Entaglement, locality, and information

In summary, according to the two notions of causality, QM is nonlocal under the realist notion, but local under the operational notion.
  • #1
91
15
As far as I understand, given that there is such a thing called entanglement, we know that there are non-local interactions. The line that this is usually followed with is "since no information traveled faster than light , locality isn't violated".

I have a few interrelated questions about this. As far as I know in GR, we demand that all real particles travel through spacetime with tangent vectors which are non-positive. No mention of information, it's a constraint on allowed tangent vectors of particles.

We have another assumed condition on "causality" which takes explicitly that two regions in spacetime which can be connected by timeliness or null curves are said to be in causal contact with each other. This also makes no reference to information, but does seem to have a notion that causal interactions do not propagate faster than light. What distinguishes causal interactions from non causal ones?

A final question, what is information, and why would it be the measure of what can and cannot travel faster than light?
 
Physics news on Phys.org
  • #2
There are two notions of causality, one not relying on information and the other one relying on information.

For the one that does not rely on information (the sense in which classical relativity is a theory of causality), quantum entanglement breaks it - ie. reality is nonlocal or weirder.

For the one that does rely on information, quantum entanglement does not break it. Here the notion of information is simple. Two observers widely separated make measurements. Can I, making measurements on my side, figure out what he is doing? It turns out that in quantum mechanics, I cannot. Since none of his actions make any difference to the outcomes of my measurements, he is unable to transmit any information to me.
 
  • #3
I'm sorry, I really don't see how that clarifies anything
 
  • #4
hideelo said:
all real particles travel through spacetime with tangent vectors which are non-positive.

That's using a Lorentz signature like ( -, +, +, +) of course
hideelo said:
causal interactions do not propagate faster than light. What distinguishes causal interactions from non causal ones?

Non-causal would be the ones that do propagate faster than light, i.e. non-local. The only one I know of is entanglement. Many physicists would argue it doesn't really qualify, but perhaps we can treat that as a nit for this question.
hideelo said:
A final question, what is information, and why would it be the measure of what can and cannot travel faster than light?

That's clarified by atyy's answer
 
  • #6
hideelo said:
I'm sorry, I really don't see how that clarifies anything

As you said in your original post, there are two notions of causality
(1) one notion involves "real" causality and does not involve any notion of information - using this notion, QM is nonlocal
(2) a different notion is usually said to involve information - a better term may be that it is an "operational" notion of causality - using this notion, QM is local

A good paper reviewing the different notions of causality is http://arxiv.org/abs/1503.06413. In section 5.1 they explain Bell's theorem in terms of the realist notion of causality, and in 5.2 they explain it in terms of the operational ("information") notion of causality.

There is some debate over the historical interpretation of Bell's work in that paper, but the physics itself is not controversial.
 

1. What is entanglement and how does it work?

Entanglement is a phenomenon in quantum mechanics where two or more particles become connected in such a way that the state of one particle is dependent on the state of the other, regardless of the distance between them. This means that the particles are intrinsically linked and any change to one will affect the other simultaneously, even if they are separated by great distances.

2. How does entanglement relate to locality?

Entanglement challenges the principle of locality, which states that physical interactions cannot occur instantaneously over large distances. As entanglement allows for instantaneous communication between particles, it suggests that the principle of locality may not hold true at the quantum level.

3. Can entanglement be used for faster-than-light communication?

No, entanglement does not allow for faster-than-light communication. While changes to one particle in an entangled pair can be observed instantaneously, it is impossible to control or influence these changes to send a message. This is due to the fact that the state of the particles cannot be predetermined and the changes occur randomly.

4. How does entanglement relate to the transfer of information?

Entanglement is often referred to as the transfer of information, but this is a bit of a misnomer. While entangled particles can exhibit correlated behavior, they do not actually transfer any information from one to the other. This is because the changes that occur in one particle are random and cannot be controlled or influenced by an observer.

5. What are the potential applications of entanglement and quantum information?

Entanglement and quantum information have many potential applications, including quantum computing, quantum cryptography, and quantum teleportation. These technologies have the potential to greatly enhance computing power, security, and communication systems. However, further research and development are needed before these applications can become a reality.

Similar threads

Replies
16
Views
994
Replies
2
Views
891
  • Quantum Physics
3
Replies
87
Views
4K
  • Quantum Physics
2
Replies
48
Views
5K
  • Quantum Physics
Replies
11
Views
2K
  • Quantum Physics
Replies
7
Views
741
  • Quantum Physics
Replies
7
Views
941
Replies
7
Views
675
Replies
276
Views
7K
  • Special and General Relativity
Replies
21
Views
2K
Back
Top