# Fifty years of Bell’s theorem

On 4 November 1964, a journal called *Physics *received a paper written by John Bell, a theoretician from CERN. The journal was short-lived, but the paper became famous, laying the foundations for the modern field of quantum-information science.

The background to Bell’s paper goes back to the late 1930s and Albert Einstein’s dislike of quantum mechanics, which he argued required “spooky actions at a distance”. In other words, according to quantum theory, a measurement in one location can influence the state of a system in another location. Einstein believed this appeared to occur only because the quantum description was not complete.

The argument as to whether quantum mechanics is indeed a complete description of reality continued for several decades until Bell, who was on leave from CERN at the time, wrote down what has become known as Bell’s theorem. Importantly, he provided an experimentally testable relationship between measurements that would confirm or refute the disliked “action at a distance”.

In the 1970s, experiments began to test Bell’s theorem, confirming quantum theory and at the same time establishing the base for what has now become major area of science and technology, with important applications, for example, in quantum cryptography.

Bell was born in Belfast, where he attended Queen’s University, eventually coming to CERN in 1960. For much of November the university is hosting a series of events in celebration of his work, including an exhibition *A**ction at a Distance: The Life and Legacy of John Stewart Bell* with photographs, objects and papers relating to Bell’s work alongside videos exploring his science and legacy. He sadly died suddenly in 1990. Now the Royal Irish Academy is calling for 4 November to be named “John Bell Day”.

### For more

The original paper: “On the Einstein Podolsky Rosen Paradox” by J S Bell [PDF]

A more general article: “On Bertlmann’s Socks and the Nature of Reality” by J S Bell [PDF]