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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The principle of Quantum indetermination

Well, it is a amazing fact that Quantum world deals with indeterminism, one can't tell about the system without directly "seeing" it. (Many novels, short stories are also formed on the same) My ...
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A mismatch in quantum mechanics

One of the basic rules of Quantum mechanics is that after a measurement of an observable, the wavefunction is an eigenstate and any subsequent measurement will give the same result. This is not so in ...
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What does “contextuality” mean in the context of partial rather than complete quantum measurements?

The Kochen-Specker theorem is often described as ruling out "noncontextual" classical hidden variable theories. I understand the math behind the theorem, but I'm a little unclear on the exact, ...
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Steps of measurement to verify the Heisenberg uncertainty principle

The answers here about experimentally verifying Heisenberg's uncertainty principle says the following. Step 3, select two operators A and B Step 4a, for some of the systems prepared in state $...
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Difference between measurement, state and system in quantum mechanics [duplicate]

This question refers to the following point made in Susskind's book Quantum Mechanics- The Theoretical Minimum: In the classical world, the relationship between the state of a system and the result ...
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283 views

What is the importance of a diagonal density matrix after measurement/decoherence?

Let $|\psi_{SA}\rangle$ be the state of a system and an apparatus, for example an electron spin and a Stern-Gerlach apparatus. If $|\psi_S\rangle=\alpha|\uparrow\rangle+\beta |\downarrow\rangle$ ...
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Does the Uncertainty Principle really rule out the existence of definite trajectory of electrons?

Excerpt from my textbook It is impossible to determine simultaneously, the exact position and exact momentum (or velocity) of an electron. It rules out the existence of definite trajectories ...
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Literature recommendation on relation between quantum and macroscopic world

I am able to solve Schrodinger's equation for particles in vacuum and know many books that describe many variations of this problem and cover al technical details of the differential equation solving. ...
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140 views

Are quantum measurement of a system only occurring with human interaction?

In quantum physics, any functional interaction like measurement/observation forces particles down to a single state. Yet when plants do their photosynthesis it's been discovered that they actual ...
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What is a 'wavicle?'

What is a wavicle? I learned in electronics class that electrons are little particles. In physics they even say that the electron orbits the nucleus thus exhibiting angular momentum. But in chemistry ...
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Potential FTL Implications of quantum “weak measurement”?

Physics noob here. I can't find a good answer to this thought experiment that is consistent with general relativity and It's really bothering me. I'm probably missing something huge but just can't ...
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Do quantum events depend on the presence of observer?

Similar to that in the Schrodinger's cat thought experiment the cat remains in superposition unless observed by an observer, also in the double slit experiment an electron is passing through both the ...
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222 views

How did people think about the Copenhagen Interpretation before decoherence was discovered?

It seems today a lot of people are happy to think about quantum mechanics in terms of collapse theories, without thinking about where in the process the collapse occurs or what triggers it etc. This ...
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Wavefunction behavior upon measurement

I am a bit confused about the idea of measurement in QM. As far as I understand, if you measure the position of a particle, the wavefunction of that particle changes into a delta function, and thus ...
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Can two entangled electrons can get disentagled in Nature, withous us making a measurement?

Imagine we have two free, entangled [by their spins, which are in this case non-separable, while $x$ (or $p$) electrons are clearly separable]. In an experimental setup, we can measure (and observe) ...
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544 views

How do we know that Quantum Mechanics isn't simply a theory of approximations? [duplicate]

From what I understand, QM is all about uncertainty. The wavefunction (or rather $|\Psi|^2$) gives us a probability of finding a particle at a certain point. Then, we measure the particle, and find ...
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Can we measure without collapsing (too much) the wave function, according to decoherence theory?

According to decoherence theory, the collapse of the wave function is a continuous process due to interaction with environment. In a measure, there are interactions with photons (for example). Can we ...
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Why do lasers in open air produce 2-slit interference patterns? [closed]

I saw this answer: Is quantum entanglement functionally equivalent to a measurement?. I had follow up questions that seem to have been addressed. Here are just a few similar topics: Why does ...
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operator invertability and measurment

In Quantum mechanics, we define for each physical quantity an operator (for example - the position operator) whose eigenvalues are the possible outcomes of a measurement (different positions in space)...
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231 views

Fluorescent screen and quantum measurement

In the well known, two slit gedanken experiment, used in most textbooks to demonstrate wave like properties of an electron, a fluorescent screen or photographic plate are used to detect interference ...
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291 views

A non-metaphysical interpretation of measurement in quantum mechanics

In quantum mechanics, measurement plays a fundamental role and to me, its role is usually described in a rather odd way. The Copenhagen interpretation states thus When that device makes a ...
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431 views

If uncertainty principle is explained by wave function, then doesn't wave function collapse when we measure position or momentum?

I have been using an app called Quantum on the play store. It says that the uncertainty principle can be explained by wave function, that is when we try to determine position , the wave function ...
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Could the collapse of a wave function be used to determine whether an object has been detected?

