38
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Why can interaction with a macroscopic apparatus, such as a Stern-Gerlach machine, sometimes not cause a measurement?
It's a very good question, since indeed if the original Stern-Gerlach machine had a well-defined momentum, then you are right that there could be no coherence upon rejoining the beams! The rule of ...
- 8,601
26
votes
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What is the quantum mechanical definition of a measurement?
Until we have an accepted solution of the Measurement Problem there is no definitive definition of quantum measurement, since we don't know exactly what happens at measurement.
In the meanwhile, ...
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20
votes
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What is the difference between classical correlation and quantum correlation?
Correlation is first and foremost a term from statistics. Given a system that consists of two (or more parts), it quantifies how much I can predict about the second system if I have knowledge of the ...
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20
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What are the strongest objections to be made against decoherence as an explanation of "collapse?"
I think most arguments in the literature can be boiled down to the point that decoherence does in no way touch the linearity of the Schrödinger equation, and thus cannot make an "or" from an "and".
...
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19
votes
Schrödinger's cat and the difficulty of macroscopic superposition state
$\renewcommand{\ket}[1]{\left \lvert #1 \right \rangle}$
$\renewcommand{\bra}[1]{\left \langle #1 \right \rvert}$
We can see how decoherence really works, why it messes up superposition states, and ...
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17
votes
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Atomic natural line width
As it turns out, the excited states of an atom are not really, strictly speaking, eigenstates. That is, they're eigenstates of the atomic hamiltonian, but they are not eigenstates of the atom-plus-EM-...
- 131k
17
votes
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Would every particle in the universe not have some form of measurement occurring at any given time?
What you describe is the process known as decoherence: any interaction of a quantum system with its environment (e.g. with photons or other particles passing by, and, yes, most likely interacting ...
- 2,032
15
votes
Accepted
Can the collapse of the wave function be modelled as a quantum system on its own?
To model the act of measurement itself as an interaction of the measurement apparatus and the measured system as quantum systems is a perfectly standard thing to do, though you might get disagreements ...
- 122k
13
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Is the wavefunction of particles inside a gas spread or localized?
Preliminaries: How do we define 'localized?'
For a single particle, or for multiple non-entangled particles, it is easy to tell from the expressions for the wavefunctions whether they are localized ...
- 7,557
13
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What is the quantum mechanical definition of a measurement?
The many-worlds interpretation defines measurement as any physical procedure in which the observer gets entangled with a quantum system. Before the measurement, the universe containing the observer ...
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11
votes
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Why doesn't gravity mess up the double slit experiment?
Yes, everything is a detector, but you need to quantify which things your system interacts with (and how strongly). Gravity is in some sense a poor example, because the quantum details of gravity are ...
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9
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Schrödinger's cat and the difficulty of macroscopic superposition state
It is because of quantum statistical irreversibility, which is closely related to entropy, as the OP suspected.
Qualitatively it is quite easy to understand this. From the laws of quantum mechanics ...
- 11.5k
9
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Why does the electron wavefunction not collapse within atoms at room temperature in gas, liquids or solids due to decoherence?
Welcome to SE -- good question! Decoherence does not mean that there won't be a wavefunction anymore, it just means that if the electron becomes coupled to the surrounding environment, its state will ...
- 837
8
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Pure dephasing $\gamma_\phi$ in a master equation and noise power spectral densities
$\def\ii{{\rm i}}
\def\dd{{\rm d}}
\def\ee{{\rm e}}
$
It turns out that the case of pure dephasing is exactly solvable, and one can obtain nice solutions under certain conditions. In particular, I ...
- 15.4k
8
votes
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Decoherence. Does it solve the measurement problem? Is it discontinuous? When does it occur?
You can find a comprehensive review of decoherence and how it fits in the QM interpretation debate in Decoherence, the measurement problem, and interpretations of quantum mechanics (Schlosshauer, 2005)...
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8
votes
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What does spontaneous symmetry breaking have to do with decoherence?
I don't think I have anything really new to say here, but saying it again in different words might have some value, so I'll give it a try.
Consider these two seemingly-contradictory statements, both ...
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7
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When does the world split in MWI
If you want to use nonrelativistic quantum mechanics you have to first start with the basics. Firstly it doesn't handle particle creation or destruction, so you need to fix how many particles you have ...
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7
votes
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Does uniqueness of the triorthogonal decomposition make quantum measurement objective?
The triorthogonal monomial basis for a Hilbert space decomposed into a triple tensor product is unique, the decomposition of the Hilbert space into such a triple product is not. So the "unique" basis ...
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7
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Are superpositions contagious?
You seem to be trying to play decoherence theory against the measurement problem, but the two are actually orthogonal. QM does not make wrong predictions, but we need to be precise about what a "...
- 122k
7
votes
Accepted
Is the Copenhagen interpretation still the most widely accepted position?
This is very hard to quantify, particularly because it's hard to define the population that should be under consideration. The best(-known) attempt to do this is reported in the paper
A Snapshot of ...
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7
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Is the Copenhagen interpretation still the most widely accepted position?
It's difficult to answer this question because there seems to be little agreement about what the "Copenhagen interpretation" is.
I don't know a lot about Heisenberg's beliefs, but Bohr's ...
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6
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Do measurements of time-scales for decoherence disprove some versions of Copenhagen or MWI?
I am not aware of any experimental evidence, so this probably does not qualify as an answer. However I can offer a reference that addresses this question theoretically:
Armen E. Allahverdyan, Roger ...
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6
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Would every particle in the universe not have some form of measurement occurring at any given time?
This topic gave me trouble as well. The fundamental basis for answering it is to look at decoherence.
Basically, any interaction in quantum mechanics yields the expected coherent result that comes ...
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6
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Decoherence: faster than light?
For the record I do not necessarily want to claim that decoherence solves all the subtle interpretational issues that go under the "measurement problem", at least not without some extra ingredient(s) ...
- 2,032
6
votes
Accepted
Relationships between elements in the density matrix
Consider a finite dimensional complex Hilbert space $H$ of dimension $d$ equipped with an inner product denoted by $(\cdot,\cdot)$ and let $\rho$ be a generic density operator, i.e. a positive semi-...
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Is there a classical quantum mechanics?
Quantum mechanics works (as far as we can tell) in all systems (although we still haven't settled on a way to get relativistic gravity to work nicely with quantum mechanics). Quantum mechanics ...
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votes
Accepted
How do Heisenberg Uncertainty and Quantum Decoherence coexist?
Note In this answer, technical asides are in italics.
There is a false dichotomy in your question, which I think comes from the way quantum mechanics is often presented at a popular science level. The ...
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6
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Why are accessible states taken as eigenstates in statistical physics? Is the resolution via decoherence?
I'll present a complement to AXensen's very good answer.
Statistical physics of quantum systems can be based on the general description of the state of a quantum system in terms of the density ...
6
votes
Can the collapse of the wave function be modelled as a quantum system on its own?
When a quantum system interacts with another system while it is undergoing interference information is copied out of the system that suppresses interference. This effect is called decoherence. Since ...
- 6,622
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