period.

 

[A. Neumaier]

You said local quantum physics. Then I have to agree, you just have declared that QM is a local theory. Period.

Local quantum physics in the sense of Haag's book is local in a very well-defined, meaningful sense. In spite of Bell's theorem (which looks for underlying hidden variables and proves that these hidden variables would have to be nonlocal) and the corresponding experiments (which agree with local quantum physics).