What is the QM description of a macroscopic event?

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    Macroscopic Qm
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Discussion Overview

The discussion centers around the quantum mechanical (QM) description of macroscopic events, exploring how these events can be mathematically defined and understood in the context of quantum theory. Participants examine the relationship between macroscopic and microscopic properties, the nature of measurement in quantum mechanics, and the implications of probability in defining events.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that macroscopic events can be defined as sets of possible measurement outcomes, drawing an analogy to classical probability theory.
  • Others argue that macroscopic events are treated as probabilities, which tend to average out towards classical behavior due to large degrees of freedom.
  • A participant questions the distinction between micro and macro in the context of measurement, suggesting that the standard formalism does not clearly define when a measurement occurs or what it means for a measurement result to be realized.
  • Some contributions highlight that both macroscopic and microscopic properties are represented by projectors in QM, with no clear demarcation between them.
  • There is a discussion about the ambiguity in the standard formalism regarding the measurement process and the observable being measured, emphasizing the need for clarity in these definitions.
  • One participant notes that the macro/micro distinction and the collapsed/uncollapsed distinction are closely related, as evidence of collapse is not observed in microscopic interactions.
  • Another participant raises concerns about the terminology used to describe quantum and classical phenomena, suggesting that clarity in definitions is necessary to avoid confusion.

Areas of Agreement / Disagreement

Participants express various viewpoints on the definitions and implications of macroscopic events in QM, with no consensus reached on the interpretation of these concepts. Disagreements persist regarding the nature of measurement and the distinctions between macroscopic and microscopic properties.

Contextual Notes

Limitations in the discussion include unresolved questions about the definitions of measurement, the nature of probability in QM, and the implications of the collapse of the wave function. The discussion also reflects a dependence on the interpretations of quantum mechanics that may vary among participants.

  • #31
Stephen Tashi said:
I didn't define a macroscopic event to be a situation where everything has been measured.

I didn't say you did. The issue I raised was not that "everything" had not been measured, but that system S had not been measured.
 
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  • #32
Stephen Tashi said:
Is " not in a quantum state" defined relative to some assumed basis of the state space? Or is "not in a quantum state" a description of something entirely different than vectors in the state space - for example, a linear operator on vector space is a distinct concept from a vector in that space.

"Not in a quantum state" is a basis independent statement.
The quantum state is a vector in the vector space (strictly speaking it is a ray or unit vector).
The measurement apparatus is represent by a self-adjoint operator on the vector space. Thus the measurement apparatus is not represented in the quantum state.
Orthodox quantum mechanics requires us to decide which part of "reality" to put in the quantum state, and which part of reality (such as the measurement apparatus) stays outside the quantum state. While we are pretty sure that measurement results are real (or classical or macroscopic), what exactly the quantum state alone represents is a puzzle.
 
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  • #33
atyy said:
"Not in a quantum state" is a basis independent statement.
The quantum state is a vector in the vector space (strictly speaking it is a ray or unit vector).
The measurement apparatus is represent by a self-adjoint operator on the vector space. Thus the measurement apparatus is not represented in the quantum state.
Orthodox quantum mechanics requires us to decide which part of "reality" to put in the quantum state, and which part of reality (such as the measurement apparatus) stays outside the quantum state. While we are pretty sure that measurement results are real (or classical or macroscopic), what exactly the quantum state alone represents is a puzzle.
The observalbes are represented by self-adjoint operators, not the measuring device.
 
  • #34
It's probably useful to distinguish between a measurement outcome–a property of the measurement apparatus–and a measurement result–a property of the measured system. The logical equivalence between them is why the formalism is so flexible re/ what must be included in the state space.
 
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  • #35
Does QM have to describe events specially? It describes all the properties and probabilities of the possible decoherent histories - which include everything to happen in our very brains - and then we observe some events and can calculate what we really need - knowing that every measurable property is formed by (means of) the universe quantum state reduction (= the actual history choice) and therefore the textbook QM is FAPP-applicable.
 
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  • #36
atyy said:
Approaches to solving the measurement problem; Bohmian mechanics and the Many-Worlds Interpretation(s).

and Objective Collapse Models..
 
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  • #37
physika said:
... Objective Collapse Models.

- why invent models? If the Collapse is truly Objective, then it itself might be causing the whole avalanche of measurements (including our brain events), the quantum state being reduced making (together with the Born rule) all the measurements to describe a consistent physical reality.

(it's like the least action principle which was used without models explaining it)
 
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