In physics, Albert Einstein is famous for two things: developing the theory of relativity, and hating quantum mechanics. Relativity is a colossal achievement, with the special theory of 1905 putting classical physics on a firmer philosophical footing, and the general theory of 1915 extending that idea into the best theory we have for understanding gravity. While Einstein’s “heuristic” model of the photoelectric effect, also from 1905, played a critical role in launching quantum physics, he ended up finding it distasteful on the same sort of philosophical grounds that made relativity so successful. In particular, he hated the indeterminacy of the full quantum theory as it developed in the 1920′s. The notion that quantum particles do not have well-defined states prior to measurement was just too radical a departure from classical physics for Einstein to stomach, and he spent the last decades of his life searching for a replacement theory that would be more philosophically congenial.
There’s no small irony, then, in the fact that the very randomness he disparaged with his famous line about refusing to believe that God plays dice with the universe (paraphrased lots of times, but originally from a letter to Max Born) is essential for preserving one of the key results of his theory of relativity. Without that indeterminacy, quantum physics would allow the sending of messages faster than the speed of light, with disastrous consequences for the whole idea of causality, which is central to the operation of physics.
How does this work out? Well, the key phenomenon is the idea we now know as “entanglement,” a phenomenon that Einstein was concerned with back in the 1920′s when he was famously arguing with Niels Bohr (I re-enacted this with puppets some years back). Its clearest expression came in a 1935 paper by Einstein with his young colleagues Boris Podolsky and Nathan Rosen (colloquially known as the “EPR paper” after the initials of the authors), in which they used entanglement to argue that quantum physics as then understood must be an incomplete version of a deeper, more deterministic theory.
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