Questions tagged [observables]

A quantum observable is a measurable operator whose corresponding property of the state can be determined by some sequence of physical operations ("observation"), such as submitting the system to various electromagnetic fields and eventually reading a value. In systems governed by classical mechanics, any experimentally observable value can be shown to be given by a real-valued function on the set of all possible system states.

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350 views

Is it possible to directly measure the expectation value of a physical observable without measuring the entire probability distribution?

The expectation value of a physical observable in a given state is just a single real scalar quantity. On the other hand, the probability distribution of the eigenvalues of the observable is a set of $...
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Is direct observation of strong and weak force ruled out by quantum field theory?

In quantum field theory electromagnetic radiation is described by a theory with an abelian gauge symmetry while the weak and strong force are described by theories with non abelian gauge symmetry. We ...
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Hermitian operator followed by another hermitian operator – is it also hermitian?

Consider the two hermitian operators $\hat A$ and $\hat H$: I can prove that the operator $[\hat A,\hat H]$ is non-hermitian as follows: $$\begin{align} \int\phi^*[\hat A,\hat H]\psi\,dx&=\int\...
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Are the components of 4-vectors the physically measured quantities?

I am very confused with the difference between components of four-acceleration and coordinate acceleration. If I was in an inertial frame observing an accelerated object I would say its four-...
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Not all self-adjoint operators are observables?

The WP article on the density matrix has this remark: It is now generally accepted that the description of quantum mechanics in which all self-adjoint operators represent observables is untenable.[...
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What are the measurable quantities in General Relativity?

What are the quantities in GR that one can actually measure in an experiment? It seems scalars, being coordinate independent, should be measurable. What about other quantities? For example, using only ...
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What is the Hamiltonian of this number theoretic system?

Background Let us have the following orthonormal basis such that: $$ \langle m | n \rangle = \delta_{mn}$$ Consider the following operators defined as: $$ \hat 1 = | 1 \rangle \langle 1 | + | 2 \...
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Is the generalized uncertainty principle dependent on the state of the particle?

The generalized uncertainty principle can be written as (where A and B are observables): $$ \sigma_A\sigma_B \geq \left| \frac{1}{2i}\langle [A,B]\rangle_\Psi \right| $$ But the average value of the ...
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QM eigenstate expansion

Why sometime we use the integral to expand the eigenstates and sometime we use the sum to expand? now i am read the modern quantum mechanics J.J.Sakurai text and confusing
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Experimentally distinguishing between topologically inequivalent physical states in gauge theory

In gauge theory, physical states are often said to be characterized by equivalence classes of gauge field configurations that differ by gauge transformations. But according to Large and small gauge ...
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What are some examples of classical observables that change with observation?

I was reading H. Moysés Nussenzveig "A course in basic physics, Volume IV" and in chapter 8, he is introducing the basic ideas of quantum mechanics, where he states that: In quantum mechanics, one ...
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What do we mean when we say “position of particle”?

a particle is in a superposition until it is observed. if observed the system collapses and chooses one position or something like that. I heard somewhere that if the particle is again observed it ...
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Eigenvalues, Hermitian operators and observables in quantum mechanics

Consider a hermitian operator. So a) in a space of infinite dimension its eigenvectors are a base. b) in a finite-dimensional space the matrix that represents the hermitian operator is always ...
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Check if operator $A$ is an observable

Given operator $A$ and following relationships: $A|2\rangle=|1\rangle+|3\rangle$ and $A|1\rangle=A|3\rangle=|2\rangle$. I know that this operator should be self adjoint to correspond to an ...
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Replacing operators in quantum mechanics

In general when we study the expectation value of any variable, which is a function of position and momentum in the way$$Q(x,p)$$ We generally do the following thing,i.e. $$\int_{x=0}^\infty \psi^*Q(x,...
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Schrodinger equation peculiar solution

This problem is generated from a problem given in Griffiths "Quantum Mechanics". The question is as follows: The initial wavefunction of a particle in an infinite square well is given by $$\phi(x,0)=...
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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 ...
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What's the deal with momentum in the infinite square well?

Every now and then a question comes up about the status of the momentum operator in the infinite square well, and while we have two good answers on the topic here and here, I'm generally not satisfied ...
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1answer
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Momentum of wave function with sum of cosines

I am struggling with question about possible outcomes of momentum measurement and their probability. I know I can calculate it with momentum operator, but a wavefunction is of form $$\psi (x)=3\cos\...
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2answers
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Measuring the polarisation of light

I've been thinking about the standard and easy "Three-polarising-filters experiment" as a demonstration of quantum phenomenon: If light is passed through two polarising filter and the filters have ...
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Finding the standard deviation of observable [closed]

I'm currently reading Quantum Computation and Quantum Information by Nielsen. I'm struggling to solve exercise 2.58. The problem is Suppose we prepare a quantum system in an eigenstate $|\psi\...
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Is energy $E$ in Schrödinger equation an observable/ Can $E$ be measured?

Take this quantum approach to estimate mean energy of a molecule: $$\langle\psi|H|\psi\rangle=\overline E$$ Question: Is $E$ an observable? How we can compare it to an experimental value? i.e how ...
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677 views

Unitary Transformations in Quantum Mechanics - Changes Meaning of Operators?

