Albert Einstein

Einstein and Niels Bohr. Photo taken by Paul Ehrenfest during their visit to Leiden in December 1925.

Bohr versus Einstein

In the 1920s, quantum mechanics developed into a more complete theory. Einstein was unhappy with the "Copenhagen interpretation" of quantum theory developed by Niels Bohr and Werner Heisenberg, wherein quantum phenomena are inherently probabilistic, with definite states resulting only upon interaction with classical systems. A public debate between Einstein and Bohr followed, lasting for many years (including during the Solvay Conferences). Einstein formulated gedanken experiments against the Copenhagen interpretation, which were all rebutted by Bohr. In a 1926 letter to Max Born, Einstein wrote: "I, at any rate, am convinced that He does not throw dice." (Einstein 1969).

Einstein was never satisfied by what he perceived to be quantum theory's intrinsically incomplete description of nature, and in 1935 he further explored the issue in collaboration with Boris Podolsky and Nathan Rosen, noting that the theory seems to require non-local interactions; this is known as the EPR paradox (Einstein 1935). The EPR gedanken experiment has since been performed, with results confirming quantum theory's predictions.

Einstein's disagreement with Bohr revolved around the idea of scientific determinism. For this reason the repercussions of the Einstein-Bohr debate have found their way into philosophical discourse as well.