# Questions tagged [measurement-problem]

DO NOT USE THIS TAG just because your question involves measurements (either quantum or classical). The measurement problem asks how wave function collapse occurs during measurement in quantum mechanics, and how it can be reconciled with unitary evolution.

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### Modelling how a macroscopic detector interacts with a quantum mechanical particle?

Background I recently came up with a math formula and wondered if could be useful for any physical kind of situation. I then, realised it might be useful for the situation of particle detector. P.S: ...
1answer
182 views

### What is the physical meaning of expectation value of the Hamiltonian operator?

I've been studying David Griffiths' Introduction to Quantum Mechanics and int that, it was explained that the expectation value of position $x$ is the average of the positions of $N$ identically ...
1answer
136 views

### Significance of cube factory paradox

I am reading a book titled QBism, a description of the Bayesian interpretation of Quantum mechanics, The author describes the “... baffling cube factory paradox...”. A factory makes perfect cubes with ...
2answers
55 views

### when I take two measure in same time, probability is product of probability?

let |φ› be a state, A the associate operator of "a" and B the associate operator of "b". I want to measure value "a" and then measure value "b", What is the probability? How is this related to the ...
1answer
222 views

### The 'Conservation of Information' myth

PBS Spacetime recently released a video talking about 'why information is never destroyed in Quantum Mechanics' I'm surprised to see this myth to persist even to this day. We know that unitarity and ...
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297 views

### Going from stochastic Schrödinger equation to master equation

I am currently reading the book "Quantum measurement and control" by Wiseman and Milburn (https://www.amazon.ca/Quantum-Measurement-Control-Howard-Wiseman/dp/1107424151) and something is really ...
0answers
98 views

### Quantum Observation

Bear with me if I present a lack of knowledge - QM is not my field. There's a common notion in QM that until a particle is observed (measured), its properties are not definite, but rather are spread ...
1answer
159 views

### Is this assertion by Landau and Lifshitz's QM equivalent to the axiom that state spaces are vector spaces?

Many expositions of Quantum Mechanics begin by stating axioms of the following type: "The state of a quantum mechanical system is given by a nonzero vector in a complex vector (or Hilbert space) ...
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73 views

### Doubt about fifth postulate of QM degenerate case

I think I didn't not really understand a comment found on "Quantum Mechanics" by Claude Cohen-Tannoudji. Talking about the fifth postulate of quantum mechanics in the case in which $a_n$ is a ...
1answer
61 views

### When does the wave function collapse in the EPR paradox experiment? Shouldn't this depend on the reference frame?

Suppose you have two entangled particles. Particle A, and Particle B. If the spin of particle A is up, then the spin of particle B must be down and vice versa. Now suppose you measure the spin of A ...
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58 views

### Generalisation of the measurement postulate in quantum mechanics

Given an observable that has a partially discrete and partially continuous spectrum of eigenvalues associated to it with the order of the spectrum's degeneracy being greater than 1, how would you ...
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45 views

### Particles trajectories in Stern-Gerlach experiment

I red this sentence on "D.J. Griffiths, Introduction to Quantum Mechanics" talking about the using of heavy particles for the experiment. "...heavy so we can construct localized wave packets and ...
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146 views

### Which experiment clearly shows us that Heisenberg Uncertainty Principle is ontological and not epistemological?

I have heard that the Heisenberg Uncertainty Principle(HUP) is not because of our limitations in measuring and the apparatus used but instead because of the very way things are I searched online in ...
1answer
94 views

### Measurement of position in quantum mechanics

I know that when you perform a measurement of position in quantum mechanics, the wave function collapses to something proportional to it, but in a small range of values of positions, depending on the ...
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137 views

### What is the relation between a measurement and an observable?

Observables are represented by Hermitian operators. First of all, it's a little strange (to me) that some measurable physical quantity is represented by a transformation (or linear map), given that I ...
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26 views

### Uncertains of diffraction mesaurement with a photodiode

I have to study the diffraction curve of a laser passed through a slit. To measure the intensity i am using a photodiode and a voltmeter; I move the photodiode and measure the intensity every $200\mu$...
2answers
283 views

### Does the quantum eraser double slit experiment disprove many worlds interpretation?

The quantum eraser double slit experiment shows that interference pattern is destroyed not by interacting with the quantum particle, but rather by getting which path information. In this experiment ...
2answers
149 views

### Finding amplitude of probability

The mathematical structure of quantum mechanics, follows almost inevitably from the concept of a probability amplitude. For James Binney and David Skinner: "With every value in the spectrum of a ...
3answers
192 views

### Superposition of position eigenstates after measurement of position?

In a discussion with someone recently, the person made the following statement: Even when we observe a particle, it is still in superposition of position states within the resolution of the ...
1answer
274 views

### All of the postulates of quantum mechanics without additional information [closed]

$\newcommand{\braket}{\left<#1|#2\right>}% \newcommand{\bra}{\left<#1\right|}% \newcommand{\ket}{\left|#1\right>}%$As an undergraduate who's trying to learn quantum mechanics it'...
1answer
63 views

### Why quantum mechanic didn't work well when length approach Planck length?

Information paradox with regard to quantum entanglement when width approach Planck length Context: Suppose I have a 3 d particle placed in a spherical box of infinite well from [-r,r] in all three ...
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103 views

### Is the Heisenberg uncertainty principle based on experimental imprecision in measurement? [duplicate]

As far as I understand, the Heisenberg uncertainty principle states that the more one of the conjugate quantities are known, the less the other is known. So for instance, if the velocity of a particle ...
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### Can Quantum Zeno effect stabilize unstable particles? [duplicate]

Quantum Zeno effect says that a continuous measurement makes the state to be "unmoved". Could we use (at least in principle) this effect to make stable unstable particles?
1answer
34 views

### What is the physical meaning of exciting a single-electron transition in a many-electron system?

Let's say I have a gas of Li atoms in the $1s^2 2s^1$ ground state configuration and I want to excite the $2s \to 3s$ transition of the outermost electron with a laser pulse on resonance. (That is the ...
1answer
73 views

### Probability of obtaining general quantum measurement outcome

The Fundamental Theorem of Quantum Measurement is stated as follows: Every set of operators $\{ A_n \}$ $n =1,...,N$ that satisfies $\sum_n A_n^{\dagger}A_n = I$ describes a possible measurement on a ...
1answer
155 views

### Measurements in the phase space picture of quantum mechanics

Suppose we are dealing with non relativistic quantum mechanics of point particles, that is we are in the realm of 'classic' quantum mechanics (no quantum fields ect.). In the Heisenberg picture, ...
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225 views

### Realism and the Copenhagen interpretation [closed]

In the Copenhagen interpretation, if some device measures the state of a quantum system, finding some property of it to be equal to some value, X, then it is then assumed that all other quantum ...
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### Measuring the polarisation of light

I've been thinking about the standard and easy "Three-polarising-filters experiment" as a demonstration of quantum phenomenon: If light is passed through two polarising filter and the filters have ...