on the assumption that all the usual versions of quantum mechanics are incomplete, and that there is an underlying layer of reality which contains additional information about the world. This additional information is in the form of the hidden variables. If physicists knew the values of these hidden variables, the arguments goes, they could predict the precise outcomes of particular measurements, not just the probabilities of getting particular results. A helpful analogy can be made with a deck of playing cards prepared by a cheating card dealer (Figure 12-06e). He knows precisely the sequence in the deck; only the ignorant gamblers insist that the chance of picking a particular card is 1 in 52. The idea on hidden variables can be traced back to the 1920s and 1930s in the form of EPR (Einstein-Podolsky-Rosen) paradox. These scientists had a lot of problems with quantum mechanics in general and entanglement in particular. Entanglement occurs for example, in the decay of the pi meson into an electron-positron pair (Figure 12-06f). Since the spin for the pi meson is 0, the spin for the electron-positron pair must be opposite according to the conservation of angular momentum. Therefore, no matter how far apart are the members of this pair, if the spin is flipped for one of the member, the spin for the other member will also be flipped to the opposite at precisely the same moment. This non-local influence (non-locality) occur instantaneously, as if some form of communication, which Einstein called a "spooky