QM Incompleteness: Can We Predict Uncertainty?

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Discussion Overview

The discussion revolves around the completeness and predictive capabilities of quantum mechanics (QM), particularly in relation to unobserved quantum systems. Participants explore the implications of QM's probabilistic nature versus classical deterministic views, questioning whether the universe can be fundamentally described by probabilistic events or if there exists an underlying order that remains unmeasured.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants assert that QM is a well-tested theory but highlight its limitations in predicting the properties of unobserved quantum systems.
  • Others argue that quantum objects do not possess well-defined properties at all times, referencing the EPR and Bell papers to discuss the completeness issue of QM.
  • One participant suggests that the probabilistic nature of quantum mechanics contradicts the classical view of a well-defined universe, proposing that perhaps all objects have well-defined properties that we cannot yet measure accurately.
  • Another participant counters that the quantum level is fundamentally different from the macro level, where properties average out, and that experimental evidence supports the probabilistic interpretation of quantum mechanics.
  • Concerns are raised about the implications of the Heisenberg Uncertainty Principle, with some questioning whether measurement limitations could account for observed phenomena like Bell's inequality violations.
  • Participants discuss the potential for local realism and the challenges of measuring quantum states without disturbing them, suggesting that technological limitations may not fully account for the behavior of quantum systems.
  • There is a mention of the Many Worlds interpretation and its implications for local realism, with some expressing skepticism about theories that cannot be tested or falsified.

Areas of Agreement / Disagreement

Participants express a range of views, with no consensus reached on whether the probabilistic nature of QM is a fundamental aspect of reality or if it could be a result of measurement limitations. The discussion remains unresolved regarding the implications of Bell's theorem and the nature of quantum objects.

Contextual Notes

Participants acknowledge the complexity of the issues at hand, including the dependence on definitions of measurement and the implications of various interpretations of quantum mechanics. There are unresolved questions about the completeness of QM and the validity of local realistic theories.

  • #31
martinbn said:
You mean that QM doesn't describe the observer in the same way as the quantum system. But it does describe him, it insist that he is a classical object.

"Classical" object does not mean a set of well-defined classical laws of physics. "Classical" object means "common sense" object. There is no known theory of a whole universe as a single system of interacting classical and quantum parts.
 
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  • #32
atyy said:
The point is that QM is intrinsically incomplete - it is incomplete even before comparison with data.
Why is it incomplete? It would be incomplete if you could show that the world is deterministic, while QM only delivers a probabilistic description, but who (or what) tells you that the world in fact is deterministic?
 
  • #33
atyy said:
"Classical" object does not mean a set of well-defined classical laws of physics. "Classical" object means "common sense" object.

Not sure what your point is?

There is no known theory of a whole universe as a single system of interacting classical and quantum parts.

What about QM?
 
  • #34
martinbn said:
Not sure what your point is?

While some parts of classical physics are complete, the classical observer in quantum mechanics is not fully described by classical laws, since he has to interact with a quantum system. Only a theory that describes both the classical observer and the quantum system as a single system with no observer is complete.

martinbn said:
What about QM?

QM does not describe the classical observer and the quantum system as a single system with no observer.
 
  • #35
vanhees71 said:
Why is it incomplete? It would be incomplete if you could show that the world is deterministic, while QM only delivers a probabilistic description, but who (or what) tells you that the world in fact is deterministic?

There is no QM without a classical-quantum cut. Does the universe have a classical-quantum cut as real?
 
  • #36
atyy said:
There is no QM without a classical-quantum cut. Does the universe have a classical-quantum cut as real?

This is irrelevant. In your terminology, the question is about internal completeness, not whether the theory can describe the actual universe.
 
  • #37
To the OP: it is uncontroversial that QM in the Copenhagen is incomplete eg. https://www.amazon.com/dp/3527403914/?tag=pfamazon01-20. One can finesse the statement, but it is essentially correct. To make QM complete, one needs another interpretation eg. MWI or Bohmian Mechanics.
 
  • #38
atyy said:
While some parts of classical physics are complete, the classical observer in quantum mechanics is not fully described by classical laws, since he has to interact with a quantum system. Only a theory that describes both the classical observer and the quantum system as a single system with no observer is complete.
QM does not describe the classical observer and the quantum system as a single system with no observer.

What do you mean it doesn't? May be you mean that it doesn't describe them in a way that you expect it should? But there is no such requirement for a theory.
 
  • #39
martinbn said:
This is irrelevant. In your terminology, the question is about internal completeness, not whether the theory can describe the actual universe.

Yes, but internal completeness is not the same as internal coherence. So you miss the point. The point is the classical-quantum cut, which is the definition of incompleteness, unless there is a theory in which classical and quantum objects co-exist and are described by a single set of laws.
 
  • #40
martinbn said:
What do you mean it doesn't? May be you mean that it doesn't describe them in a way that you expect it should? But there is no such requirement for a theory.

QM does not say when a measurement is made.
 
  • #41
atyy said:
Yes, but internal completeness is not the same as internal coherence. So you miss the point. The point is the classical-quantum cut, which is the definition of incompleteness, unless there is a theory in which classical and quantum objects co-exist and are described by a single set of laws.

This is a strange definition of incompleteness!

Why should there be a single set of laws? You are imposing arbitrary restrictions on the theory.
 
  • #42
atyy said:
QM does not say when a measurement is made.

So?
 
  • #44
To all who are still interested in the topic, I'd like to recommend our archive (use the search function), which already contains a vast number of threads about quantum theory and this one won't certainly be the last one. So thread remains closed.
 
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