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.

-1
votes
1answer
45 views

A possible way to test whether integrated information theory causes collapse?

One criticism, of the hypothesis that consciousness causes collapse, is that 'consciousness' has not been well defined. Perhaps, in order to test whether consciousness causes collapse, it is best to ...
-2
votes
1answer
73 views

Quantum mechanics limits to understanding the Universe [closed]

By definition, a wave function does not describe a particle's state exactly, we can only know that information when we make measurements and thus collapse the wave function. This gives us a lot of ...
6
votes
1answer
270 views

How does one actually measure the position or momentum of a quantum object?

How is the position or momentum of a Quantum particle is measured experimentally in laboratory? Suppose we want to know the position or momentum of quantum particle which is kept in a box i.e. an ...
0
votes
1answer
206 views

Do the eigenstates of the Pauli operators correspond to the six directions of the 3D world?

I understand that the six eigenstates of the three Pauli operators $X, Y, Z$ correspond to the six poles of the Bloch sphere. By fixing an orthonormal basis of our physical word, does "measuring Pauli ...
5
votes
3answers
1k views

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 ...
1
vote
1answer
263 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 ...
4
votes
3answers
398 views

Do quantum wave functions curve spacetime before they are measured

Do wave functions cause spacetime curvature before they are measured, or would curvature only happen upon measurement? I guess the question becomes, do quantum wavefunctions carry energy while they ...
0
votes
1answer
120 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 ...
17
votes
5answers
3k views

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 ...
3
votes
1answer
71 views

Can we craft a Hamiltonian such that the measurement is consistent with the discrete measurement taught in Quantum physics?

So the way I understand this, the way measurement is taught is that you have a wave function $\Psi(t)$. It's evolution over time is : $$i \hbar \frac{d}{d t}\vert\Psi(t)\rangle = \hat H(t)\vert\Psi(t)...
0
votes
2answers
197 views

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 ...
-2
votes
0answers
71 views

How Do We Know That It Is Measurement

How do we know that it is measurement that collapses wave functions, and not just the other forces at play in all of the different measurement devices that collapse wave functions? Thanks
5
votes
2answers
2k views

What counts as a measurement?

In quantum mechanics, an elementary particle does not have a well defined position until a measurement is performed on it (right?). Such a "measurement" is any sort of interaction with other ...
0
votes
1answer
556 views

How to construct the operator and the physical experiment needed to perform an arbitrary 'measurement in a basis'?

I have taken an introductory level course in QM and have covered some advanced topics by myself and don't really understand what it means to 'measure in a particular basis'. A projective measurement ...
2
votes
1answer
276 views

How to think of matrices as observables?

I'm reading Nielsen and Chuang. In one of the early chapters, they introduce some matrices such as $$X = \begin{bmatrix} 0 & 1 \\ 1 & 0 \end{bmatrix}.$$ They interperet this as a gate that ...
1
vote
4answers
408 views

The double slit experiment - methods used to observe single photons prior to striking the target

I can accept that when single photons are used in the double slit experiment that a diffraction pattern results at the target due to their wave property. What I am puzzled about is exactly what ...
2
votes
1answer
130 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 ...
3
votes
3answers
1k views

Solution to the quantum measurement problem? [closed]

What can be a scientific solution to the quantum measurement problem (other than many worlds idea)? Can it be somehow verified through experiment?
-1
votes
3answers
603 views

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 ...
1
vote
3answers
300 views

Is the probability current an observable?

Is the probability current in Quantum Mechanics an observable? If so, how can it me measured (directly or indirectly)?
0
votes
2answers
232 views

Does the quantum eraser double slit experiment disprove many worlds interpretation?

The quantum eraser double slit experiment shows that interference pattern is destroyed not by interacting with the quantum particle, but rather by getting which path information. In this experiment ...
0
votes
1answer
75 views

In QM does the observer really perturb the system to make a measurement?

It is commonly taught in introductory QM courses that in order to get to know the position or momentum of a particle, be it by "sending a photon" or similar experiments, the measurement necessarily ...
6
votes
1answer
460 views

Is the Born rule indeed wrong?

This is a question about the validity of a preprint, arXiv:quant-ph/0509089, which claims that the "Copenhagen Interpretation of QM is incorrect" (same title, authored by Guang-Liang Li and Victor O.K....
33
votes
7answers
5k views

Isn't the uncertainty principle just non-fundamental limitations in our current technology that could be removed in a more advanced civilization?

From what I understand, the uncertainty principle states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on ...
5
votes
2answers
189 views

When will a quantum measurement occur?

