The principle of quantum mechanics that associates the physical properties of particles into pairs, such that both together cannot be measured to within more than a certain degree of accuracy. If A and V form such a pair (called a conjugate pair) then: ∆A∆V > k where k is a constant and ∆A and ∆V are variances in the experimental values for the attributes A and V. The best-known instance of the equation relates the position and momentum of an electron: ∆p∆x > h where h is Planck’s constant. This is the Heisenberg uncertainty principle. See also Bell’s theorem, Einstein-Podolsky-Rosen thought experiment.

http://www.aip.org/history/heisenberg/p08.htm An online exhibit on the principle’s history, with a bibliography

http://www.philosophytalk.org/TheStrangeWorldofQuantumReality.htm An audio discussion of quantum indeterminacy by three philosophers