An Entangled Parable of Acausal Connections
One of the most surprising aspects of quantum entanglement is the possibility of access to shared information between widely separated particles, enabling them to coordinate their state parameters (for example spin or polarization states) over great distances. Bell’s inequality, proposed by (Northern) Irish physicist John Stewart Bell in 1964, offers a litmus test to see if a greater deterministic framework with local realism or the possibility of “hidden variables” (advocated by Albert Einstein and David Bohm, respectively) might have somehow conveyed information between the particles in an entangled state, or if such a back-channel might be ruled out.
To understand the difference between the presence or absence of local realism (as gauged by the two possible outcomes for Bell’s inequality), let’s tell a story two different ways. In the first version of the tale, akin to a detective story, we’ll be as realistic as possible — leading to a conclusion Einstein and Bohm would support. In the second telling, we’ll make it a fantasy story and add an element of magic. Quantum entanglement is not magic, of course, but if we didn’t posit the notion of instant, non-local anticorrelation due to an entangled quantum state, it would sure seem that way.
First the realistic version. Imagine a family with two identical twin boys, Fred and George. They look so much alike, that they sometimes fool their friends. Therefore they are raised with a strict rule. No matter which way you dress, pick some characteristic that is different. In their town, which is somewhat isolated, clothing options are limited. Shirts come in red or blue, but not other colors. They are either plaid or striped, long-sleeved or short-sleeved. Thus, if, on a certain day, Fred chooses plaid as his distinguishing characteristic, George must wear striped to distinguish the two, no matter what the color. But if, on the other hand, George decides one day that he absolutely must wear a different colored shirt that day than Fred’s, if he dons a blue shirt, George is slated for red, no matter what the pattern.
We have used color, pattern, and length as metaphors for the three axes, x, y, and z, in which the magnetic fields might be directed to measure spin. The binary options in each case — red or blue…