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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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Doing Stern-Gerlach experiment without blocking the atoms in the $-x$ direction

In quantum mechanics, in general, it is stated that the act of measurement changes the state of the system. For example, consider the following Stern-Gerlach setup; A beam of silver atoms first ...
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1answer
75 views

Is is possible to have a pair commuting observables only in a single direction?

In quantum mechanics, for two observables to be compatible, successive measurements of the observables, say $A$ and $B$, should yield the same result as earlier, i.e if we do the measurements with the ...
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Some thoughts on Wave particle duality [closed]

The double slit experiment proves that electrons are in fact waves but if you try to measure it--- hit it by a photon--the wave function collapses and it behaves as a particle. The human brain cannot ...
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2answers
62 views

What’s a measurement qualifier in the double-slit experiment

In the double-slit experiment why does the light hitting the back screen not result in a measurement? How far away from the slit can the measurement take place and still cause a wave collapse and is ...
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1answer
107 views

Simultaneous measurements in Quantum Mechanics

In the notes I am using, it states that if two observables A and B are measured simultaneously, then the measurement of A does not affect the measurement of B, and vice versa. However, why does the ...
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1answer
62 views

Does the Hermitian Operator Reduce the Non-eigenstate State to the Self-Eigenstate?

Let's consider $\left| \Psi \right> $ some state of quantum system. Let's also consider some Hermitian Operator $\hat{Q}$, with the discrete specrum: $$ \hat{Q}\left| Q_n \right> = Q_n\left| ...
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1answer
67 views

Finding an energy of a particle in an infinite potential well

This question arises from a discussion I recently had with my friend. We were talking about a particle in an infinite potential well. The particle is in an arbitrary wavefunction $\Psi$. When one ...
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1answer
58 views

Difference between Wavefunction collapse and throw of dice

It might be a really stupid question and I think I am trivializing it. I am not able the understand the big issue with collapse of wave function. So we have set of probabilities and when you measure ...
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1answer
56 views

The common wavefunction and annihilation of 1 photon [closed]

QM says that if we have many particles they have a common wavefunction. Also QM says that when you measure a particle or observe it, you collapse its wavefunction. That must be a logical mistake. Now ...
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2answers
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Time evolution of stationary states [closed]

Let's say we have a state $ \phi=\sum_i c_i \phi_i $ where the $ \phi_i $ denote energy eigen vectors with non degenerate eigen values. Now if a measurement of the energy is done this state ...
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1answer
122 views

Heisenberg choice of the observables and different outcomes [duplicate]

Somebody could help me to clarify how its possible that different choices of the observable to measure can lead to different outcomes of the observed system state?
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1answer
70 views

The many world interpretation and immortality [closed]

This question might not be appropriate as it is philosophical on a similar level as the Schrödinger's cat thought experiment. In any case it might be interesting to think about it nonetheless. The ...
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3answers
177 views

What constitutes measuring in the double slit experiment? [duplicate]

In the double slit experiment attempting to measure which slit the particle passed through causes the wave function to collapse. According to the question: What is the quantum mechanical definition ...
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2answers
194 views

Decoherence: faster than light?

The answer to this question suggest that one can solve the measurement problem by decoherence. If I understand it correctly the decoherence appears when the quantum state interacts with the ...
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3answers
3k views

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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1answer
82 views

Is there a quantum analogue of Mean Value Theorem theorem?

Background I was thinking of Mean Value Theorem in the context of classical mechanics I have $2$ points $A$ and $B$ and my particle goes from $A$ to $B$ then I know the velocity of the particle at a ...
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2answers
107 views

Does Von Neumann entropy fail to account for projective quantum measurements?

