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That doesn't make sense to me. We are lost in semantics. I think we should pause the discussion a little to cool it down.
Then state your notion of causality. My notion of causality is:vanhees71 said:That doesn't make sense to me. We are lost in semantics.
But it is determined by x(s), not by f[x(s)]).vanhees71 said:If ##x(t)## is determined by ##x(s)## for ##s<t## then also ##f[x(t)]## is determined for any ##t>s##.
vanhees71 said:It describes the observable properties of an individual quantum system, particularly the probabilities of observables that don't take determined value by the preparation. You may question that this is a complete description, but there's no hint that it's not complete.
There is a distinction that can be drawn between the following questions:martinbn said:It may be that it is in principle impossible to describe the quantum properties of an individual quantum system, or even meanigless to talk about them. So, this need not be a deficiency of this interpretation.
Lynch101 said: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.
Because physical reality is what we actually observe, with or without measurements.AlexCaledin said: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.
- then physical = actual? why then use the two words with identical meaning?A. Neumaier said:physical reality is what we actually observe
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.AlexCaledin said:- 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.AlexCaledin said:- then physical = actual? why then use the two words with identical meaning?
From the 1st paper cited above:vanhees71 said: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
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?Interested_observer said: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.
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.vanhees71 said: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
Does the term 'observe', here, imply measurement?A. Neumaier said:Because physical reality is what we actually observe, with or without measurements.
No, but it includes it. We observed long before we were able to measure.Lynch101 said:Does the term 'observe', here, imply measurement?
Does it imply then that physical reality is limited to what we can observe and/or measure?A. Neumaier said:No, but it includes it. We observed long before we were able to 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 said:Does it imply then that physical reality is limited to what we can observe and/or measure?
Thanks for the clarification.A. Neumaier said: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.
? What is nice about this paper?vanhees71 said: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
vanhees71 said: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.
See the book 'Local quantum physics' by Rudolf Haag. This is the most far reaching notion of locality, and holds without known exception.facenian said: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.facenian said: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.
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 said:? 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.
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.vanhees71 said: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.
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.vanhees71 said: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.
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.facenian said: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.
The collapse is not a nonlocal interaction; nobody claims that when they use this term.vanhees71 said: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.
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.A. Neumaier said: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.
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.A. Neumaier said:Indeed, Bell's assumption are completely independent of quantum mechanics!
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 said:It is about evaluating whether certain spooky predictions made by QM can find a local explanation by shifting to another theory.
You said local quantum physics. Then I have to agree, you just have declared that QM is a local theory. Period.A. Neumaier said: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.
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).facenian said:You said local quantum physics. Then I have to agree, you just have declared that QM is a local theory. Period.