Consider a ship in space that is attempting to stealthily travel between two locations. I hypothesize that it could use the collapse of a wavefunction to tell if it's been detected by using a ...
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1answer
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Causality according to Dirac

Dirac writes in page number 4 of his "Principles of Quantum Mechanics": It is usually assumed that, by being careful, we may cut down the disturbance accompanying our observation to any desired ...
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Quantum measurement in experiment

According to standard quantum mechanics, after a measurement, the original state will "collapse" to one of the eigenstate of the observable, and if one measures it immediately after , the result will ...
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Why Einstein was always opposed to uncertainity? [closed]

Einstein always believed that everything is certain, and we can calculate everything. And he always rejected Quantum mechanics due to it's factor of uncertainty. But still Quantum physics was right....
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From uncertainty to commutation relations

Consider the famous problem of measuring both the position and momentum of an electron. We start with two photographs at different times, then worry about the momentum of the photon, the wavelength of ...
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Measurement on density operator

Question: A system in a mixed state $\rho$ is measured with the measurement described by a projection operator $P$. What is the probability of the outcome? What is the density operator ...
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If a wave function collapses into one state, does it ever go back to a superposition of states?

It is my understanding that after an observation, the wavefunction collapses to one state. Thus, if you do an observation right after an observation (that collapsed the wavefunction), you get the same ...
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Fields: Fundamental and Physical, yet Unobservable?

I'm currently working through Robert Klauber's Student Friendly Quantum Field Theory, which by the way is much more accessible than other texts like, say, Peskin and Schroeder, for others also coming ...
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544 views

Definition of the words “macroscopic/classical object” vs “quantum object”

Let's take the Schrodinger's cat as an example. In what sense is the cat different from the isotope ? We say that the isotope is "microscopic/quantum" and the cat is "macroscopic/classical" object ...
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What processes cause the collapse of a wavefunction and break entanglement?

This question states that measuring the spin of an entangled particle causes the collapse of the wavefunction and thus the entanglement is broken. Then this question states that we don't know what ...
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218 views

Do measurements of commuting obervables really commute?

Say I have two commuting operators $A$ and $B$, with joint eigenvectors $|n\rangle$. Say I have a state $S = \sum_n a_n | n \rangle$. If I measure $A$ first, I pick out say an eigenstate $|k\rangle$ ...
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Why does the universe need quantum superposition? [closed]

My question is not about the nature or reality of quantum superposition, superposition is a fact and no argue about that. My question is: What is the purpose of superposition and why does the ...
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State of a system after $L_z^2$ been measured

A system is initially in a state given by $$|{\psi_i}\rangle = \begin{pmatrix}\frac12\\1\over2\\1\over{\sqrt2}\end{pmatrix}$$ corresponding to the angular momentum $l=1$ in the $L_z$ basis of states ...
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When exactly is “God playing dice”? (Question on Hamiltonian and ground state, actually)

There is something I don't get: So I read that by applying an operator to the wavefunction (aka. measuring stuff), it is as if the wavefunction collapses onto one defined state which is an eigenstate....
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Improvement of Von Neumann's measurement scheme [closed]

I would like to know some references on possible improvements of Von Neumann's measurement scheme beyond the use of POVMs. Specifically, I am interested in the possibility of implementing a sort of ...
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279 views

Do the solutions to the Schrodinger Equation tell us the allowed outcomes of a quantum experiment and the associated probabilities of those outcomes?

Is it accurate to say that the solutions to the Schrodinger Equation tell us the allowed outcomes of a quantum experiment and the probability of observing that outcome?
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Quantum mechanics, uncertainty and measurement

I'm trying to learn the very basics of quantum mechanics and trying to grasp the concepts of uncertainty. My claim, which may be false is; the energy of a particle, lets say in a box, may be any ...
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4answers
620 views

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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Wavefunction Collapse and Energy Conservation (and Stationary Tunneling)

Let's say I have a simple isolated quantum system of one particle, 1-D, without spin or relativistic effects. The system was prepared in a way that it begins in an eigentstate of the Hamiltonian, with ...
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231 views

Is the wave function the Radon-Nikodym derivative of a complex measure?

I read somewhere (latest version of a webcomic to be honest) that "superposition" means: a complex linear combination of a $0$ state and a $1$ state... Quantum mechanics is just a certain ...
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How to distinguish “system” and “environment” in quantum decoherence

Quantum decoherence distinguishes the whole big system into "system" and environment, and shows how system, when density matrix is traced over environment, comes to be decoupled from environment. But ...
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What is evaluating our universe? [closed]

When I was talking to my physics professor, he went on an interesting tangent that I did not understand. His Words (Mostly) He said he had a colleague that recently died. His colleague learned a ...
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Decoherence time of quantum decoherence and appearance of wavefunction collapse

If decoherence time of quantum decoherence is zero, or if off-diagonal elements of density matrix of quantum system (without environment considered) vanished immediately after interaction with ...
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Non-zero angular momentum

I have understood the commutator relationship between $L^2$ and any $L$ component namely $L_x, L_y, L_z$. Also the fact that commutator of these two components would not commute. However I find it ...
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Are all kind of measurements of properties in the two slit-experiment destroying interference?

Usually when by measurement is found out through which slit the particle went then there is no interference. But suppose you don’t measure through which slit it went but for example his spin, will it ...
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When does Quantum Mechanics Measurement lead to Mixed State

Consider the context of the Stern-Gerlach experiment. As is stated on numerous sources (e.g. Feynman Lectures, MIT Lecture), a silver atom in the state of $\vert+z\rangle$ that is put through a Stern-...
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The Von Neumann-Wigner interpretation as the explanation for the fine-tuning problem and the existence of free-will [closed]

Disclaimer: Some of the concepts I'm using here are considered by some to be pseudo-science. I do not intend to have a hocus-pocus discussion and fairies and wizards here, my intention is to have a ...
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Is my baby's gender an example of Schrodinger's cat?

At some time in the conception of a child, gender is determined by the X or Y chromosomes given by the male. This is, from my perception, a random selection, like the decaying atom. Like Schrodinger'...