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 can't we measure non-linear operators in QM using single shot measurements?
Consider an operator like the modular Hamiltonian $K$ defined by $$\rho=e^{-K}$$ where $\rho$ is some (reduced) density matrix. So $K$ depends non-linearly on $\rho$ in general. It can be shown that ...
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Does quantum mechanics derive statistical independence for repeated measurements, or is it an assumption beyond the Born rule? [closed]
In standard quantum mechanics, the Born rule gives marginal probabilities for individual measurement outcomes, such as $P(\uparrow) = P(\downarrow) = 1/2$ for a qubit in the $|+\rangle$ state measured ...
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How does the path integral for the Universal Wavefunction account for future internal decoherence events?
I'm trying to solidify my understanding of the path integral formalism when applied to the entire universe, and I've run into a conceptual point that I'd like to clarify.
Let's assume a purely ...
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What happen with wavefunction immediately after a measurement?
One of the postulates of quantum mechanics, introduced by Dirac, says that immediately after a measurement of an observable $\hat A$ the wavefunction abruptly becomes an eigenfunction of $\hat A$. In ...
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Do quantum entities lack definite positions because of measurement limits, or is it a fundamental feature of nature? [duplicate]
In quantum mechanics, particles such as electrons do not have a fixed position until they are measured
Questions:
Is this indeterminacy simply due to the limitations of our current measurement ...
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Wave Function and Measurement (Landau-Lifschitz Volume 3)
I believe Landau is trying to motivate the collapse of a wavefunction. However, there are some parts which are confusing.
He claims that the apparatus is in an initial state $\Phi_0(\xi)$ while the ...
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Is the collapse of the wavefunction really part of the quantum theory?
The collapse of the wavefunction by comparing it with the Schrodinger equations has some differences: it is higly non-linear while the Schrodinger equation is linear, it is non-local as proven by Bell'...
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Does the collision between two molecules count as a "measurement"?
When two reactant molecules collide in a reaction (according to the collision theory of chemical kinetics) do their wavefunctions collapse? That is, does the collision count as a measurement? If they ...
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Quantum measurement when passing through a slit?
Consider a quantum particle like an electron or photon that hits a screen (say it's fluorescent, or generally involves some interaction with one or few atoms to isolate the location) that can ...
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Multimeter cannot read the capacitance
I am making a DIY Paralel Plate Capacitor with the dimension of 27.5cm times 4cm seperated by 80 gsm paper. Assuming paper thickness is 0.1mm I should get around 3.6nF.
But when I test using my cheap ...
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To what degree should macroscopic objects be in superposition of position states according to standard quantum theory?
In this review article on objective collapse theories, which is also linked from this Phys.SE post, at least in the part I've read so far, a deal of fuss seems to be made about the fact that we don't ...
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How much can an observer know about its own quantum state?
Let me state my background: I am familiar with the usual formalism of QM, of how measurements are modeled, measurement problem, decoherence, etc.
In thinking about the process of measurement, I was ...
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Continuous Sample Paths for distributions determined by Schrodinger equation?
Let us first recall how Brownian motion is described in pure mathematics. For all $t\in [0,\infty)$, we have a probability distribution $\mathscr D_t:= \text{Normal}(0,t)$, and random variables $B_t$ ...
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Uncertainty of measurement in Bohmian mechanics
Let's have two non commuting operators $\hat A$ & $\hat B$ corresponding to the physical quantities $A$ & $B$. Also let's assume we are given a wave function $\Psi(\vec{r})$. Because $\hat A \...
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Solution to the measurement problem - explanation
Has the measurement problem been resolved? Could someone explain the current state-of-the-art understanding of why deterministic evolution results in a random measurement outcome? Is there a model ...
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Can the Many-Worlds Interpretation survive without assigning multiple copies of worlds for the same observation?
In Everett's original paper, he postulates a measure $\mu (x)$ to the branches of the universal wavefunction which correspond to measurement outcomes $x$, and argues that it should be given by the ...
