Science Tribune -Letter - March 1998
The E.P.R. paradox
Newcastle University, UK
E-mail : Max.Hammerton@newcastle.ac.uk
The letters E, P and R stand for Einstein, Podolsky and Rosen, three physicists who published, in 1935, a paper (1) which has been more widely misconstrued than any six others I have ever heard of.
When I was a student - long ago - and you asked about it, you would be told that it expounded a supposed weakness in Quantum theory, but that Bohr (2) had shown that it was misconceived. Nowadays, you will almost certainly hear that Einstein & co thought that they had found an hiatus in Quantum theory, but that Bell's theorem (3) (4) showed that Einstein - somehow the two others get forgotten - had it wrong.
Further questioning will usually elicit the fact that your informant has never read the E.P.R. paper. In the whole of my life, I have only met 3 working physicists who have, and I have no reason to suppose that my sample is unrepresentative. It is, therefore, not wholly surprising that what you have been told is misleading. It is, nevertheless, very widely held. When Bell died lamentably young, the semi-popular journal New Scientist referred to him as 'the man who proved Einstein was wrong' whereas, in an important way, the case is exactly the reverse. Well, a man - even a journalist - is not, as Sam Johnson reminded us, on oath in a lapidary inscription (a).
So what is all the fuss about ?
First, let me explain a technical (NOT a jargon) term which must be used. This term is 'correlated'. In quantum theory, two particles are said to be correlated when their states are interdependent and, roughly speaking, mirror images of one another. That is, if one is in some state or undergoes some change, the other is in an opposite state or undergoes a reverse change. So far so good; but what did E., P. and R. have to say about this ?
Stripped of a lot of highly technical matter, their argument ran as follows:
1. Quantum theory requires that, under certain special circumstances, particles can remain correlated however far apart they are, i.e., if one undergoes a change, the other instantaneously does so.
2. This clearly violates speed-of-light separability, i.e., the principle that no effect can travel faster than light.
3. Therefore, there is something wrong with the theory.
Now, what did Bell and his followers demonstrate ?
In fact, it was that Quantum theory really must predict precisely what E.P.R. said it did. In technical terms, it is necessarily non-local (b). In other words, part 1 of the argument is dead right.
Since Bell's paper (3), there have been two more developments :
First, and following on from Bell's work, it has been shown that it is part 2 of the argument which is not wholly watertight. The correlation may exist; but, if it does, the phenomenon could not be used to transmit information. As I heard one theorist put it:"you can't have a Bell telephone".
Secondly, and most excitingly, experimental studies seem to have confirmed that the phenomenon really does happen (c). So Einstein & co were correct in their deductions from the theory but, to paraphrase a well-known remark of the distinguished British biologist and political nitwit J.B.S. Haldane, the Universe is even stranger than Einstein imagined it to be.
What of Bohr's original reply ?
Although I tremble to say it of one of the most distinguished - and, by all accounts, one of the nicest - physicists of the century, it really fails to deal with argument (2) at all. I suspect that this is what Einstein was hinting at, ever so politely, when he said that he failed to follow what Bohr was driving at.
A case of misreporting ?
So why do so many people get it wrong ? Well, I strongly believe that almost all working physicists, when they are not being consciously 'philosophical', adhere to Richard Feynman's (d) implied view that we know damn-all about what may be happening at the quantum level; but the equations work, so let's stick to them until experiment throws up something which compels a rethink.
In other words, forget.
1. Einstein A, Podolsky B, Rosen N, Can quantum mechanical description of physical reality be considered complete ? Phys Rev 47, 777 ff, 1935.
2. Bohr N. A reply to the Paradox of E., P. and R. Phys Rev 48, 696 ff., 1935.
3. Bell JS. On the E.P.R. paradox. Physics 1, 195 ff, 1964.
4. Bell JS. Physics World 3 (8), 33 ff, 1990.
(a) Samuel Johnson (1709-1784): British writer and lexicographer, leading literary figure in the second half of the 18th century. When someone remarked to Johnson that an inscription on a gravestone erred on the side of generosity, he observed that "A man is not upon oath in a lapidary inscription".
(b) For the neophyte, it may be useful to know, for the sake of comparison, the current way of presenting E., P. and R.'s arguments. It is said that they criticised quantum mechanics for not being a complete theory and for having to be supplemented by additional variables to restore causality and locality. They are accused of using locality as an assumption (For more information). This is why they were said to be in the wrong when Bell proved that the world according to quantum mechanics is non-local. Dr. Hammerton, on the contrary, stresses that they were right in drawing attention to the non-local nature of quantum mechanics. Thus, if they were right as regards space, they must have been wrong about time.
(c) The most recent experiment was performed by Nicolas Gisin of the University of Geneva. Two photons were emitted in two different directions by a single source, a KNbO3 (Nb = nobium) crystal excited by a laser. They each travelled along an optic fibre until they reached a semi-reflecting mirror. Statistically, the behaviour of the photon is random. Its path crosses the mirror as often as it is deviated. There is, therefore, "logically" no reason to suppose that the two photons will behave similarly. However, although the photons were 10 km apart, they behaved identically as if they "communicated instantaneously" although, according to the theory of relativity, no information can travel faster than the speed of light (300 000 kms/sec).
(d) Richard P. Feynman : American physicist who received the Nobel prize in 1965 for his work on quantum electrodynamics. He devised the "sum over all histories" method. In some of his calculations, time runs backwards and this has been used as an argument in some interpretations of quantum theory.