Our everyday experience tells us that physical objects have properties independent of whether or not we measure them. As Albert Einstein put it: the Moon is there, whether you look at it or not. However, this is different in the quantum world; the mere act of observing this world seems to change it.
Einstein wasn’t a fan of this view of physics, and suggested there must be something missing from the quantum description. This view became most apparent in his treatment of quantum entanglement. When two particles are entangled they behave as if they are one. No matter how far apart they are in time or space, observing one of them seems to have an instantaneous influence over the other. Einstein called this effect “spooky action at a distance” and suggested that other hidden variables were at play.
In 1964, CERN physicist John Bell precisely formalised Einstein’s view as the concept of “local realism”. Here “realism” says that objects have well-defined properties even when we are not looking at them, while “locality” states that objects can only be influenced by causes in their immediate vicinity.
Bell also put forth a powerful argument to show that Einstein’s view, even when including his suggested missing pieces, cannot explain the results of experiments performed on entangled quantum particles. In doing so he made it possible to test Einstein’s...
