Questions tagged [locality]
273 questions
142
votes
11 answers
How fast does gravity propagate?
A thought experiment: Imagine the Sun is suddenly removed. We wouldn't notice a difference for 8 minutes, because that's how long light takes to get from the Sun's surface to Earth.
However, what about the Sun's gravitational effect? If gravity…
Stefano Borini
- 3,970
125
votes
10 answers
Why is quantum entanglement considered to be an active link between particles?
From everything I've read about quantum mechanics and quantum entanglement phenomena, it's not obvious to me why quantum entanglement is considered to be an active link. That is, it's stated every time that measurement of one particle affects the…
Andrey Tatarinov
- 1,431
109
votes
12 answers
Why are differential equations for fields in physics of order two?
What is the reason for the observation that across the board fields in physics are generally governed by second order (partial) differential equations?
If someone on the street would flat out ask me that question, then I'd probably mumble something…
Nikolaj-K
- 8,873
98
votes
10 answers
Quantum Entanglement - What's the big deal?
Bearing in mind I am a layman - with no background in physics - please could someone explain what the "big deal" is with quantum entanglement?
I used to think I understood it - that 2 particles, say a light-year apart spatially, could affect each…
Pete Oakey
- 1,574
45
votes
5 answers
Why are infinite order Lagrangians called 'non-local'?
And in what sense are they 'non-local'?
WIMP
- 2,685
40
votes
5 answers
Can the Hubble constant be measured locally?
The Hubble constant, which roughly gauges the extent to which space is being stretched, can be determined from astronomical measurements of galactic velocities (via redshifts) and positions (via standard candles) relative to us. Recently a value of…
burgerking
- 445
39
votes
2 answers
Definitions: 'locality' vs 'causality'
I'm having trouble unambiguously interpreting many answers here due to the fact that the terms locality and causality are sometimes used interchangeably, while other times seem to mean very different things to the author.
My current understanding…
user1247
- 7,509
34
votes
4 answers
What combinations of realism, non-locality, and contextuality are ruled out in quantum theory?
Bell's inequality theorem, along with experimental evidence, shows that we cannot have both realism and locality. While I don't fully understand it, Leggett's inequality takes this a step further and shows that we can't even have non-local realism…
John
- 2,185
32
votes
6 answers
Bell's theorem for dummies, how does it work?
I've been reading up on theoretical physics for a few years now and I feel like I am starting to get an understanding of particle physics, at least as much as you can from Wikipedia pages. One thing I have tried to understand but fails to make…
jeffpkamp
- 503
28
votes
10 answers
How do we know quantum entanglement works no matter the distance?
It is said quantum entanglement works regardless of distance. 2 particles can be entangled and information is shared instantaneously, even if they are lightyears away from each other.
But how do we know this still works with such a vast distance…
Sven Deckers
- 389
23
votes
6 answers
Quantum entanglement and spooky action at a distance
When quantum entanglement is explained in "layman's terms", it seems (to me) that the first premise, that we have to accept on faith, is that a particle doesn't have a certain property (the particle is not IN one state or another) until that…
DWalker
- 333
- 2
- 4
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
20
votes
2 answers
Locality in the scattering amplitude
Early in this talk by Nima Arkani-Hamed, he describes what locality means for scattering amplitudes. "Locality tells you that the only poles in the scattering amplitude occur when the sum of a subset of the momenta of the particles goes on…
Tim Goodman
- 2,352
20
votes
1 answer
Is string theory local?
By locality I mean something like the Atiyah-Segal axioms for Riemannian cobordisms (see e.g. http://ncatlab.org/nlab/show/FQFT). I.e. to any (spacelike) hypersurface in the target we associate a Hilbert space and to any cobordism an S-matrix.
I am…
Pavel Safronov
19
votes
3 answers
Uncertainty principle - momentum so precise that uncertainty of position is outside light-cone?
Thought experiment: what happens if we measure momentum of a particle so precisely, that the uncertainty of its position becomes absurd?
For example, what if the uncertainty of the position exceeds 1 light year? We know for a fact that the particle…
mbeckish
- 645