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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Why is wave function collapse mysterious?

There are lots of questions and answers on this site about wave function collapse (for example, How does a Wavefunction collapse?, Why does a wavefunction collapse when observation takes place?, How ...
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Is Uncertainty from a Scalar object using Momentum/Direction from Vector? [closed]

A Scalar Object is a Physical Scalar Volume. I'm asking what causes Uncertainty, not what we already know. Are objects tossed in the double slit, that can't display fringes, subject to the Measurement ...
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Why does particle measurement cause quantum wavefunctions to collapse

When we attempt to measure a certain property of a particle, how and why does its wave function collapse? I've tried to find answers on my own, but they've been far too complicated for me to ...
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Why does commutativity mean that two observables can be measured together?

BACKGROUND As far as the Heisenberg uncertainty principle is concerned, my understanding of commuting observables $\hat{A}$ and $\hat{B}$ is that the measurement outcome $a_i$ does not perturb (or ...
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Can someone explain the measurement problem with little bit of mathematics?

Can someone mathematize the statement of the quantum measurement problem? I am only interested in the statement of the problem (and not its solutions). Thanks. Still confused. Stated in this way (as ...
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2answers
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Does the Bell test preclude localism, realism, both, or just one of either (indeterminate)?

I saw this excerpt from the wikipedia article on EPR paradox They postulate that these elements of reality are, in modern terminology, local, in the sense that each belongs to a certain point in ...
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Can quantum measurements be the origin of thermodynamic arrow of time?

We can practically consider that the microscopic interactions are symmetric with respect to time(as we can neglect weak force for many cases which is the only interaction that can violate $T$ symmetry)...
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Determine initial permeability from experimental data

How to determine initial permeability of the sample having only its dimensions (ring with $R$ - external radius, $r$ - internal radius, and thickness $h$), the number of coils around it $N$, and only ...
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312 views

Wigner's friend experiment - How is there an apparent paradox?

Wigner's friend thought experiment mentions that there is an apparent paradox - 'when exactly did the collapse occur?'. Whether it occurred when the friend made the measurement, or when Wigner asked ...
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Wigner's friend question [closed]

This is probably a dumb question, but does anyone know whether both Wigner and Wigner's friend observe the same result once both have been in the room and measured?
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Electron interference

As I have understood, as per QM, electrons could behave as waves in a double slit experiment, i.e. form dark and bright bands, albeit after sufficient electrons have been shot from the source. Also, I ...
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Measuring momentum in a 2 height well

In a 2 height finite square well we know that qualitatively the position wavefunction will have a lower amplitude in the side of the well with a smaller potential energy (since it will be travelling ...
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Many-world interpretation and absolute time

Consider a quantum system $S$ in superposition of two state,with an observer $A$, forming a bigger quantum system $S'$, with observer $B$. At $t=0$, observer $A$ measure $S$ and according to MWI, the ...
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Quantum measurement paradox

let's define 'a measurement device' as a system which is highly sensitive to the eigenstate of an observable. The sensitivity is quantified let's say by how irreversible and grand the small changes in ...
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Difference between Phenomenal and Phenomenological in the Context of Physics

I was going through a conference presentation on System of Systems Engineering. In the presentation entitled "Macroscopic Quantum Mechanics and the SoSE Design Approach", I came across a ...
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2answers
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Why is $\Delta x$ or $\Delta p$ constant for a particular $\psi_n$?

We were asked to calculate $\Delta x \Delta p$ for the $\psi_0,\psi_1$ of the harmonic oscillator.And so we calculated the answers and verified that $$\langle T \rangle +\langle V\rangle = (n+1/2)\...
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Does the particle interfere with itself, or the observer?

In the double slit experiment, the observer doesn't know which slit the photon went through so the wavefunction is modelled as going through both slits at once and thus there's interference on the ...
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60 views

How could one ever settle with a coin flip?

Suppose that one could "flip" a coin, and that it settled onto a platform. For simplicity, let the coin processed with the nonzero angular momentum on $y$ axis only. To further simplify the ...
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1answer
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Can we think about a particle trapped in a potential well in terms of “quantum measurement”?

