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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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51 views

Unitarily reversing a projective measurement

We start with a particle in a pure superposition state. Let's say it is, $$\vert\psi\rangle = \frac{1}{2}(\vert 0\rangle + \vert 1\rangle)$$ Alice sends this particle inside a box and the box ...
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Uncertainty and error in a measurement

I do not find a clear and simple explanation of the difference between uncertainty and error of a measurement. Basically, in a measurement, uncertainty is defined as the standard deviation of the ...
5k views

Isn't the detector always measuring, and thus always collapsing the state?

I have a radioactive particle in a box, prepared so as to initially be in a pure state $\psi_0 =1\ \theta_U+ 0\ \theta_D$ (U is Undecayed, D is Decayed). I put a Geiger counter in the box. Over ...
72 views

Does measurement of quantum system always collapses the state of system?

Does measurement in quantum mechanics always disturb the system? The measurement postulate states that "when we do a measurement on a quantum system, the state of the system is collapsed to one of ...
60 views

Measurement postulate and black hole information paradox: are they related somehow?

The measurement postulate states that whenever we make a quantum measurement, we select (projection) from the general superposition state a single pure state. Thus, as a general quantum state is a ...
6k views

How can Schrödinger's cat be both dead and alive? [closed]

So, this goes to something so fundamental, I can barely express it. The Schrödinger's Cat thought experiment ultimately asserts that, until the box is opened, the cat is both dead AND alive. Now, ...
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What is in physical term meaning of penetration of a potential barrier?

While solving the quantum mechanical case of potential barrier meaning - $$\text{E} <\text{V}$$ The transmission coefficient is nonzero. My problem is what is happening with particle motion Has ...
123 views

Are superpositions contagious?

Does quantum mechanics really predict that a particle prepared in a state of superposition of spin will result, after being measured by an appropriate instrument (Stern-Gerlach device), in a ...
84 views

Quantum mechanics limits to understanding the Universe [closed]

By definition, a wave function does not describe a particle's state exactly, we can only know that information when we make measurements and thus collapse the wave function. This gives us a lot of ...
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Finding an energy of a particle in an infinite potential well

This question arises from a discussion I recently had with my friend. We were talking about a particle in an infinite potential well. The particle is in an arbitrary wavefunction $\Psi$. When one ...
64 views

Difference between Wavefunction collapse and throw of dice

It might be a really stupid question and I think I am trivializing it. I am not able the understand the big issue with collapse of wave function. So we have set of probabilities and when you measure ...
75 views

The common wavefunction and annihilation of 1 photon [closed]

QM says that if we have many particles they have a common wavefunction. Also QM says that when you measure a particle or observe it, you collapse its wavefunction. That must be a logical mistake. Now ...
123 views

Time evolution of stationary states [closed]

Let's say we have a state $\phi=\sum_i c_i \phi_i$ where the $\phi_i$ denote energy eigen vectors with non degenerate eigen values. Now if a measurement of the energy is done this state ...
146 views

Heisenberg choice of the observables and different outcomes [duplicate]

Somebody could help me to clarify how its possible that different choices of the observable to measure can lead to different outcomes of the observed system state?
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The many world interpretation and immortality [closed]

This question might not be appropriate as it is philosophical on a similar level as the Schrödinger's cat thought experiment. In any case it might be interesting to think about it nonetheless. The ...
778 views

What constitutes measuring in the double slit experiment? [duplicate]

In the double slit experiment attempting to measure which slit the particle passed through causes the wave function to collapse. According to the question: What is the quantum mechanical definition ...
239 views

Decoherence: faster than light?

The answer to this question suggest that one can solve the measurement problem by decoherence. If I understand it correctly the decoherence appears when the quantum state interacts with the ...
3k views

Would every particle in the universe not have some form of measurement occurring at any given time? [duplicate]

I know this is probably a common point of confusion, but I have a specific question about measurements in Quantum Mechanics. I read an explanation on this, but still have a point of confusion. The ...
93 views

Is there a quantum analogue of Mean Value Theorem theorem?

Background I was thinking of Mean Value Theorem in the context of classical mechanics I have $2$ points $A$ and $B$ and my particle goes from $A$ to $B$ then I know the velocity of the particle at a ...
174 views

Does Von Neumann entropy fail to account for projective quantum measurements?

I'm trying to understand if this line of reasoning is correct: For a two-state pure quantum system $\left|\psi \right\rangle=\frac{1}{\sqrt{2}} (\left|0 \right\rangle+\left|1 \right\rangle)$, The ...
22 views

Measuring change in magnitude of electric current due to change in conducting material characteristics

I am new to this community and hence may not be aware of all the rules. Please do let me know if you think I am violating any. I will try to get it corrected. I am trying to get an answer to the ...
81 views

Information Centric Interpretation of the Delayed Choice Experiment

The photon travels from the emmitter to the detector. Along the way it bounces off a few mirrors and off or through a beam splitter or two. This journey takes time, but only from OUR perspective/frame....
120 views

A problem in three sequential selective measurements in case of incompatible observables

In case (a) the B filter takes care of B's eigenvalues and we sum all of their probabilities to calculate the probability of obtaining $|c'\rangle$. In case (b) B filter is not used and this creates ...
57 views

Probability of a particle to go from one position to another

In Feynman's lectures on physics Volume:3 Ch:21. Feynman states The amplitude that a particle goes from one place to another along a certain route when there’s a field present is the same as the ...