How strange is quantum mechanics? An introduce to local-realism violation

Even though quantum mechanics does not allow two experimenters to exchange information acausally, quantum mechanics does allow the responses of independent detectors to be more strongly correlated than allowed in a classical world in which the response of a detector is determined solely by the causal flow of signals from the transmitters to the detectors. Since the original proposals of Bell for experiments that could display this incompatibility between quantum mechanics and the classical world view, a variety of new proposals have been put forward and considerable progress has been made in exploring the extent of the incompatibility. Local-realism violating phenomena discovered by Greenberger, Horne, and Zeilinger and by Hardy are particularly graphic at displaying this incompatibility and are well-suited for pedagogical purposes. In light of the increasing variety of local-realism violating phenomena being uncovered it is natural to ask whether quantum mechanics places any restrictions on the variety of such phenomena that might be imagined within the constraint that two observers not be allowed to communicate with each other acausally. It has been shown that quantum mechanics does indeed place restrictions on the degree to which local realism can be violated. The strangeness of quantum mechanics is bounded. Recent work in exploring the limits to this strangeness will be described.