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What are some experimental facts/logical reasons to dismiss arguments for consciousness collapse interpretations? [closed]
Let's start by defining consciousness collapse interpretation as the claim that, when we a person performs a measurement of a quantum system, a definite outcome is realised only when the person ...
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Weak measurements, non-demolition measurements and interaction-free measurements vs. wave function collapse [closed]
Let $\psi \in \mathcal{H}$ be a (pure) state of some physical system and suppose we measure an observable $A$ (represented as self adjoint (say bounded) operator, i.e. an element of $\mathcal{B}(\...
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How do I calculate the probability distribution of momentum assuming that my instrument has a small spatial extension?
Let us consider a particle in a one-dimensional space (X-axis) whose state, at a given instant of time, is described by a given wave function.
Let us assume that we measure the momentum p using a ...
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On the wave function collapse
I'm particularly concerned with the conceptual consequences of this postulate, which I never quite pondered enough.
In quantum mechanics, wave function collapse, also called reduction of the state ...
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Has quantum measurement and particle appearance ever been modelled as a resonance effect created by the measuring device on the quantum wave?
Has anyone ever modelled quantum measurement as a resonance effect, that is created by introducing a measuring device into the quantum system?
An analogy may explain what I mean: if you take the free ...
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How does wave function collapse relate to uncertainty in position when measurement intervals approach zero? [closed]
In quantum mechanics, measuring the position of a particle causes the wave function to collapse, fixing the particle at a measured position. Given this collapse, how can it be claimed that as the ...
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Can the measurement problem be overcome? [closed]
I was listening to some physicists discuss the issues with measurement in quantum mechanics and some of the earlier philosophical repercussions. However in most cases where measurement affects a ...
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Is there a resolution to the "Wigner's Friend" paradox?
Suppose that two observers $A$ and $B$ are mutually isolated, observer $A$ measures the state $|0\rangle + |1\rangle$ and concludes that the result is $|0\rangle$.
However, observer $B$ concludes that ...
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What happens if two people have different knowledge about a state in a quantum mechanical system?
Let’s say I measure the spin of an electron, but I don’t tell you what it is and you don’t measure it yourself. Does that change the wave function for you or does it remain the same either way? If it ...
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Does a Quantum System Really "Jump" to an Eigenstate When Observed?
Warning: This is a highly hypothetical question.
I am bothered with Dirac's description of the system when making a measurement. Without quoting his statement (from The Principles of Quantum Mechanics,...
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On the Measurement Problem
In the orthodox interpretation of quantum mechanics, the following three assumptions are made (please correct me if I am wrong):
Every physical system is completely specified by a state $\lvert\psi\...
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Is the collapse of the wavefunction relativistic?
If a stationary observer, 'A', observes the collapse of a wavefunction, does an observer, 'B', traveling at relativistic speed observe a different collapse of the same wavefunction?
What do all the ...
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Position operator action on a wavefunction [closed]
In a 1 dimensional infinite potential well with width $a$, the ground state wave-function is given by
$$\psi(x) = \sqrt{\frac{2}{a}}\sin(\frac{\pi}{a}x)$$
The action of the position operator in the ...
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Exact time evolution of Stern-Gerlach (SG) apparatus
Background: I was always under the impression that when considering the Stern-Gerlach (SG) Experiment, the interpretation of the split of the beams is that the spin $1/2$ particle get measured the ...
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Doubts on my interpretation of Wigner’s friend scenario
I have some doubts regarding my personal interpretation that i was contemplating about in the context of Wigner's friend experiment (also tested in the laboratory).Could it be that a system is always ...
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Understanding Wigner’s friend scenario
I recently watched this video https://youtu.be/Wsjgtp9XZxo?si=sIRlAbvAm2wjXRNP
and tried (unsuccessfully, because i don’t have the knowledge required) to read Bruckner’s paper.
What i am missing, is ...
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Ontologically speaking, does single-shot quantum interference occur in pairs of possibilities?
