Questions tagged [wavefunction-collapse]

Wavefunction collapse amounts to the apparent reduction of a wavefunction consisting of a superposition of several eigenstates to a single eigenstate (by "observation"). It underlies measurement in quantum mechanics and connects the wave function with classical observables, in a thermodynamically irreversible interaction with a classical environment, normally disfavoring future QM interference.

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Does an electron move from one excitation state to another, or jump?

I'm wondering, when an electron changes state, does it move from one state to another over some (very small) time period? Or does it change from one state to another in no time? If the former, what ...
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What constitutes an observation/measurement in QM?

Fundamental notions of QM have to do with observation, a major example being The Uncertainty Principle. What is the technical definition of an observation/measurement? If I look at a QM system, it ...
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On the nature of the collapse of the wave function

The collapse of the wave function by measurements is one of the most mysterious properties of quantum mechanics. At what scale does the wave function collapse? What are the conditions for a collapse?...
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How do we know a quantum state isn't just an unknown classical state?

When an observer causes the wave function of a particle to collapse, how can we know that the wave function was not collapsed already before the measurement? Suppose we measure the z-component of the ...
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When light reflects off a mirror, does the wave function collapse?

This question is specific to the Copenhagen interpretation, which states that the wave function collapses on interaction. If we have a beam of light reflected off a mirror, whether you see this light ...
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Does measurement, quantum in particular, always increase the total entropy?

Measurement of a quantum observable (in an appropriate, old-fashioned sense) necessarily involves coupling to a system with a macroscopically large number of degrees of freedom. Entanglement with this ...
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What is the difference between a measurement and any other interaction in quantum mechanics?

We've learned that the wave function of a particle collapses when we measure a particle's location. If it is found, it becomes more probable to find it a again in the same area, and if not the ...
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Is the collapse of the wave function inherently time asymmetric?

Schroedinger's equation, as we all know, is time symmetric. In quantum field theory, we have to come up with a more sophisticated CPT reversal, but the essential point remains unchanged. However, the ...
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Momentum of particle in a box

Take a unit box, the energy eigenfunctions are $\sin(n\pi x)$ (ignoring normalization constant) inside the box and 0 outside. I have read that there is no momentum operator for a particle in a box, ...
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How isolated must a system be for it's wave function to be considered not collapsed?

As an undergrad I was often confused over people's bafflement with Schodinger's cat thought experiment. It seemed obvious to me that the term "observation" referred to the Geiger counter, not the ...
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Why does observation collapse the wave function?

In one of the first lectures on QM we are always taught about Young's experiment and how particles behave either as waves or as particles depending on whether or not they are being observed. I want to ...
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Is the wave function of a particle re-created after a measurement stops?

Yeah, I haven't quite understood, or been told, what happens to, for example an electron and its wavefunction, when you stop to measure it. I mean, an electron has a wave function describing its ...
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Is there a difference between observing a particle and hitting it with another particle?

First, let me state that I'm a lot less experienced with physics than most people here. Quantum mechanics was as far as I got and that was about 9 years ago, with no use in the meantime. A lot of ...
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How does a Wavefunction collapse?

How does a wavefunction collapse into one state? More specifically, what conditions cause a wavefunction for a quantum particle to collapse? Does this have to do with density matrices? Please ...
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How is it possible that quantum phenomenons (e.g. superposition) are possible when all quantum particles are being constantly observed?

I don't understand how quantum mechanics (and therefore also quantum computers) can work given that while we work with quantum states, particles that this quantum state consist of cannot be observed, ...
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Decoherence and collapse

It is said that the decoherence does not solve the problem of measurement and/or the emergence of classicality, can somebody explain it with simple analogies or in a manner accessible to a non-...
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Is there an objective asymmetry between a collapsed and un-collapsed wave function?

In a quantum double slit experiment, one starts at t0 with a wave function that propagates through two slits, interferes, and probabilities for various positions at the final stage at t1 can be ...
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State collapse in the Heisenberg picture

I've been studying quantum mechanics and quantum field theory for a few years now and one question continues to bother me. The Schrödinger picture allows for an evolving state, which evolves through ...
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Inexact measurement and wavefunction collapse

As is usually said, measurement of an observable $q$ leads to collapse of wavefunction to an eigenstate of the corresponding operator $\hat q$. That is, now the wavefunction in $q$ representation is $\...
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Meaning of $\int \phi^\dagger \hat A \psi \:\mathrm dx$

While analysing a problem in quantum Mechanics, I realized that I don't fully understand the physical meanings of certain integrals. I have been interpreting: $\int \phi^\dagger \hat A \psi \:\...
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How does a state vector be projected onto an eigenspace after measurement

In http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#Degenerate_spectra, it is said that If there are multiple eigenstates with the same eigenvalue (called degeneracies),..., The ...
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In the double-slit experiment of electrons (observed by photons), is it correct to say the collapse is caused by the momentum of the photons?

I'm working off the article, The Double Slit Experiment Demystified. Disproving the Quantum Consciousness connection. I think it's well-written, but I'm not convinced about this part: So what is ...
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Does $\sigma_x\sigma_p = 0 \cdot \infty$ after a measurement of particle position?

I feel this question has an obvious answer that I should have been able to find independently, but I've searched for a while now it hasn't clicked. When position is measured, the uncertainty of the ...
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Is a photon always in a state of superposition while traveling through space?

In the double-slit experiment, we emit a photon that is in a state of superposition (wave form) which travels through both slits to interfere with itself. When we measure which slit it went through, ...
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Is Schrödinger’s cat misleading? And what would happen if Planck constant is bigger?

