# 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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### If I want to measure an electron how would I do it?

If I look at the Electron to see it then a Photon must of hit the Electron. So I have no idea of what the Electron was doing before I looked at it. To find its position again I need to perform a ...
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### Are results of "quantum eraser experiment" same both for particles moving at the speed of light and slower ones?

Recently I've encountered a video about the quantum eraser experiment. I was curious, if results of this experiment are the same for entangled particles moving with the speed of light and slower ones?
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### Is it possible to use Bayesian method in improving the measuring conditions and accuracy of an electron's (or photon) momentum and position?

for you who need a definition for what a "Bayesian method" is, and as per wiki's easy definition. it's a method of statistical inference in which Bayes' theorem is used to update the probability for a ...
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### Is the future set? [duplicate]

If we know the state of the universe at a certain point in time, is the future set? There have been quite a few similar questions on here and some of the answers were quite useful to me. But there is ...
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### Realtivity and the measurement problem

If a quantum system is prepared and the eigenvalues are known but the experimenter gets in a spaceship and rockets off in a particular direction at relativistic speeds and then performs the ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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 ...