James Rose:
Non-locality has been presented in all discussions and equations as the unimpaired transmission of information... at faster than light speed...
Keith Sutherland:
I'm not a physicist, but I have an uneasy feeling that this is, in the words of Pat Churchland, a "pile of horse manure". I remember hearing Penrose point out that the EPR is a correlation effect and nothing to do with information transfer...
I think the truth about nonlocality is slightly more peculiar than either of these summaries, though it includes important aspects of each of them. Perhaps it is true that what EPR noticed when contemplating multiparticle quantum systems was a "correlation effect" to use Keith's term - but the whole point of Bell's 1964 demonstration is that it cannot be an ordinary correlation.
Ordinary correlations are worked out within space and time; if the number written down in my pocket book next to my friend's name corresponds to his telephone number then it is because i wrote it down when i visited him, or else he told me when he called once (in which case the information passed by electrical signal and the vibration of air). If there is a correlation between the facial structures or eye colors of two twin sisters then it is because they were born nearby each other, and at that time each received a nearly identical genetic template which she has carried since.
John Bell brought up the example of his colleague Bertlmann's correlated socks.
Dr. Bertlmann likes to wear two socks of different colours. What colour he will have on a given foot on a given day is quite unpredictable. But when you see that the first sock is pink you can already be sure that the second sock will not be pink...
Knowledge of one sock along with the correlation function allows us to gain information about the second sock. Of course, Bertlmann dresses with his feet near each other so once again there is no mystery to this correlation (it is mediated by causal links).
What about correlated multi-particle systems? If they are correlated then Einstein, Podolsky and Rosen assumed they must be correlated in what i've called the ordinary way. For example, the particles might carry "hidden" information much like the genetic templates of identical twins. That way, when we interview the particles later, naturally they would agree.
However, this is not what is going on in quantum systems! Bell skillfully showed that the ordinary kinds of explanation are in this case just plain wrong. Simple quantum systems are in fact more correlated than Bertlmann's socks (or any other system correlated in the "ordinary" way) can ever be.
...these additional variables were to restore to the theory causality and locality. In this note the idea will be formulated mathematically and shown to be incompatible with the statistical predictions of quantum mechanics. It is the requirement of locality, or more precisely that the result of a measurement of one system be unaffected by operations on a distant system, that creates the essential difficulty. (Bell 1964)
Quantum systems really are nonlocal in the sense that instead of 'working it out ahead of time,' they 'make it up as they go along,' and then when push comes to shove (often known as a "measurement") their component parts suddenly improvise a consistent, correlated configuration, even if they are by then widely separated. (Wavefunctions "collapse" at a distance as surely as they do locally.) And for our purposes the important point is that the correlation really is very strong - we can apply a wide variety of experiments to the system, sort of like approaching the correlation from various directions. At the same time the state is nonlocalized. Of course, such an intricate physical state is a uniquely quantum mechanical phenomenon.
Before the measurement there is no particle pair; there is only a gigantic atom. This atom pervades all space. The experiment dematerializes the atom, and in its place two particles appear. Each materializes, as it must in the universe, so as to preserve the laws of nature. Marvin Chester, A Primer of Quantum Mechanics
Imagine if you suddenly had to jump out of bed and put on color matched socks while one foot was in the milky way and the other several galaxies away! What would poor Dr. Bertlmann have done in a situation like that?
Anyway, the point is that the connections in an entangled quantum system are not ordinary correlations and this is precisely what Bell showed. An entangled quantum system is actually both more correlated and nonlocally correlated than Bertlmann's socks or any other purely classical system.
In a famous paper on Bell's theorem in 1977 Henry Stapp put it this way:
The central mystery of quantum theory is 'how does information get around so quick?'. How does the particle know that there are two slits? How does the information about what is happening everywhere get collected to determine what is likely to happen here?
Given the prima facie evidence of quantum phenomena, Stapp concluded that the superluminal transfer of information is "a priori, not unreasonable," and in fact apparently necessary.
Everything we know about Nature is in accord with the idea that the fundamental process of nature lies outside space-time...
Does this get us back to "the unimpaired transmission of information at faster than light speed" that Keith was uneasy about? No, i agree with Keith. Talk of superluminal signaling is generally off track. For one thing, a signal surely has a vectorial direction to it, whereas that is something the quantum connection lacks completely: for even if you did have magic socks which always emerged properly matched while your feet were in separate galaxies, still who could say whether a signal passed from left foot to right or (on the other hand) vice versa? All we can say is that by the time the measurement dust settles the whole quantum system is consistent.
Roger Penrose recently summed up his own view by writing that "quantum entanglement is a mysterious thing that lies somewhere between direct communication and complete separation." (Shadows, p. 274)
Where does this leave us from a practical standpoint?
Jean Burns described the general EPR-Bell situation particularly clearly...
"Nonlocal correlations" refer to correlations between the results of measurements at two different places. The result of a measurement, e.g., spin component in some specified direction, at location A determines the result of the same measurement (i.e., spin component with the same orientation) at location B, and influences the probability of measurements at other orientations at location B. This influence is instantaneous -- as soon as the person at A makes the measurement, all the correlations are determined. But it is not possible for the person at B to discover what measurement A made from looking at his own data, even though he can verify the quantum correlations after A has sent him (via email say) a copy of that data.
Even so, i think we should be very cautious in agreeing to Jean Burns' conclusion:
...I want to make it specific that there is no information transfer either.
Granting for the moment that signaling is not in the cards, in my view it is a mistake to conclude that the information in the quantum state is irrelevant. Of course, the opposite is true. We know very well quantum systems do evolve in completely nonclassical ways. Superfluid and super- conducting states undergo macroscopic quantum transitions and behave in ways that are classically impossible - how are such miracles possible if there is no quantum mechanical "information transfer"? In fact, though no signals may pass from any point to another, nevertheless a coherent quantum system will physically evolve in nonlocal and highly correlated ways.
The question that's really been waiting in the wings throughout is what about an object like the brain, or perhaps even other structures which somehow serve as substrates of consciousness? Is any nonlocality in the brain significant to the way it is synchronized or memories are stored in it? When we think about superluminal information, are we touching on consciousness?
The answer I am taking pains to offer forward is that we might be, and that in any case the usual arguments against superluminal signaling do not have any force here at all - quantum systems are indeed internally constituted via something closely akin to information which is outside of spacetime. They do this (perhaps we do this) not by signaling from any one place to another but simply by being and physically evolving in accord with quantum mechanics.
Rhett Savage
Bell, J.S. (1964) "On the Einstein Podolsky Rosen paradox" Physics, 1, 195-200
Bell, J.S. (1981) "Bertlmann's socks and the nature of reality" Journal de Physique, 3, 42, C2 41-61
Penrose, R. (1994) Shadows of the Mind, Oxford
Stapp, H.P (1977) "Are Superluminal Connections Necessary?" Nuovo Cimento 40B, 1, 191-204
Sometimes it is proposed that information transfer of psi takes place via nonlocal correlations. Because of the above, I argued for years that this cannot be the mechanism by which psi works. However, I've had the reply that if consciousness is acting in a given situation, the limitations which apply to a strictly physical situation may not hold. And, if we accept the laboratory findings that show psi exists, I think the point of view that its explanation will be similar to present physics, but with some differences due to the action of consciousness, is reasonable. This doesn't mean that nonlocality *is* the means by which psi works, but it's an idea worth exploring.
Jean Burns jeanbur@netcom.com
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