In my quantum-optics lecture, my Professor wanted to derive some quantities regarding the time evolution of an electromagnetic field, coupled to an electron. He started with the usual Hamiltonian in ...
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Observable Vs Projector in case of Neutrinos

For a spin $s$ system, we can define the observable $A=S_z$, such that $A(t) = S_z(t) = U^\dagger S_z U$, where $U$ is the unitary operator $e^{-iHt}$. We can always define a projector $\Pi_m = |s,m \...
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Is every observable a function of position and momentum?

In the first answer of this question it is said that every quantum observable, let's say $\hat{A}$, can be represented as a function of position and momentum observables. In other words, as I ...
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Questions about the formalism around QFT

There are some things I don't get in QFT. I will take the scalar free field : $$\phi(x)=\int d\tilde k (a(\vec k) e^{-ikx}+a^\dagger(\vec k) e^{ikx})$$ My first question is: In QFT, do we consider ...
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Born rule and expectation value integral for $L = x \times p$

Physical interpretation of the wave function $\psi (x,t)$ is a probability amplitude for location $x$ and the Fourier transform of $\psi (x,t)$ can be interpreted as a probability amplitude for ...
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Spectrum of an operator

Why does the mathematical definition of a spectrum of an operator namely the set of complex numbers without the resolvent set, agreed with the real physical spectrum of an observable? is this a ...
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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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Why does the time-evolution operator $U(t)$ depend explicitly on time in the Schrodinger picture?

Schrodinger's picture is that operators are time-independent. But time evolution operator $U(t)$ is time-dependent. Why is that?
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Uncertainty Principle: Commutators [duplicate]

How are commutators the mathematical basis for uncertainty principle? What makes one say that commutators imply uncertainty principle or vice-versa?
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Commutativity vs Compatibility

As far as I know, two compatible observables have a complete set of common eigenvectors, and using this fact, one can prove that their corresponding operators are commutative. Well now is the converse ...
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In what sense (if any) is Action a physical observable?

Is there any sense in which we can consider Action a physical observable? What would experiments measuring it even look like? I am interested in answers both in classical and quantum mechanics. I ...
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What happens when one "observes' a quantum field, and how do particles get involved?

I've recently begun my journey to understand QFT. I apologize in advance for the length of the post, but there are gaps in my understanding of how I, as an experiementalist, interact with fields to ...
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Can particles be in positional eigenstate in reality?

Do i understand quantum right in the following description? what we observed as particle is just the phenomenon they have some kind of quality corresponding to the macroscopic stones in experiments ...
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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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1answer
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understanding resolving power in QM

Consider the definition of resolving power of two states in quantum mechanics as being the absolute value of the difference between the probabilities of two states following the measurement averaged ...
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Paradox about the Stern-Gerlach experiment with $B=0$, $\nabla B\ne 0$

In a modern interpretation of the historical Stern-Gerlach experiment, a beam of neutral silver atoms, each with spin 1/2, was sent in the z direction through a nonuniform magnetic field having both a ...
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1answer
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Common eigenkets of spherically symmetric Hamiltonian

In a QM text it states: "Consider a spinless particle subjected to a spherical symmetrical potential. The wave equation is known to be separable coordinates, and the energy eigenfunctions can be ...
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Kinetic Energy operator in Quantum Mechanics

Firstly, I'd like to point out clearly that I'm not a physicist but I'm a nano engineer studying quantum mechanics so that I understand my work on surface sciences better, so please don't presume my ...
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1answer
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Is it obvious that the Hamiltonian observable in Quantum Mechanics should also be the Energy observable?

In Quantum Mechanics, the Hamiltonian observable is defined as the generator of time translations. It's easy to show that if we take this to be the definition of the Hamiltonian, then it is of the ...
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Are eigenvectors always orthogonal each other?

When an observable/selfadjoint operator $\hat{A}$ has only discrete eigenvalues, the eigenvectors are orthogonal each other. Similarly, when an observable $\hat{A}$ has only continuous eigenvalues, ...
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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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Diffeomorphism invariance and correlation functions

Consider the following paragraph taken from page 15 of Thomas Hartman's lecture notes on Quantum Gravity: In gravity, local diffeomorphisms are gauge symmetries. They are redundancies. This means ...
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1answer
280 views

Gauge transformations at infinity

Consider the following paragraph taken from page 15 of Thomas Hartman's lecture notes on Quantum Gravity: In an ordinary quantum field theory without gravity, in flat spacetime, there two types of ...
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1answer
112 views

Analogy expectation of an observable / random variable

I'm trying to figure out the analogies between the expectation of a random variable $X$ and the expectation of an observable of a quantum mechanical system $A$ (using this wikipedia article). The ...
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1answer
660 views

Significance of imaginary momentum in the finite potential well

In the case of the finite potential well, when the particle enters the classically forbidden region its wavenumber, $k$ is imaginary. However $k=\frac{2\pi}{\lambda}=\frac{p}{\hbar}$ so we get an ...
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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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QM: Find expectation value of measurements

We have an observable $$O \mapsto \begin{pmatrix} 0 & 2 \\ 2 & 4 \end{pmatrix}$$ We find the eigenvalues and eigenvectors by O$\psi$ = o$\psi$. The eigenvalues will give us the possible ...
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1answer
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Lie Algebra of Classical Observables under Poisson Bracket

I am confused with understanding the fundaments of classical mechanics. All classical observables commute since they are represented by regular functions on phase space. All classical observables form ...