When observed in space, subatomic particles move in irregular, unpredictable ways. For example, the 53pm distance between a hydrogen nucleus and its lone orbiting electron is a calculated average of where an electron will show up in the electron cloud during any observation. In the Copenhagen interpretation (a consensus of physicists in the 1920s about quantum mechanics that is still used today), the movement of these particles is assumed to be nondeterministic, that is, the randomness of the exact location of a particle at any time it is observed is baked in to the nature of spacetime, and there is no underlying mechanism that can be used to determine where exactly the particle will be.
Some physicists have developed theories that this nondeterminism is really a function of other, deterministic things that happen below the level of observable spacetime, most notably the pilot wave theory of deBroglie and Bohm (which we will cover in the future). The presumed deterministic data that generates the seemingly nondeterministic movement of particles are called “hidden variables.” Bell’s Theorem shows that, if hidden variables exist, they cannot exist alongside the notions of realism (a particle has a predetermined location before being observed) and locality (the hidden variables for particles or systems of particles can exist independently of those for others).
In classical mechanics, local realism is intuitive. I can place a ball on the ground, turn around, throw another ball, and turn back to find the original ball where I left it. The nondeterministic movement of particles violates the realism part of this intuition immediately (if the ball were a quantum particle I’d have to observe it to find out where it was), but the local part is also violated by the phenomenon known as quantum entanglement. Two particles may become “entangled,” which makes the properties of each particle dependent on each other. When one particle is measured, then the other one shows properties based on the original correlation between the two particles (usually, for demonstration, having the exact opposite). This happens even if the two particles are moved far from each other, and it happens instantaneously, even faster than light travels. In other words, if there were local hidden variables in the universe, they’d have to violate general relativity by propagating faster than light across space to update the hidden variables of the other entangled particle. The crux of Bell’s theorem, then, is that local hidden variable theories cannot explain everything that was predicted and later experimentally validated about how quantum mechanics behaves.
Born and raised in Belfast, John Stewart Bell received his Ph.D. in physics at the University of Birmingham in 1956. While most of his career was spent at CERN, he took leave in the early ’60s to lecture and work at several American universities. A result of this was the 1964 paper “On the Einstein-Podolsky-Rosen Paradox” which established Bell’s eponymous theorem. Bell married the physicist Mary Ross in 1954 and the two contributed substantially to the design of the CERN particle accelerator systems and the field of particle physics in general. After the first accelerator tests demonstrated correlations in entangled particle systems higher than local hidden variable theories would allow for (“violation of Bell’s inequalities”), Bell was conciliatory toward the late Einstein, who was famously opposed to the idea of the universe being nondeterministic at its core (and by extension to the Copenhagen interpretation).
Bell died unexpectedly of a cerebral hemorrhage in 1990, and it is claimed that he was at the time in the nomination process for the Nobel prize (which is not awarded posthumously). 25 years after Bell’s death, in 2015, several independent, carefully designed, high energy particle experiments closed the last possible loopholes that would have still allowed local hidden variable theory to be plausible in the face of Bell’s inequalities.