I'm trying to understand if this line of reasoning is correct: For a two-state pure quantum system $\left|\psi \right\rangle=\frac{1}{\sqrt{2}} (\left|0 \right\rangle+\left|1 \right\rangle)$, The ...
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1answer
17 views

Measuring change in magnitude of electric current due to change in conducting material characteristics

I am new to this community and hence may not be aware of all the rules. Please do let me know if you think I am violating any. I will try to get it corrected. I am trying to get an answer to the ...
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0answers
73 views

Information Centric Interpretation of the Delayed Choice Experiment

The photon travels from the emmitter to the detector. Along the way it bounces off a few mirrors and off or through a beam splitter or two. This journey takes time, but only from OUR perspective/frame....
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1answer
55 views

A problem in three sequential selective measurements in case of incompatible observables

In case (a) the B filter takes care of B's eigenvalues and we sum all of their probabilities to calculate the probability of obtaining $|c'\rangle$. In case (b) B filter is not used and this creates ...
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1answer
51 views

Probability of a particle to go from one position to another

In Feynman's lectures on physics Volume:3 Ch:21. Feynman states The amplitude that a particle goes from one place to another along a certain route when there’s a field present is the same as the ...
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1answer
91 views

Would Schroedinger's cat smell 50% like death? [duplicate]

If we left Schroedinger's cat in a superposition for a week would it begin to start smelling 50% like a dead cat and 50% like a living cat? Would the smell of death immediately disappear if we opened ...
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2answers
175 views

How is it possible that a universal wave function is predicted to simulate a continuously branching timeline?

If there was no mistake and it actually is the case that a universal wave function that's in a very ordered state at the beginning of time is predicted to simulate a continuously branching timeline ...
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1answer
135 views

Is the Everettian interpretation the only 'realist' interpretation which has been actively researched for relativistic quantum theories? [closed]

I take it that quantum interpretations can be roughly classified into realist and instrumentalist interpretations. In the former category we have the De Broglie-Bohm interpretation, Everettian ...
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3answers
189 views

Does Newton's first law of motion apply to a single moving electron?

That is in a zero electric field, if an electron moving, does it follow rectilinear propagation? Does it follow Newton's first law of motion? I not asking about newtonian mechanics. I am asking ...
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1answer
106 views

Two Time Correlation function calculated from Born rule

Update Below I'm having a hard time reconciling two different calculations of the quantum two time correlation function. Consider quantum operator $A$ with eigenvectors $\{|\phi_i\rangle\}$ and ...
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1answer
70 views

Collapse of the wave function and switching off measurement device before particles hit the wall detector

Would collapsing wave function by measuring/observing and then quickly turning off measurement device cause to uncollapse wave function? so it goes like this : wave--->particle---> wave(hits the wall)...
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2answers
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Current quantum theory does not offer a dynamical mechanism to account for energy transfer in position-measurement, WHY?

In Quantum Mechanics (vol. 1) by Tannoudji et al., there are two pages discussing "perturbation created by a position measurement" (p.278-279). They consider a particle in an infinitely deep potential ...
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49 views

Does it matter whether or not $\delta(x)$ is a valid wave function for a particle on the real line?

We model the wave-functions of a particle on the line by vectors $\psi \in L^2(\mathbb{R})$, and the position operator $X:D(X) \rightarrow L^2(\mathbb{R})$ as the operator such that $X\psi(x) = x\psi(...
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1answer
59 views

What is the relation between uncertainty and information obtained from a measurement?

For ex. if a measurement gives a position with twice the uncertainty as another measurement, how much less information regarding position are you getting? In other words, if uncertainty doubles, is ...
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7answers
203 views

What do positions in Schrodinger Equation mean (Remember: the particle never has definite position)?

In position or configuration representation, the Hamiltonian operator, and thus the Schrodinger Equation, is expressed in terms of positions. But the particle never has definite position, what do ...
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In-homogenous magnetic field 'measures' the spin projection, whereas a homogenous magnetic field simply evolves the spin system.Why?

If a spin-half particle in the $ \lvert+\rangle_z $ state is made to pass through an inhomogeneous magnetic field in the x-direction, $S_x$ gets measured. However, there is no time evolution of the ...
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Hilbert space of an ensemble of identically prepared systems

In order to verify experimentally the quantum mechanical predictions when we measure observable $\hat O$ on a given system A, it is usual to prepare an ensemble of identically prepared systems ($N$ ...
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Measuring quantum mechanics changes the answer and energy?