Usually, when I'm thinking about a quantum measurement, I see a sort of particle that is being hit by a photon. The more energy the photon carries, the more the momentum of the particle is disturbed, ...
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Different realizations of single-electron double-slit experiment

Recently I was reading about the real world realization of the double-slit experiment, in which we shoot single electrons. The final detection on the rear screen is done "by collecting the transmitted ...
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1answer
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Resolving the measurement problem with mathematical theorems? [closed]

The key debate, around the measurement problem is whether collapse should be interpreted as a physical process(Bohmian Mechanics) or as an immaterial process(e.g. Copenhagen Interpretation, ...
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When you observed an quantum measurement, do you know the measurement result?

Consider a quantum system A with state $|a\rangle$. Scientist B use an instrument to measure A. The state collapse and obtained an result. Suppose there is an observer C, treating B and its ...
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Is it possible to observe a quantum probability distribution?

Is it possible to observe the probability distribution of a quantum particle in real time? So not to observe A state, which would collapse the wavefunction, but observe the whole wave and its ...
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2answers
106 views

Definition of an “Observer” in regards to quantum physics and consciousness involvement [duplicate]

Each time I look video in regards to quantum physics there is almost always one point where they claim small matter can be represented by a wave function (can be everywhere but the item in question ...
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Does the energy and/or mass of an electron change during the act of measurement and so the wavefunction collapse?

In the double-slit experiment, if we want to know which slit did the electron go through, we have to use a laser that gives an energy to the electron. Now, I have two questions: 1. Does the energy of ...
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Many worlds & non-local correlation

I just finished Sean Carroll's Something Deeply Hidden, and found it the best explanation of MWI I've ever seen, and even find that I have no good arguments against it; the parts I understand seem ...
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Quantum mechanics and the “measurement problem,” any recent advancement? [closed]

Lately, I've tried to use Google in an attempt to understand the concepts of wave function collapse and quantum decoherence. So far though, things sound a bit contradictory. If the the actual ...
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2answers
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In the electron's double slits experiment, what causes the electron's wave function to collapse at the screen?

In this figure from Wikipedia, we know that electron's wavefunction collapse at screen F, causing an interference pattern. Does it mean that in this case when the wavefront arrives at the screen, the ...
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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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Decoherence and measurement in simple terms?

I don't know about physics. Say there are only two electrons in the universe, so there is a probability distribution of their position, speed etc. They are in each other's observable universe so they ...
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103 views

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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66 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 ...
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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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84 views

Entropy of measurements; is the Von Neumann entropy lying?

Pre-measurement Let $\left| \psi \right>$ be a pure state: $$ \left| \psi \right> = a \left| 0 \right> + b \left| 1 \right> $$ The density matrix of $\left| \psi \right>$ is: $$ \...
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Are all processes time/CPT-reversible, e.g. measurement, stimulated emission, state preparation, Big Bang?

"The CPT theorem says that CPT symmetry holds for all physical phenomena, or more precisely, that any Lorentz invariant local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry." ...
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Penrose experiment

I just read about Penrose interpretation theory about the wave function collapse: https://en.wikipedia.org/wiki/Penrose_interpretation, which could be confirmed/infirmed by the following experiment: ...
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Measuring small changes in length using the micrometer screw gauge

Which of the following method/instrument cannot be used to measure small changes of the order of a millimeter occurring in a length of about 50cm. 1)spherometer 2)travelling microscope 3)meter ...
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220 views

If the wavefunction is continuous how can the many-worlds be discrete?

Preamble for clarity: The many worlds interpretation is usually used to explain the measurement of a 2 level system ($|0\rangle$ or $|1\rangle$) as: $$\frac{1}{\sqrt{2}}(|0\rangle+|1\rangle)|\text{...
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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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93 views

Can we craft a Hamiltonian such that the measurement is consistent with the discrete measurement taught in Quantum physics?

So the way I understand this, the way measurement is taught is that you have a wave function $\Psi(t)$. It's evolution over time is : $$i \hbar \frac{d}{d t}\vert\Psi(t)\rangle = \hat H(t)\vert\Psi(t)...
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In QM does the observer really perturb the system to make a measurement?

It is commonly taught in introductory QM courses that in order to get to know the position or momentum of a particle, be it by "sending a photon" or similar experiments, the measurement necessarily ...

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