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Is here any progress on explaining Bell's Inequality? I do not mean explaining what it is, I mean how it works.
Elroy said:I believe that Einstein said that "nothing can travel faster than the speed of light". However, it turns out that that's not quite right. What he should have said is that "no meaningful information can travel faster than the speed of light".
Ah, now we are getting to the part that is fascinating. I agree it seems to say "there is some 'space fabric' connection." If we could derive some measurable consequences from that idea and they panned out we would win the Noble prize. ;)Elroy said:The fact that the Bell inequalities are experimentally violated shows that there is some "space fabric" connection between the entangled (EPR pair) qubits.
edpell said:small delay
Elroy said:Your (or possibly Smolin's) use of these words is potentially quite fascinating though. With respect to the collapse of superposition (even with entangled states), I personally haven't heard of anyone talk about it in terms of anything but "instantaneous".
edpell said:Is here any progress on explaining Bell's Inequality? I do not mean explaining what it is, I mean how it works.
edpell said:It is that "cannot be explained locally" part that I feel needs explaining. How does that work with or without violating the speed of light?
Elroy said:What is quite fascinating is that, as soon as Alice (making her the first to "read" her entangled qubit) "reads" hers, then the state of Bob's qubit is determined "instantaneously" (faster than light). The fact that the Bell inequalities are experimentally violated shows that there is some "space fabric" connection between the entangled (EPR pair) qubits.
Nugatory said:The problem is that if Alice's and Bob's measurements are spacelike-separated, there is no way of saying which one happened first. Some observers moving at some speeds relative to the experimental apparatus will find that Alice measured her particle before Bob measured his; others will find that Bob's measurement came first and determined the state of Alice's particle.
It doesn't work. Quantum mechanics is phenomenological theory that can't have realistic model at it extremes.edpell said:It is that "cannot be explained locally" part that I feel needs explaining. How does that work with or without violating the speed of light?
Nugatory said:It's almost impossible to resist the temptation to think that Alice's measurement determines the state of Bob's particle through some faster-than-light connection (perhaps messages carried by flying pigs, perhaps as you say "some 'space fabric'"). Nonetheless, you must resist this temptation.
The problem is that if Alice's and Bob's measurements are spacelike-separated, there is no way of saying which one happened first. Some observers moving at some speeds relative to the experimental apparatus will find that Alice measured her particle before Bob measured his; others will find that Bob's measurement came first and determined the state of Alice's particle.
Elroy said:It's generally agreed that both of the entangled qubits are still in superposition if neither has been read.
It proves that a "hidden variable" which transports information only slower than light is not sufficient as an explanation. Thus, there has to be some hidden communication faster than light.Elroy said:I can't completely outline the reasoning here, but the fact that the Bell inequalities have been experimentally violated also supports the position that there is no "hidden variable" or "hidden classical communication" between the two entangled qubits.
The "it cannot be explained locally" is the generally accepted language, but it is very misleading.edpell said:It is that "cannot be explained locally" part that I feel needs explaining. How does that work with or without violating the speed of light?
I would object to naming this "more fundamental". Instead, I would name it "less fundamental", because it has a much more direct connection with observation.atyy said:It is important to note that although quantum mechanics does not respect relativistic causal structure if it is used to explain the nonlocal correlations, quantum mechanics does respect the more fundamental relativistic constraint that no classical information is transmitted faster than light.
Ilja, good point. I have never heard this before.Ilja said:If the first explanation, with maximal speed c, is named "local", the second one, which is qualitatively of the same type, deserves to be named "local" too, because the word "local" in no way refers to the particular special choice of c as the maximal speed.
Ilja said:I would object to naming this "more fundamental". Instead, I would name it "less fundamental", because it has a much more direct connection with observation.
bhobba said:Superposition is not the best way of expressing it - entangled is much better.
Elroy said:It's interesting to think about whether you can have entanglement without superposition, or vice-versa.
According to this paper, the hidden/private quantum signals that exist between entangled particles/systems cannot remain hidden if the speed is anything less than instantaneous:Ilja said:The problem is that they could be explained with some type of information transfer much much faster than the speed of light, but nonetheless with finite maximal speed. In such an explanation everything would be similar to what is named today "local", only the maximal speed has another value, not c but say 10000000000000 c.
Quantum nonlocality based on finite-speed causal influences leads to superluminal signalingThe 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 SignallingThe 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.