A thought occured to me thinking about the quantum slit experiment. Upon firing an electron or particle it would impart an opposing force on the machine shooting it. The direction if this force should ...
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1answer
42 views

Modelling how a macroscopic detector interacts with a quantum mechanical particle?

Background I recently came up with a math formula and wondered if could be useful for any physical kind of situation. I then, realised it might be useful for the situation of particle detector. P.S: ...
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1answer
108 views

What is the physical meaning of expectation value of the Hamiltonian operator?

I've been studying David Griffiths' Introduction to Quantum Mechanics and int that, it was explained that the expectation value of position $x$ is the average of the positions of $N$ identically ...
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1answer
94 views

Significance of cube factory paradox

I am reading a book titled QBism, a description of the Bayesian interpretation of Quantum mechanics, The author describes the “... baffling cube factory paradox...”. A factory makes perfect cubes with ...
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2answers
51 views

when I take two measure in same time, probability is product of probability?

let |φ› be a state, A the associate operator of "a" and B the associate operator of "b". I want to measure value "a" and then measure value "b", What is the probability? How is this related to the ...
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1answer
153 views

The 'Conservation of Information' myth

PBS Spacetime recently released a video talking about 'why information is never destroyed in Quantum Mechanics' I'm surprised to see this myth to persist even to this day. We know that unitarity and ...
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1answer
141 views

Going from stochastic Schrödinger equation to master equation

I am currently reading the book "Quantum measurement and control" by Wiseman and Milburn (https://www.amazon.ca/Quantum-Measurement-Control-Howard-Wiseman/dp/1107424151) and something is really ...
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Quantum Observation

Bear with me if I present a lack of knowledge - QM is not my field. There's a common notion in QM that until a particle is observed (measured), its properties are not definite, but rather are spread ...
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1answer
140 views

Is this assertion by Landau and Lifshitz's QM equivalent to the axiom that state spaces are vector spaces?

Many expositions of Quantum Mechanics begin by stating axioms of the following type: "The state of a quantum mechanical system is given by a nonzero vector in a complex vector (or Hilbert space) ...
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Loss of relative phase through ignorance and what is measurement?

Two online lectures I found by Prof. Wojciech Hubert Zurek and Nima Arkani-Hamed made me try to understand the meaning of: "It is our ignorance of the environment that leads to the loss of information ...
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1answer
66 views

Doubt about fifth postulate of QM degenerate case

I think I didn't not really understand a comment found on "Quantum Mechanics" by Claude Cohen-Tannoudji. Talking about the fifth postulate of quantum mechanics in the case in which $a_n$ is a ...
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29 views

Is local realism an inaccurate description of the Universe? [duplicate]

Has the big Bell tests proven local realism wrong? References: The Big Bell Test: Global physics experiment challenges Einstein Crowdsourced quantum reality-check gets crowdsourced peer review
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1answer
51 views

When does the wave function collapse in the EPR paradox experiment? Shouldn't this depend on the reference frame?

Suppose you have two entangled particles. Particle A, and Particle B. If the spin of particle A is up, then the spin of particle B must be down and vice versa. Now suppose you measure the spin of A ...
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1answer
56 views

Generalisation of the measurement postulate in quantum mechanics

Given an observable that has a partially discrete and partially continuous spectrum of eigenvalues associated to it with the order of the spectrum's degeneracy being greater than 1, how would you ...
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1answer
44 views

Particles trajectories in Stern-Gerlach experiment

I red this sentence on "D.J. Griffiths, Introduction to Quantum Mechanics" talking about the using of heavy particles for the experiment. "...heavy so we can construct localized wave packets and ...
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1answer
130 views

Which experiment clearly shows us that Heisenberg Uncertainty Principle is ontological and not epistemological?

I have heard that the Heisenberg Uncertainty Principle(HUP) is not because of our limitations in measuring and the apparatus used but instead because of the very way things are I searched online in ...
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1answer
61 views

Measurement of position in quantum mechanics

I know that when you perform a measurement of position in quantum mechanics, the wave function collapses to something proportional to it, but in a small range of values of positions, depending on the ...