Misreading EPR: Variations on an Incorrect ThemeHe then gives examples of famous authors who get EPR wrong.
Notwithstanding its great influence in modern physics, the EPR thought-experiment has been explained incorrectly a surprising number of times.
He gets to the heart of the Bohr-Einstein dispute:
EPR write, near the end of their paper, "[O]ne would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted."This is well put. The aim of EPR is to explain a simple example of entangled particles, and to argue that no reasonable definition of reality would permit two observables that cannot be simultaneously measured.
The response that Bohr could have made: "Yes."
EPR briefly considered the implications of this idea and then dismissed it with the remark, "No reasonable definition of reality could be expected to permit this."
But that is exactly what Bohr did. A possible reply in the Bohrian vein: "Could a `reasonable definition of reality' permit so basic a fact as the simultaneity of two events to be dependent on the observer's frame of reference? Many notions familiar from everyday life only become well-defined in relativity theory once we fix a Lorentz frame. Likewise, many statements in quantum theory only become well-defined once we have given a complete description of the experimental apparatus and its arrangement."
This is not a quote from anywhere in Bohr's writings, but it is fairly in the tradition of his Warsaw lecture, where he put considerable emphasis on what he felt to be "deepgoing analogies" between quantum theory and relativity.
In spite of all differences in the physical problems concerned, relativity theory and quantum theory possess striking similarities in a purely logical aspect. In both cases we are confronted with novel aspects of the observational problem, involving a revision of customary ideas of physical reality, and originating in the recognition of general laws of nature which do not directly affect practical experience. The impossibility of an unambiguous separation between space and time without reference to the observer, and the impossibility of a sharp separation between the behavior of objects and their interaction with the means of observation are, in fact, straightforward consequences of the existence of a maximum velocity of propagation of all actions and of a minimum quantity of any action, respectively.
And yet that is a core teaching of quantum mechanics, from about 10 years earlier. Two non-commuting observables cannot be simultaneously measured precisely. That is the Heisenberg uncertainty principle.
Theories that assign definite simultaneous values to observables are called hidden variable theories. All the reasonable ones have been ruled out by the Bell Test Experiments.
Complaining that the uncertainty principle violates pre-conceptions about reality is like complaining that relativity violates pre-conceptions about simultaneity. Of course it does. Get with the program.
There are crackpots who reject relativity because of the Twin Paradox, or some other such surprising effect. The physics community treats them as crackpots. And yet the community tolerates those who get excited by EPR, even tho EPR makes essentially the same mistake.
Saying "maximum velocity of propagation" is a way of saying the core of relativity theory, and saying "minimum quantity of any action" is a way of saying the core of quantum mechanics. The minimum is Planck's constant h, or h-bar. The Heisenberg uncertainties are proportional to this constant. That minimum makes it impossible to precisely measure position and momentum simultaneously, just as the finite speed of light makes it impossible to keep clocks simultaneous.