# Entanglement happens at what speed?

boffinwannabe
when one entangled particle is measured and its state known the other particle of the entangled pair is described instantaneously collapsing into the same state. What exactly does this mean? does this mean zero time for the pair to take on the same state? I thought the minimum amount of time you can have is Planck time 10−43 seconds. So do we have a mathematical description for the time this takes or it truly taking zero time?

alexepascual
This question is somewhat controversial as its answer may depend on interpretation. I think those who are in favor of the Copenhagen Interpretation would say that there is no paradox because the wave function is not something that exists at all these points where we would assume it to "be" but that it only represents our "knowledge". Of course not everybody agrees with this. Going back to the instantaneity concept, two events separated in space can be considered to be instantaneous in a particular frame of reference, but if they are seen from another frame (such as a moving spaceship) they are not instantaneous anymore. I don't think there would be any problem in having two events being totally instanataneous in a particular frame of reference. But if the time difference can be made very small I don't think is relevant. If we consider "collapse" as something happening at both places where the entangled particles are located, then just having this thing (whatever it is) happening close enogh in time so that there can't be a signal traveling at the speed of light or slower connecting them then we do have a somewhat paradoxical situation and as far as I know there is no agreement on this yet as none of the proposed interpretations explain al the paradoxes in a satisfactory way.
I think you should be able to find plenty of threads in this forum where things like the EPR experiment, causality, realism, non-locality etc. are discussed.
With respec to the title of your post, I think it is somewhat confusing. Before reading the post, I thought you would ask about the speed at which entanglement is generated. Entanglement is assumed to exist from the time the particles interacted, were generated or got correlated through some other mechanism. What you are asking is about the measurement events.

boffinwannabe
but if the event is zero secs all observers would see the event the same. Since the point is you are not observing both particles at the same time, just observing one particle at which point you instantly know the state of the other. So i don't see how frame of reference changes anything.

Gold Member
but if the event is zero secs all observers would see the event the same. Since the point is you are not observing both particles at the same time, just observing one particle at which point you instantly know the state of the other. So i don't see how frame of reference changes anything.

Frame of reference has nothing to do with the speed of entanglement decoherence. There is no measurable "difference" present in the effect. Please recall that there is nothing obviously different after collapse related to one particle alone. You have to look at the entire system together.

bohm2
According to this paper, the hidden/private quantum signals that exist between entangled particles/systems cannot remain hidden if the speed of these "private lines" is anything less than infinite velocity/instantaneous.
The new hidden influence inequality shows that the get-out won't work when it comes to quantum predictions. To derive their inequality, which sets up a measurement of entanglement between four particles, the researchers considered what behaviours are possible for four particles that are connected by influences that stay hidden and that travel at some arbitrary finite speed. Mathematically (and mind-bogglingly), these constraints define an 80-dimensional object. The testable hidden influence inequality is the boundary of the shadow this 80-dimensional shape casts in 44 dimensions. The researchers showed that quantum predictions can lie outside this boundary, which means they are going against one of the assumptions. Outside the boundary, either the influences can't stay hidden, or they must have infinite speed.

Quantum nonlocality based on finite-speed causal influences leads to superluminal signaling
http://arxiv.org/pdf/1110.3795v1.pdf

Quantum non-locality based on finite-speed causal influences leads to superluminal signalling
http://www.nature.com/nphys/journal/v8/n12/full/nphys2460.html

Looking Beyond Space and Time to Cope With Quantum Theory
http://www.sciencedaily.com/releases/2012/10/121028142217.htm

One of the authors (Gisin) discusses other ramifications in a follow-up piece recently posted in arxiv:

Quantum correlations in Newtonian space and time: arbitrarily fast communication or nonlocality
http://lanl.arxiv.org/pdf/1210.7308.pdf

Perimeter lecture from the lead author:
The experimental violation of Bell inequalities using spacelike separated measurements precludes the explanation of quantum correlations through causal influences propagating at subluminal speed. Yet, it is always possible, in principle, to explain such experimental violations through models based on hidden influences propagating at a finite speed v>c, provided v is large enough. Here, we show that for any finite speed v>c, such models predict correlations that can be exploited for faster-than-light communication. This superluminal communication does not require access to any hidden physical quantities, but only the manipulation of measurement devices at the level of our present-day description of quantum experiments. Hence, assuming the impossibility of using quantum non-locality for superluminal communication, we exclude any possible explanation of quantum correlations in term of finite-speed influences.
Quantum Nonlocality Based on Finite-speed Causal Influences Leads to Superluminal Signalling
http://pirsa.org/displayFlash.php?id=11110145

Last edited by a moderator: