This tag is for questions about the exact nature of wavefunction collapse.

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What is the meaning of “ Ψ is not a measurable quantity in itself”?

I want to know that why the wavefunction Ψ as a complex quantity (i.e $A+iB$ form) in quantum mechanics and somewhere I have studied that Ψ is not a measurable quantity in itself that's why we ...
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3answers
217 views

How does De Broglie–Bohm theory or pilot wave theory explain the results of the Stern–Gerlach experiment?

The Copehagen interpretation of QM explains the Stern–Gerlach experiment by asserting that a particle is in a superposition of states and doesn't have a definite spin until measured. However, the de ...
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8answers
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What is an observer in quantum mechanics?

My question is not about (pseudo) philosophical debate; it concerns mathematical operations and experimental facts. What is an observer? What are the conditions required to be qualified of observer, ...
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3answers
85 views

Physical meaning of quantum interpretations

Do interpretations of quantum mechanics have physical meaning? An argument for no would be the fact that no matter the interpretation, one gets the same measurements. They also do not follow logical ...
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2answers
277 views

Double slit experiment with animals as observers

I was searching about the double slit experiment, reading and watching videos, etc. If I understood correctly, when they measure the photon it behaves like a particle. On the Youtube video Tom ...
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6answers
362 views

How do we know that there isn't a classical solution to the measurement problem/Quantum Mechanical uncertainty?

It was mentioned to me that it can be shown that there is no classical explanation for the uncertainty in Quantum Mechanics -- i.e. that there are no hidden workings that we have just not yet seen, ...
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2answers
83 views

What makes quantum decoherence different from dissipation?

From my understanding quantum decoherence and dissipation are completely different ways of modelling information loss to the environment. Dissipation can be modeled using the Caldeira-Leggett model ...
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1answer
56 views

Quantum state of a system after measurements with non-commutative operators

a) Assume two operators $A$ and $B$. 1) Assume $$[A,B]=0 $$ and $$ ψ= \sum c_n u_n ~~~~\text a~ wavefunction~ describing~ the~ state~ of~ the~ system $$ with $$Aψ=a_n u_n $$ $$Bψ=b_n u_n$$ If we ...
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1answer
231 views

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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2answers
67 views

Classical notion of trajectory [closed]

Why the classical notion of trajectory is meaningless in quantum mechanics? I am asking here about notion of trajectory from classical mechanics and why in quantum mechanics we cannot use it or is ...
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1answer
138 views

Are the authors saying that the observer effect plays no role in Bohr's thought experiment of the Heisenberg uncertainty principle?

Here is an excerpt from Eisberg & Resnick's Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. Here is introducing Bohr's though experiment to establish a physical origin for the ...
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42 views

Why position and momenta are fluctuating quantities?

In a coordinate basis we have $$\langle \Psi \mid \Psi \rangle = \int \prod_{i=1}^N d^3q_i |\Psi(\textbf{q}_1,\dots,\textbf{q}_N)|^2=1$$ This means that for any quantum state $\mid \Psi ...
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7answers
762 views

Measurement problem in the orthodox interpretation

Let's look at the measurement problem in the orthodox interpretation of quantum mechanics as an inconsistency between inner and outer treatment of the measurement apparatus. You can always push your ...
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7answers
537 views

Born's rule and Schrödinger's equation

In non-relativistic quantum mechanics, the equation of evolution of the quantum state is given by Schrödinger's equation and measurement of a state of particle is itself a physical process. Thus, ...
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1answer
34 views

Are measurement results only orthogonal?

Are all measurement operators on a quantum mechanical system defined by a Hilbert space, such that all possible post-measurement states are orthogonal? For example measuring a qubit in some ...
3
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1answer
128 views

Has this experiment really demonstrated wave-function collapse?

My question is: why did the following experiment claim that it had demonstrated the wave-function collapse? http://www.nature.com/ncomms/2015/150324/ncomms7665/full/ncomms7665.html I would have no ...
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3answers
308 views

Looking for a simple example of generating unequal probabilities in QM

I am trying to understand the problem of branch counting in Everettian interpretations of QM, so I thought I would try to analyze a simple example of starting with equal branch amplitudes that evolve ...
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1answer
86 views

Superposition and simultaneous observation

Trying to understand superposition. Ok, so double slit experiment. The multiple paths the particle simultaneously travels interfere with each other but as it is absorbed, it chooses one "actual" ...
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2answers
230 views

Measurement of the energy of an atom using a cold substance

An atom was prepared in a superposition of ground state and excited states.I propose to measure the state by coupling the system to a cold enough substance. By cold enough I mean $$kT\ll E_1,$$ where ...
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2answers
147 views

Unitarity and measurement

I used to believe that the wavefunction collapse came from the interaction of the system we want to measure {S} with the measurement apparatus {M} : {S} undergoing a non unitary transformation, but ...
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1answer
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In a Stern-Gerlach measurement, does passing particles through the B field influence their spin state?

This question is similar to this post and was influenced by this discussion as well. Assume a Stern-Gerlach apparatus oriented along the z-axis acts as a measurement on the basis of the z-component ...
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2answers
108 views

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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1answer
180 views

How does many-worlds interpretation make measurement unitary?

Does many-worlds interpretation of QM make the process of measurement (wavefunction collapse) be an unitary transform? If so, how does it do this? If we have an "object" qubit in state ...
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3answers
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Measurement of quantum state

Consider a particle in a box system. Assume its state to be a superposition of the ground and the first excited energy states. Consider two observers A and B (rest of the world). A made the ...
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4answers
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Entanglement, real or just math?

