Undergrad What is the QM description of a macroscopic event?

[PeterDonis]

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.

 

[atyy]

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.

 

[martinbn]

"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.

 

[Morbert]

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.

 

[AlexCaledin]

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.

 

[physika]

Approaches to solving the measurement problem; Bohmian mechanics and the Many-Worlds Interpretation(s).

and Objective Collapse Models..

 

[AlexCaledin]

... 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)