The EPR Paradox and Entanglement

A Founder Turned Critic

Einstein helped create quantum theory. Light quanta, the quantum theory of solids, stimulated emission, and Bose–Einstein statistics are all his. But when the finished theory of quantum mechanics arrived in the mid-1920s, he was dissatisfied with it.

The new theory predicts only probabilities. It does not say when a given atom will decay, only how likely decay is. Most physicists, led by Niels Bohr, came to accept that this is simply how nature is. Einstein believed the theory was correct as far as it went, but incomplete: a statistical summary of some deeper description not yet found. His famous remark that God does not play dice comes from a 1926 letter to his friend Max Born.

At the Solvay conferences of 1927 and 1930 Einstein challenged Bohr with ingenious thought experiments designed to expose a flaw in the theory. By the usual account Bohr answered each one, though neither man conceded. Einstein changed his line of attack.

The EPR Argument

In 1935, with Boris Podolsky and Nathan Rosen at Princeton, Einstein published "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?"

Consider two particles that interact and then fly far apart. Quantum mechanics describes them with a single shared state. Measure the position of the first, and you immediately know the position of the second. Measure its momentum instead, and you know the momentum of the second.

Nothing was done to the second particle. So, the authors argued, it must have had a definite position and a definite momentum all along. But quantum mechanics forbids describing both at once. Therefore the theory leaves something out. (Podolsky wrote the paper, and Einstein complained afterward that the main point had been buried under formalism.)

The only escape was to suppose that measuring one particle instantly affects the other, however far away. Einstein dismissed that, in a 1947 letter to Max Born, as "spooky action at a distance." Erwin Schrödinger, responding to the paper, gave the shared state its name: entanglement.

The Test

For decades the debate seemed to be philosophy. Then in 1964 the Northern Irish physicist John Bell proved a theorem. If particles carry definite hidden properties and cannot influence each other faster than light, then the correlations between measurements on them must stay within a certain limit, written $|S|\le 2$ in the version above. Quantum mechanics predicts that the limit is broken, with values up to $2\sqrt{2}$.

That made the question experimental. John Clauser and Stuart Freedman ran the first test in 1972. Alain Aspect closed important gaps in the early 1980s, and Anton Zeilinger and others refined the experiments further. With one early exception, later traced to an experimental fault, every test has agreed with quantum mechanics. The three shared the 2022 Nobel Prize in Physics.

What It Means

The kind of theory Einstein hoped for, with local hidden properties, is ruled out. Entanglement is real. It cannot be used to send a message faster than light, so relativity survives, but nature is connected across distance in a way he found unacceptable.

What People Get Wrong

• Einstein did not misunderstand or reject quantum mechanics. He accepted its predictions and understood it well enough to find its strangest consequence.
• He did not simply lose the argument. The question he asked led to Bell's theorem, and from there to quantum cryptography and quantum computing. See Quantum Technology.