I'm new here, actually this is my first question so I'll just get to it. In quantum entanglement when something acts on one particle the other one reacts also, just in reverse (more or less). From ...
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2answers
484 views

Are Thomas Breuer's subjective decoherence and Scott Aaronson's freebits with Knightian freedom the same things in essence?

In his remarkable works (1,2 and their recent development 3) Thomas Breuer proves by diagonalization the phenomenon that the observer cannot distinguish all phase space states of a system where he is ...
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4answers
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Electrons - What is Waving?

If an electron is a wave, what is waving? So many answers on the internet say "the probability that a particle will be at a particular location"... so... the electron is a physical manifestation of ...
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1answer
145 views

Are Forces Involved Non-Local?

Below is a copy of a answer given to this Phys.SE question asked previously: Does every material thing just consist in forces? In short, assume that we have two labs A and B, in each one there is ...
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3answers
546 views

Uncertainty principle and measurement

I would like to really understand how the uncertainty principle in QM works, from a practical point of view. So this is my narrative of how an experiment goes, and I'm quickly in trouble: we prepare ...
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3answers
107 views

Are there two aspects of Born's rule?

I am having some problem understanding Born's rule. I am getting a little bit confused. Here it goes; Let $f(x,t)$ be a solution of Schrodinger equation. Then Born's rule says that the square modulus ...
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4answers
131 views

What does “spread of momentum” actually mean?

I was reading Feynman's lecture in which Feynman invoked his own way of explaining the uncertainty principle using single-slit experiment. There I found: To get a rough idea of the spread of ...
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2answers
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Quantum measurement problem with eigenvectors (Dirac notation) [closed]

Ok so I've got two state vectors related to two other state vectors. $$|\alpha_1\rangle= (1/5)(3|\gamma_1\rangle+4|\gamma_2\rangle)$$ $$|\alpha_2\rangle= (1/5)(4|\gamma_1\rangle-3|\gamma_2\rangle)$$ ...
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2answers
49 views

Quantum fortune teller

A diffraction pattern in a double slit experiment only occurs if randomness is preserved for which way the photon goes and once certainty is determined by actual measuring the pattern is lost. Can ...
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2answers
89 views

Heisenberg uncertainty and probabilistic nature of QM

I am trying to understand whether the HUP and the probabilistic nature of QM are orthogonal or not. By that I mean that the HUP fundamentally derives from operators not commuting, which is the ...
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1answer
47 views

Time reversal on superposition: I think [duplicate]

Imagine I have a box, and in it, I have a photon in a superposition of state |1> and |0>. I look into the box and register that the photon is in state |1>. Now, if I have ALL information in the ...
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2answers
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Eigenstate vs collapsed wave function

An eigenstate, or determinate state, is a state where the measurement of some observable always yields the same result. This means that the standard deviation of the observable is zero. If a ...
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53 views

Books on Quantum Measurement

I have been trying to understand clearly the concept of non locality, hidden variables, quantum measurement etc through research papers. I also read Quantum Theory and measurment by Wheeler and Zurek ...
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Are universally valid possibilistic theories possible?

This is a spin-off of the following question: Are Thomas Breuer's subjective decoherence and Scott Aaronson's freebits with knightian freedom the same things in essence? Given that Thomas ...
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2answers
101 views

The uncertainty principle and spin

I realize that this may be a very basic question, but I've been unable to find the answer elsewhere so thanks in advance for the help. Suppose an electron's spin is measured about an axis, and then ...
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6answers
539 views

Explanation for the EPR-like paradox

I am trying to understand the process of Quantum Entanglement for use in Quantum computers. The problem I have is this: Suppose some nuclear process emits an electron-positron pair. Now after ...
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2answers
52 views

Eigenstates into which a system can be projected after a measurement

I'm currently reading Dirac's Principles of Quantum Mechanics, on page 36, he says: Another assumption we make connected to the physical interpretation of the theory is that, if a certain real ...
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0answers
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finetuned quantum experiments by Murch lab, do any have dynamics outside of basic QM formalism/ axioms?

a series of very finetuned quantum experiments have been reported by the Murch lab eg in 2 articles in Nature & analysis there,[1][2][3] some leading to dramatic accounts in the media.[4] do ...
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1answer
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Would a pair of independent quantum coin tosses be perfectly anti-correlated?

Background Suppose we attach a button to an electronic flip flop such that an LED will toggle when we press the button with 50% probability, where the source of the randomness is a quantum event, ...
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1answer
65 views

If I want to determine a particle's momentum or position, do I get this information from the wave function?

I am confused about how one measures the dynamical variables (eg position) of a particle. I thought the wave function $\Psi(x,t)$ was the probability amplitude and $|\Psi(x,t)|^2$ represents the ...
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So far, what has been proven about the definition of an observer? [duplicate]

I've seen people reason that nearly any particle could be an observer, causing electrons in the double-slit experiment to have definite locations. The idea of what exactly the universe/quantum physics ...
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3answers
691 views

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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4answers
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Why is the Heisenberg uncertainty principle stated the way it is?

I spent a long time being confused by the Heisenberg uncertainty principle in my quantum chemistry class. It is frequently stated that the "position and momentum of a particle cannot be ...
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2answers
59 views

Is there enough information in a given quantum state to determine the state beforehand?

If I knew all the information about a state, and I knew the laws of physics in their complete totality, could I "reverse engineer" it to find, with 100% certainty, the state before it?
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How to infer the wave function $\psi$ of a EM wave?

There was a claim I read somewhere that said the wave function $\psi$ of a EM wave is measurable via $\vec E$ and $\vec H$ Can someone show how this works?
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Uncertainty Principle - measuring momentum on one entangled particle, position on the other

If two entangled particles are sent far apart and then at exactly the same time the position of one, and the momentum of the other, is measured, won't this mean that, because the corresponding values ...