Interpreting the Quantum World

Einstein's Last Stand

Einstein moved to the USA after the rise of nazism. He continued to be dissatisfied with the formulation of quantum mechanics and tried to find ways to challenge the theories. Seemingly unable to use the uncertainty principle for this purpose, he developed the EPR paradox with two younger colleagues at Princeton, Boris Podolsky and Nathan Rosen. Their argument centered around a quantum system consisting of a pair of particles (A and B) which are created with opposite spin, i.e.: the overall system has no net spin.

The particles move apart at the speed of light and when they are widely separated, the spin of one of the particles, say A, is measured and found to point 'up'. The classical interpretation is simple: particle 'B' was always in a spin 'down' state. The quantum mechanical interpretation is more tricky as the Copenhagen Interpretation says that the spin of particle 'A' has no definite value until it is measured. Or, to quote Bohr: '… the state of an atomic system before a measurement is not defined but only has the possibility of certain values with associated probabilities'. At this point, it must produce an instantaneous effect on particle 'B' to give the opposite spin. This requires 'action at a distance' or faster than light communication! Faster than light action was completely disallowed by Einstein's theory of special relativity. Einstein and his colleagues said that there must be hidden information which is not contained in the wave function of the system. Einstein debated this at length with Bohr until his death in 1955.

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