Schrödinger’s cat, the thought experiment, makes it seem like as if measurement can cause a system to stop being in a superposition of states and become either one of the states (collapsed). So does ...
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Wavefunction collapse in relativity

It is well accepted that quantum theory has well adapted itself to the requirements of special relativity. Quantum field theories are perfect examples of this peaceful coexistence. However I sometimes ...
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Wavefunction collapse to a state that seems impossible

Let $\Omega$ be an observable and let $\Psi = \sum a_i\phi_i$ be a decomposition of a state of a system (satisfying the Schrödinger equation) in eigenfunctions of $\Omega$ (assume for simplicity it is ...
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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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What are the strongest objections to be made against decoherence as an explanation of “collapse?”

When we measure an observable A of a quantum system, we get an eigenvalue of A. Without worrying about connotations of Copenhagen vs. MWI, etc., let's just call this "collapse." Question: Among ...
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Paradox of the wavefunction collapse into un unphysical state

"A measurement always causes the system to jump into an eigenstate of the dynamical variable that is being measured, the eigenvalue this eigenstate belongs to being equal to the result of the ...
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Stochastic processes and wavefunction collapse

Some time ago I had an idea that, as the unitary evolution of the wavefunction is described by a deterministic equation (PDE, simply), could be the collapse of it be described by some kind of a ...
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What does it mean to apply an operator to a state?

Let's say I have an operator $\hat{A}$ and a state $|\psi\rangle$. What exactly is the state $\hat{A}|\psi\rangle$? Is it just another different state that I am describing using my $\hat{A}$ and $|\...
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Is it really right to say that we never measure anything exactly in QM?

In reference to this elaborate answer by @DanielSank, I would like to pose the following question(s) in order to verify my understanding of the subject matter--in particular, that of the nature of ...
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Confusion about wavefunction separability

A wavefunction is inherently a multi-particle function. If you have a container that is perfectly isolated from the external universe (not possible, but just imagine it) and filled with $n$ particles,...
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Does the wave function/density state actually exist?

I have been reading with interest the debates here on whether the wave function/density state actually collapses or not, or whether it is subjective Bayesian or objective with actual complex numbered ...
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Why isn't transformation, caused by measurement, unitary?

It is said, that when measured, a quantum system undergoes "wave function collapse", which is a non-unitary transformation. Why? The wave function is $\Psi = \alpha \left|0\right\rangle + \beta \...
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Wave function collapse and Schrodinger's equation without measurement

Will wave function collapse without measurement? Since all matters are described by wave functions, then in principle, I should be able to describe wave function collapse by Schrodinger's equation. (...
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Is the Copenhagen interpretation merely an approximation to quantum mechanics?

So, I'm reading Max Tegmark's Our Mathematical Universe (Knopf edition, p. 229). He's discussing Everett/MWI for a bit and I'm not really paying attention and then I wake up to this: [I]t's time to ...
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How can two electrons repel if it's impossible for free electrons to absorb or emit energy?

There is no acceptable/viable mechanism for a free electron to absorb or emit energy, without violating energy or momentum conservation. So its wavefunction cannot collapse into becoming a particle, ...
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What happens after the collapse of a wavefunction?

If I have a quantum system which I prepare in a certain state, this state then evolves unitarily via a Hamiltonian. Suppose an observer provokes a collapse of the wave function by a certain ...
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How do probabilities emerge in the many-worlds interpretation?

My understanding is that at each quantized unit of time that a split occurs, every possible recombination of particles occurs in the 'objective' universe. If this is the case, what relevance to ...
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The quantum state just after a position measurement

The wave function of a free particle is given as, $$\psi(x) ~=~ e^{-{ x }^{ 2 }/{ a }^{ 2 }}.$$ Then a position measurement is made and the position of the particle is found to be at $x=a$. My ...
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Quantum Collapse

When we observe a quantum object does it collapse into a point? Or does it collapse into a smaller wave of area that is out of our range of accuracy? My gut tells me the latter.
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Why is wave-function collapse still being taught in quantum mechanics? [closed]

I don't really understand why wave-function collapse is still being taught while we seem to have better interpretations of QM available nowadays. During the early development of quantum mechanics the ...
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Does the observer or the camera collapse the wave function in the double slit experiment?

Ok so if we setup a camera before the slit we will find a single photon and will follow through accordingly, likewise by having a camera setup after the slit, we can retroactivly collapse the wave ...
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Historical background of wave function collapse

I wonder what were the main experiments that led people to develop the concept of wave function collapse? (I think I am correct in including the Born Rule within the general umbrella of the collapse ...
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wavefunction collapse and uncertainty principle

We all know that wavefunction collapse when it is observed. Uncertainty principle states that $\sigma_x \sigma_p \geq \frac {\hbar}{2}$. When wavefunction collapse, doesn't $\sigma_x$ become $0$?, as ...
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What exactly causes quantum wave function collapse? [duplicate]

I've heard many things, such as the wave function collapses when the particle or system it determines is observed. But then I've heard that if we consider that to be the case, it's incomplete. What ...
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Measurement of position of particle

Say a particle was found to be at $C$. If I measure the position of particle immediately, its given in Griffith's book that it will still be at $C$. In the macroscopic world, isnt this true only if ...
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How is wavefunction probability redistributed after partial wavefunction collapse?

Suppose I set up the double-slit experiment using photons as my particle. Behind the left slit I place a beam splitter that points some of the light off in the direction of a camera (represented as <...