an international team of physicists from the Massachusetts Institute of Technology (MIT) in Cambridge, the University of Vienna in Austria, and elsewhere provided strong evidence for the existence of a strange phenomenon termed quantum entanglement. This seemingly “spooky” idea proposes that a duo pf particles, no matter how distant they are from one another in Spacetime, can actually be inextricably bound to one another. Indeed, according to this theory, when pairs or groups of particles are generated, interact, or share spatial proximity in a way that the quantum state of each individual particle cannot be described independently of the state of the others–even when the particles are separated by a vast distance–a quantum state must be described for the entire system as a whole. For example, if a pair of particles are haunting opposite edges of the observable Universe, if they are really entangled, then their physical properties should be related in a way that any measurement made on one particle must instantly dispatch information about any future measurement outcome to the other entangled particle. For this reason, Albert Einstein was highly skeptical of this theory, referring to it dismissively as “spooky action at a distance.”
This strange phenomenon was the subject of a paper by driving distance Albert Einstein and two other physicists: the Russian-American Boris Podolsky (1896-1966), and the American-Israeli Nathan Rosen (1909-1995). Their theory is generally referred to as EPR, in honor of the first letters of the three authors’ last names. In addition, several papers were written by the Austrian physicist Erwin Schrodinger (1887-1961). Schrodinger’s research papers were published soon after the publication of EPR. All of the papers describe what ultimately came to be known as the EPR Paradox. However, Einstein thought that this ghostly behavior on the quantum level could not possibly occur in nature because it violated the view of causality–which is why he called it “spooky”. Briefly, in physics, causality refers to a genetic connection through which the cause gives rise to the effect.
In the 1960s, the American physicist John Stewart Bell (1928-1990) calculated a theoretical upper limit beyond which such “spooky” correlations must possess a quantum, rather than a classical, explanation–which would render these correlations impossible (Bell’s inequality). Bell’s inequality addressed the strength of correlations that can be created in any theory that obeys local realism. Bell showed that quantum theory predicts violations of this limit for specific entangled systems. His inequality is experimentally testable and, for this fortunate reason, there have been a number of relevant experiments. These experiments were first conducted with the pioneering work of the American physicists Stuart Freedman (1944-2012) and John Clauser (b. 1942) in 1972, and later by the French physicist Alain Aspect (b.1947), who conducted his experiments a decade later. All of the experiments have shown agreement with quantum mechanics, as opposed to the principle of local realism.