Questions tagged [non-locality]

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A Venn diagram for: (non-)locality, (non-)realism, (non-)contextuality, (non-)signalling, (dis-)entanglement

I recently asked a (yet-unanswered) question about the relationship between state-dependence and violations of realism. The more I read on the subject, the more I find myself digging deeper in a ...
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Can disentangled states violate local realism?

I (re-)learned about Bell tests from the ground up based on this very intuitive lecture by Spekkens. In a nutshell, realism is violated whenever measurement statistics cannot be reproduced by ...
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Question on hidden variable theory of QM [closed]

my understanding from careful study of Bells thm is that there may be a deep subtlety that is not ruled out, but is probably very minimally studied in the literature. however the literature is vast, ...
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1answer
95 views

Is Bell nonlocality compatible with, or even predicted by, quantum field theory?

Bell's theorem, together with experiments confirming the ordinary quantum mechanical distant correlation predictions which violate its conclusion for local theories, show that any entirely local ...
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1answer
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Are following statements valid for quantum gravity?

In the concluding section of this post user Chiral Anomaly states following: On the other hand, since any stable marriage of quantum theory and gravity (in the sense of general relativity, not just ...
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1answer
51 views

Non-analytic functions and non-local Lagrangians

Infinite sums of increasingly higher-order derivatives, when present in Lagrangians, are typically taken as a sign of nonlocality. This is supposed to rule out fractional, negative and exotic (for ...
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What does it mean for an extended operator to possess “local excitations”?

In the context of defect conformal field theory, we consider in operator product expansions "local excitations" of the defect (see e.g. text between eq. $(1.1)$ and $(1.2)$ in the paper ...
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What is an “optimal ensemble” used to demonstrate quantum steering?

I am reading Wiseman et al. (2007) and Jones et al. (2007) papers on quantum steering. They relied on the existence of "optimal ensembles" for a Local Hidden State theory. They make use of ...
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Locality & Effective field theories

I know that in general locality is a much desired/needed feature of a field theory, e.g. Standard Model. What I would like to know is which principle forbids non-local terms in a Lagrangian density. ...
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1answer
119 views

Blocking quantum effects

A Faraday cage blocks electromagnetic fields, provided they are not intense enough to change the state to something non-conductive (e.g. slicing it in two with a laser). Is there any analogous system ...
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How to deal with integral operators in the action, in the path integral of a field theory?

One could imagine adding to the free action of a scalar field theory some non-local operators given as integrals over the base manifold (or over the boundary) of some smooth function of the scalar ...
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2answers
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Why quantum mechanics doesn’t break locality in entanglement but hidden variables theories will?

What makes it so that quantum mechanics doesn’t break locality in entanglement yet hidden variable theories will? In Bell‘s inequality said that hidden variables theories need to break locality in ...
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Light in action-at-a-distance formulation of Electromagnetism

If it is possible to use Jefimenko's equations to formulate Electromagnetism as a non-instantaneous action-at-a-distance theory, what would electromagnetic radiation be in such a theory? Would it be a ...
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Concrete Example of Quantum Entanglement, Quantum Steering and Bell Nonlocality

I was reading the wikipedia page for quantum steering. The following is the mathematical way they have described the three notions: Is there any concrete example of these three concepts? Like, any ...
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What is locality?

In QFT and statistical mechanics, one is usually interested in studying integrals of the form: $$Z(\phi) =\int d\mu_{C}(\phi')e^{-V(\phi+\phi')}$$ where $\mu_{C}$ is Gaussian measure with mean zero ...
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1answer
94 views

Functional Analytic Square Root of Hamiltonian Alternative to Dirac

I was thinking about the history of the Dirac equation and asked myself, what happens if one simply considers the Schrödinger equation $i\hbar\frac{\partial\phi}{\partial t}=\sqrt{-c^2\hbar^2\Delta+m^...
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1answer
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Argyres-Douglas CFT

Adding of mass in supersymmetric gauge theories will affect structure of moduli space by creating new singular point (picture and some statements from Matteo Bertolini: Lectures on Supersymmetry): ...
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Is there a deep reason why action comes from a local lagrangian?