I've been perplexed by the semantics used in Science 329, 418-421(2010), where they state that according to
Born’s rule and its square exponent, interference always occurs in pairs of possibilities.
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Andromeda paradox and quantum mechanics
Roger Penrose introduced the Andromeda Paradox as a thought experiment that delves into the implications of relativity and quantum mechanics on our understanding of simultaneity and reality. The ...
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Fundamental randomness unprovable? [closed]
Is it too strong a claim (i.e. unprovable) to say that no being (hypothetical or otherwise) could possibly predict the outcome of every individual measurement of any quantum system with certainty? ...
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How can the Copenhagen and Everett interpretations of quantum mechanics make the same predictions?
Suppose we have a spin $\frac{1}{2}$ particle in the spin-up state along the $z$-axis, $\lvert \uparrow \rangle$, and after $t$ seconds of evolution under the Schrodinger equation it is in state $\...
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Shouldn't any measurement cause all wavefunctions to collapse?
Given the fact that every wavefunction exists everywhere in space, shouldn't a measurement at any location cause all wave functions to collapse since a measurement at any point measures all ...
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What exactly is a "measurement" in quantum mechanics?
*I can't see immediately whether or not this post constitutes as a duplicate, but I wouldn't be surprised if it does. If that is the case, then please reference me the post that I'm duplicating so ...
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Do objective collapse equations actually collapse the state?
Why are objective collapse theories stated to collapse the state from a superposition to a single eigenstate (corresponding to the measured eigenvalue)? For this discussion, we are focusing on the ...
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Quantum Collapse during the Measurement of the spectrum of hydrogen [closed]
We have hydrogen inside a tube, and we induce a voltage on it; a current passes through it and light is emitted. The frequencies of light correspond to the differences of the eigenvalues of the energy ...
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Measurement problem and precise mathematical calculation
The infamous measurement problem is a problem in the foundations of quantum mechanics: different people have different views how to understand this problem: some people even deny that there is any ...
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Are there any other nondeterministic processes than measurement in quantum theory?
Nondeterministic refers to a system or process that does not have a single predictable outcome. In other words, when a system is nondeterministic, it means that multiple outcomes are possible for a ...
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How to obtain quantum-state purity and concurrence by measurement on quantum computer?
My whole question is aimed at the implementation on quantum computers utilizing quantum circuits, i.e. we can assume, that I can prepare several "copies" of the same state, without really ...
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Quantum Measurement and the law of thermodynamics
When discussing the conceptual issues of quantum mechanics, concepts like Bell's inequality, non-locality, and the Kochen-Specker theorem are often brought up. Many physicists have dedicated time to ...
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Why is this a correct explanation for the Uncertainty Principle? [duplicate]
There's this really common explanation for Heisenberg's Uncertainty Principle, but I quote it from Classical Dynamics by Marion and Thornton:
The wave character of the photon precludes an exact ...
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Is projective measurement a channel that results from a unitary evolution on both the system and the apparatus?
I was researching the motivation behind introducing quantum channels and this is essentially what I've gathered.
Suppose we have two subsystems, the system we're interested in where states exist in ...
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Will the electron interference remain if the photon scattering method doesn't include a photo-detector? Is it an example of quantum entanglement?
It is actually not a question. I am giving a chain of arguments here , I believe at some point I made a mistake. I want the mistake to be pointed out.
/1. The probability of an event in an ideal ...
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Nielsen and Chuang general measurements and POVM
I'm reading Nielsen and Chuang's famous book on Quantum Computation and Information. In section 2.2 on the postulates of quantum mechanics, they talk about quantum measurements starting with
Postulate ...
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What is the 'charge' and 'mass' of an electron means before measurement?
What I have learnt: We can't talk about an electron's position,momentum,angular momentum,energy...anything 'before the measurement'. An electron simply doesn't have these physical parameters before ...
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Ideal quantum measurement
Schlosshauer (in Decoherence and the Quantum-to-Classical Transition) defines ideal quantum measurement as a von-Neumann measurement in which
(1) the apparatus states correspond 1-to-1 to given system ...
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