I am confused about the nature of quantum measurements. In short: if a particle is coupled with a field, when will the wave collapse. A field might be coupled to a particle for a long time, and ...
2
votes
0answers
90 views

What is the simplest model of a quantum measurement on a 2 level system

What is the simplest physical system which can be used to model the quantum measurement of a 2 level system? For example, can the following, spin coupled to a harmonic bath, be used to model a ...
0
votes
1answer
124 views

Probabilities with the Density Matrix

The density matrix of the system is given by: $$ [\rho_{S}(t)]_{mn} = [\rho_{S}(0)]_{mn} e^{-i\omega_{0}(m - n)t} e^{-i \delta(t)(m^2 - n^2) - \gamma(t)(m - n)^2}, ...
2
votes
2answers
90 views

Does observation in quantum theories always imply interaction (affecting quantum system with photons, electromagnetic fields, etc.)?

The term observation is obscure. As I see so far, observation is always done by means of affecting (!) the quantum system by some means - often photons or electromagnetic waves or whatever else. ...
0
votes
0answers
85 views

Measurement problem: Origin of probabilities in Many-Worlds Interpretation

As far as I can tell there appears to be an active group of academics (including the likes of Sean Carrol) who believe in the Many-Worlds Interpretation of quantum mechanics, but feel that the origin ...
0
votes
0answers
61 views

Can wave collapse during measurement be avoided with a better experiment / measurement apparatus?

Or to restate the question, does quantum mechanics or the quantum field theory state or imply that we can only observe the wave in a collapsed state? And if they don't, do we have any promising ...
0
votes
1answer
129 views

How is measurement on system in a Hilbert space seen?

I am a bit confused about different kinds of measurements on a system in state $W$ where $W$ is the density operator in Hilbert space $H$. A general measurement can be given by POVM's, let $E_1,E_2......
3
votes
1answer
73 views

Why does the interaction with environment remove the basis ambiguity : decoherence theory

In decoherence theory, we try to remove the measurement postulate from Q.M to replace it by unitary evolutions. Consider a two level system $S$, an apparatus $A$ and the environment $E$. A first ...
0
votes
1answer
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 ...
19
votes
10answers
2k views

On a measurement level, is quantum mechanics a deterministic theory or a probability theory?

Quantum mechanics is a non-commutative probability theory. As such, it fundamentally behaves differently from classical probability theories. This manifests itself most pronouncedly in the uncertainty ...
2
votes
2answers
100 views

Is this decoherence?

I have a very basic understanting of decoherence (i.e. I,ve read the Wikipedia page), but I was recently reading Heisenberg's The Physical Principles of the Quantum Theory and I came across a thought ...
3
votes
1answer
109 views

Full Hilbert space of a particle

Given the Hilbert space $H$ of a single particle, we know we can write \begin{equation} H = H_{spatial}\otimes H_{other} \end{equation} where $H_{spatial}$ is spanned by the possible position states ...
1
vote
1answer
156 views

Measurement and change of quantum state

Suppose we have a quantum system that is in a certain initial state, together with a measuring device capable of limited precision measurement of some observable. After the measurement is complete, we ...
4
votes
9answers
1k views

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 ...
51
votes
5answers
6k views

Why aren't particles constantly “measured” by the whole universe?

Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' ...
1
vote
1answer
213 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 ...
9
votes
5answers
222 views

Conceptual understanding of operators in QM

Do operators in QM represent in some fashion the action of the measurement apparatus on a state being measured? Usually operators in QM are introduced as abstract transformations whose eigenvectors/...
3
votes
1answer
51 views

Is sole projection onto a subspace of a system associated with a quantum mechanical measurement?

I'm a little confused about a recent discussion, where a one dimensional wave-function $\Psi(x)$ was considered, associated with a state $$|\Psi\rangle = \int dx \, \Psi(x) |x\rangle .$$ Now ...
0
votes
2answers
62 views

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 ...
3
votes
1answer
84 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 ...
2
votes
3answers
108 views

How is valid qbit information retrieved when measurement spoils it?

As layman I couldn't yet conceive how one could know that a given qbit, if it is said to have certain probability amplitudes alpha and beta, actually have (fairly accurately) these amplitude values, ...
0
votes
3answers
747 views

Probability of measuring eigenvalue of non-normalised eigenstate

This came up while working on a question about measuring the angular momentum of a particle in a superposition of angular momentum eigenstates: Given that: $$\langle\theta,\phi|\psi\rangle \propto \...
3
votes
2answers
80 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 ...
2
votes
1answer
74 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| ...
4
votes
2answers
222 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 ...
3
votes
1answer
62 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 ...