In both classical and quantum physics Lagrangians play a very important role. In classical physics, paths that extremize the action $S$ are the solutions of the Euler-Lagrange equations, and the ...
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1answer
100 views

Particle picture in position space in quantum field theory

When I operate $a^{\dagger}_k$ on vacuum, $|0\rangle$, I get a particle created in momentum space with a 4-momentum equal to $(\omega_k, \vec{k})$ where $\omega_k=\sqrt{m^2+\vec{k}^2}$ here I'm only ...
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1answer
52 views

Local and non-local interactions

When I look for the van der Waals interactions, they are defined as they are non-local interactions but no explanation for what they mean by non-locality. What would be the best way to understand this ...
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What is the difference between non-local optical response and local optical response?

Does anyone knows what is the difference between non-local optical response and local optical response in metal-dielectric-metal?
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2answers
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Does Quantum Entanglement have a preferred reference frame?

What I mean by this is, with wave function collapse,--and by extension, collapse between two entangled particles--being nonlocal (instantaneous across space), in what reference frame does the ...
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1answer
130 views

What is the Hilbert space of the SYK model?

In reading a recent preprint [1] contrasting bosonic models with local (tensor product) Hilbert spaces with SYK-like models of fermions, I realized I was confused about something. While I have a vague ...
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375 views

Non-local potential

I read that the Schrodinger equation for a non-local potential is given by $$-\frac{\hbar^2}{2m}\nabla^2\psi(x)+\int V(x,x')\psi(x')dx'=E\psi(x).$$ In case of a local potential, $$ V(x,x')=V(x)\delta(...
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Are there any nonlocal one-body operators?

In case I have a one-body operator given by $$ \hat{O}=\int d^3r\int d^3r' \,\hat\psi^\dagger(\mathbf{r})\langle \mathbf{r}|\hat O|\mathbf{r}'\rangle\hat\psi(\mathbf{r}'), $$ are there any operators ...
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What is recessional nonlocality?

In laymen’s terms, what is recessional nonlocality? I understand recessional means to recede or retract away, and (correct me if I’m wrong) I understand locality has something to do with quantum ...
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1answer
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Why does implementing the annihilation operator $\hat a$ not allow action at a distance?

I was reading a paper (link) in which they implement action at a distance using the annihilation operator, and in it they say that "this observation does not imply superluminal signaling because ...
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Don't these experiments suggest that locality has to be abandoned in the quantum realm?

First a quick recap because maybe my understanding/assumptions are flawed (you can jump to the question highlighted in bold below if you like to): Bell's inequalities require that an accurate(ly ...
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1answer
72 views

Energy and canonical momentum conservation in non-local classical field theory

Assume we have the following Lagrangian field density where $x, x'$ both three dimensional real vectors are coordinates and $t$ represents time, field is given by $\phi$. Assume for the sake of ...
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1answer
66 views

Replication of Single Particle QM with Local Hidden Variables

In learning device-independent QM from [https://arxiv.org/abs/1303.3081], Scarani tells us that all quantum statistics involving one particle can be reproduced with local (hidden) variables. He then ...
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2answers
741 views

What is a local operator in quantum mechanics?

In quantum mechanics, what exactly is meant by "local" operator? What about a "global" or a "non-local" operator? Are these the same? Can you also also help me understand what exactly is a local ...
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1answer
104 views

Quantum Mechanics - How do we know that the observed locations of electrons are random? [duplicate]

How do we know that the observed location of a electron (or any quantum object) is purely random (there is no way to predict it) within the probability-function instead of normal randomness (we don't ...
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Do any quantum interpretations not include nonlocality?

