Questions tagged [non-locality]
240 questions
45
votes
5 answers
Why are infinite order Lagrangians called 'non-local'?
And in what sense are they 'non-local'?
WIMP
- 2,685
26
votes
5 answers
Are fermions intrinsically non-local?
Background:
When one studies quantum mechanics of more than one particle, one learns that all fundamental particles can be classified as either bosonic or fermionic. Fermions have a spinor structure, and pick up a phase of -1 when rotated by 2$\pi$…
Rococo
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24
votes
2 answers
Bell polytopes with nontrivial symmetries
Take $N$ parties, each of which receives an input $s_i \in {1, \dots, m_i}$ and produces an output $r_i \in {1, \dots, v_i}$, possibly in a nondeterministic manner. We are interested in joint conditional probabilities of the form $p(r_1r_2\dots…
Marcin Kotowski
- 1,339
22
votes
2 answers
Locality in QFT vs "non-local" in QM
In quantum mechanics (QM), teacher always emphasizes on the "weird" parts, like EPR paradox, Bell inequality and so on. The Bell inequality tells us that QM is either nonlocal or non-realistic or both.
However in quantum field theory (QFT), teacher…
maplemaple
- 2,217
16
votes
4 answers
Why can't noncontextual ontological theories have stronger correlations than commutative theories?
EDIT: I found both answers to my question to be unsatisfactory. But I think this is because the question itself is unsatisfactory, so I reworded it in order to allow a good answer.
One take on contextuality is to develop an inequality on…
Mateus Araújo
16
votes
2 answers
How to understand locality and non-locality in Quantum Mechanics?
What actually is the definition of locality and non-locality?
Does non-locality in Quantum Mechanics mean however far you separate 2 entangled atoms in space, the 2 atoms can still influence each other? Doesn't non-locality mean action at distance?…
Lynn
- 271
14
votes
1 answer
Local versus non-local functionals
I'm new to field theory and I don't understand the difference between a "local" functional and a "non-local" functional. Explanations that I find resort to ambiguous definitions of locality and then resort to a list of examples. A common explanation…
Doug
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12
votes
1 answer
Is this field redefinition for free scalar field theory non-local?
The action of free scalar field theory is as follows:
$$
S=\int d^4 x \frac{\dot{\phi}^2}{2}-\frac{\phi(m^2-\nabla^2)\phi}{2}.
$$
I have been thinking to redefine field as $$\phi'(x)=\sqrt{m^2-\nabla^2}\phi(x),$$ where I assume that I can separate…
Howard
- 123
- 5
12
votes
2 answers
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 perturbation and a local symmetry?
João Bravo
- 794
12
votes
2 answers
The derivation of fractional equations
Recently I saw some physical problems that can be modeled by equations with fractional derivatives, and I had some doubts: is it possible to write an action that results in an equation with fractional derivatives? For example, consider a…
gsAllan
- 1,317
12
votes
1 answer
CHSH violation and entanglement of quantum states
How is the violation of the usual CHSH inequality by a quantum state related to the entanglement of that quantum state?
Say we know that exist Hermitian and unitary operators $A_{0}$, $A_{1}$, $B_{0}$ and $B_{1}$ such that
$$\mathrm{tr} ( \rho (…
Earl
10
votes
3 answers
How exactly does the proof of Bell's theorem fail if you remove the locality assumption?
In this paper Bell derives his famous inequality using the assumtions of locality and realism. In order to understand how the locality assumption affects the derivation of the inequality, and why it is needed for the equality, I have attempted to…
The Ledge
- 432
10
votes
2 answers
What's wrong with the square root version of the Klein-Gordon equation?
The Wikipedia article has a derivation of the Klein-Gordon equation. It gets to this step:
$$\sqrt{\textbf{p}^2 c^2 + m^2 c^4} = E$$
and inserts the QM operators to get
$$\left( \sqrt{ (-i \hbar \nabla)^2 c^2 + m^2 c^4 } \right) \psi =…
Nick
- 3,039
9
votes
1 answer
What is meant by a local Lagrangian density?
What is meant by a local Lagrangian density?
How will a non-local Lagrangian look like?
What is the problem that we do not consider such Lagrangian densities?
SRS
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9
votes
11 answers
What does it mean when folks say that universe is not "Locally real"?
I've read somewhat about the matter but can't quite picture it. Is this a property that only applies at the quantum level and not the classical level like us?
So far I've seen some rather strange takes on what it means. Some suggesting the moon…
user464384