Quantum mechanics, fundamental theory of small-scale physical phenomena (such as the motions of electrons within atoms). This theory was developed during the 1920s, when it became clear that the existing laws of classical mechanics and electromagnetic theory were not successfully applicable to such systems. French physicist Louis De Broglie suggested (1924) that particles have a wavelike nature, with a wavelength h/p (h being the Planck constant, and p the particle momentum). This wavelike nature is significant only for particles on the molecular scale or smaller. These ideas were developed by Erwin Schrödinger and others into the branch of quantum mechanics known as wave mechanics. Werner Heisenberg of Germany worked along parallel lines with a theory incorporating only observable quantities, such as energy, using matrix algebra techniques. His uncertainty principle (that a subatomic particle's momentum and position cannot both be accurately known) is fundamental to quantum mechanics, as is Wolfgang Pauli's exclusion principle (that each electron in an atom is in a quantum state shared by no other electron in that atom). Paul Dirac incorporated relativistic ideas into quantum mechanics.
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