Are there any interpretations of quantum theory that have a mechanism such that there is no need to invoke nonlocality?
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Is nonlocality more fundamental to quantum theory than superpositions? [closed]

What is more fundamental to quantum mechanics: nonlocality or superpositions?
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How is Mermin's magic square computed?

BACKGROUND The Peres-Mermin magic square consists of different combinations of tensor products of Pauli operators being applied on an arbitrary bipartite state. Suppose we're considering the entry in ...
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1answer
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Is there a systematic method to derive Bell's inequalities?

BACKGROUND The purpose of Bell's inequalities is to put bounds on certain combinations of classical measurement probabilities. One can then show that quantum entanglement can yield probabilities that ...
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Does reality express non-local realism, local non-realism or non-local non-realism? [closed]

So I read that Bell's tests prove that reality cannot follow the requirements of both realism and locality. My understanding is that: Realism: Objects have definite properties before they are ...
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1answer
113 views

How are contextuality and locality related?

I know how quantum non-locality is defined in Wikipedia quantum nonlocality is a characteristic of some measurements made at a microscopic level that contradict the assumptions of local realism ...
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1answer
208 views

Can a single photon behave in a non-local way?

Most discussions of quantum non-locality center on entangled pairs of particles which are described by a single wave function and thus, in some way, are to be regarded as one quantum object such that ...
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1answer
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Fractional derivatives in a QFT Lagrangian

There are is at least one question asking about fractional powers of fields in QFT (and why they're not expected to occur), and several others asking about the physical relevance of fractional ...
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1answer
186 views

Does Bell's theorem imply nonlocality using a false assumption? [closed]

In https://arxiv.org/abs/1409.5158, the author concludes that Bell tests cannot refute local realism, because they employ a wrong analysis. He says: "The quantum joint prediction cannot be ...
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335 views

Non-local field redefinition and effects on path-integral measure

Consider the partition function $$ Z[0] = \int \left[\mathcal{D}A_\mu\right]\left[\mathcal{D}\pi\right] e^{-i \int d^4x \left(-\frac{1}{2}(\partial\pi)^2-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}+ \frac{a}{M^2}...
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Non-locality of the Madelung equations?

As far as I understand, the Madelung equations $$ \frac{\partial \rho}{\partial t} + \nabla \cdot \rho \mathbf{u} = 0 \\ \frac{\partial \mathbf{u}}{\partial t} + \mathbf{u} \cdot \nabla \mathbf{u} = -\...
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Does a pilot-wave theory need to be stochastic in a discrete space-time?

I have found an article about Bohmian mechanics on a lattice with discrete space and time, the link of which is given below: https://arxiv.org/abs/1606.02883 Here the motion of quantum particles is ...
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Have fractional order differential models been explored as an alternative to standard gravitational field theory?

Since Einstein introduced his field equations and general theory of relativity, experimental evidence, at least on the cosmic scale has repeatedly supported the theory. Nevertheless, many seeking to ...
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280 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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1answer
125 views

The non-locality of Quantum Mechanics

I often heard physicists mentioning the EPR paradox or the quantum entanglement when talking about the non-locality of quantum mechanics, but wasn't quantum mechanics already non-local from the ...
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How exactly does the proof of Bell's theorem fail if you remove the locality assumption?

In this paper Bell derives his famous inequality using the assumtions of locality and realism. In order to understand how the locality assumption affects the derivation of the inequality, and why it ...
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1answer
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Recharacterizing spooky action at a distance as a time loop invariant?

Caveat: I'm not a physicist and am a little fuzzy on what exactly what I'm asking here. But I'm hoping someone smarter than me can at least clarify (or dismiss) the question as stupid... :) This ...
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1answer
110 views

Is there a relation between the factorisation of the joint conditional probability distribution and Bell inequality?

I'm approaching the study of Bell's inequalities and I understood the reasoning under the Bell theorem (pdf : https://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf) and how the postulate of ...