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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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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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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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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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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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 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 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 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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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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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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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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How does wave function collapse when I measure position?

Text books say that when you measure a particle's position, its wave function collapses to one eigenstate, which is a delta function at that location. I'm confused here. A measurement always have ...
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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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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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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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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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How does wave function collapsing fit into QFT?

I followed a basic course on QFT and know some basic concepts but I focussed mainly on calculations and understanding the Feynman rules e.g.. However, sometimes I have the idea that there a quite some ...
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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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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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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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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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How do we show that no hidden variable theories can replace QM?

I've always hit two big stumbling blocks in conceiving of the proof or disproof of hidden variable theories as being even valid idea, let alone an answerable question... I feel I must be ...
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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 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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Why does a wavefunction collapse when observation takes place?

Why does a wavefunction collapse when observation takes place? Can this question be explained in non mathematical terms? I have tried finding the answer but couldn't find a clear explanation.
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Does Quantum Entanglement happen between two wavefunctions? [closed]

Does the entanglement happen between two particles or two wavefunctions? If it's wavefunction then what happens to the two wavefuntions after getting entangled?
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After measuring momentum, it seems like the particle's position could be literally anywhere?

Once measuring momentum, the wavefunction "collapses" into something that looks like this If you were to then measure the position, couldn't it be literally anywhere? What am I missing? Is it even ...
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Has it been practically proven that quantum superposition exists ? If yes, how does it even work?

I was wondering if quantum particles do actually exists in two different states simultaneously and if it has been proven they do indeed exists in a superposition of states. How has it been figured ...
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Faster than light information paradox? [closed]

Suppose there is a moving particle with a continuous and well spread wavefunction. Then it is surely possible for 2 separate measurements of position to have spacelike separation. Even though the ...
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Is there a way to detect the collapse in wave function of entangled particles instantaneously? [closed]

Is there a machine or instrument which will notify us instantly when the wave-function has collapsed if we have access to only one of the entangled particles? Note that the wave function collapse will ...
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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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Practically, how does an 'observer' collapse a wave function?

I have been reading/learning about the double slit experiment, its implications in quantum theory, and how it explained that “particles” can behave as both waves and particles. I know that the wave ...
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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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Paradox of wavefunction collapse into an 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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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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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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Has the collapse of wave function due to observation been recorded?

I've seen pictures like this one, which depict the outcome of the Double-slit experiment with wave-like or particle features, depending how measurement has taken place. The graphic showing multiple ...
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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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Why aren't transformations caused by measurements 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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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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Measuring the position of identical particles and wavefunction collapse

I'm working through Shankar's Principles of Quantum Mechanics, and I think I have hit a confusion over identical particles. The book refers to 'measuring the position' of two bosons to be $x_1$ and $...
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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 ...