The Copenhagen Interpretation and Entanglement
The phenomenon of entanglement dictates that if one entangled partner, say an electron has spin up when measured in a particular direction, it instantly determines the behaviour of its related particle and the other must ipso facto be spin down and vice-versa, this being the case no matter how far apart the two may be, even galaxies distant from each other[1]. Einstein did not much like the idea, which he termed “spooky action at a distance”, because it implied that action and reaction must necessarily be happening at greater than the speed of light. He thought that local factors must be influencing the behaviour of the particles, “local” meaning that “hidden variables” limited by the speed of light must be at work[2]. The certainty of predicting the outcome for each particle also appeared to violate Heisenberg’s uncertainty principle. Einstein intimated as much in a paper he wrote in 1935 along with two co-authors Boris Podolsky and Nathan Rosen: the EPR paper.
The EPR postulate as explained by JP McEvoy [3] goes something like this: imagine a pair of particles say electrons A and B in a so-called singlet state amounting to zero. The two particles move widely apart. The spin of A in one direction is measured and found to be spin up. The other particle B traveling in the same direction must therefore have spin down. In classical physics, this would not be a problem. One would conclude that it always had spin down from the time of separation.
However, according to the Copenhagen Interpretation, the spin of A has no definite measurement until it is measured at which point it must produce an instantaneous effect at B, collapsing its spin wave function into the opposite or down state. This "bizarre situation" demands action at a distance or faster than speed of light communication, neither of which is acceptable. The big issue was Einstein's idea of separateness: the locality principle. If two systems are in isolation from each other for some time, then a measurement of the first can produce no real change on the second.
Bohr's response was that this separateness or locality was not allowed, that quantum mechanics does not permit a separation between the observer and the observed. The two electrons and the observer are part of a single system. In other words, the EPR experiment does not demonstrate the incompleteness of quantum theory, but the naivete of assuming local conditions in atomic systems. Once they have been connected, atomic systems never separate.[4]
In 1964, a physicist by the name of John Bell formulated a way to test whether Einstein's “local hidden variables” could account for the entanglement phenomenon. However, there were two weaknesses in his analysis. In some instances, the measurement stations were close enough to be able to facilitate communications with each other at sub-light speed during a test, and in others the detectors may only have the ability to measure some but not all of the entangled particles, each leaving some leeway for hidden variables. The big question was whether this remarkable property of non-locality could ever be experimentally tested. Or could the existence of Einstein’s separateness be proven instead?
In 2015, different groups of scientists, one at Delft University of Technology in the Netherlands and others in the United States, Austria and Germany, formulated a way to foreclose on both these weaknesses, in the process demonstrating that local hidden variables cannot explain the consequences of entanglement, which therefore remains a fundamental attribute of the quantum world which cannot simply be explained away, confronting as that may be to our normal perception of reality.[5]
[1] Ronald Hanson, Krister Shalm, “Spooky action”, Scientific American, December 2018, 51-57.
[2] Ibid, 53.
[3] Introducing Quantum Theory: A Graphic Guide, Icon Books Ltd, Kindle Edition, locations 1596-1722.
[4] Ibid.
[5] The intricacies of the experiments involved are described in Hanson and Shalm, 56-57.