<P> Quantum mechanics (QM; also known as quantum physics or quantum theory), including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles . </P> <P> Classical physics (the physics existing before quantum mechanics) is a set of fundamental theories which describes nature at ordinary (macroscopic) scale . Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale . Quantum mechanics differs from classical physics in that: energy, momentum and other quantities of a system may be restricted to discrete values (quantization), objects have characteristics of both particles and waves (wave - particle duality), and there are limits to the precision with which quantities can be known (uncertainty principle). </P> <P> Quantum mechanics gradually arose from the necessity of discrete energy values in Max Planck's solution in 1900 to the black - body radiation problem, and from the necessary correspondence between energy and frequency in Albert Einstein's 1905 paper which offered a quantum - based theory to explain the photoelectric effect . Early quantum theory was profoundly re-conceived in the mid-1920s by Erwin Schrödinger, Werner Heisenberg, Max Born and others . The modern theory is formulated in various specially developed mathematical formalisms . In one of them, a mathematical function, the wave function, provides information about the probability amplitude of position, momentum, and other physical properties of a particle . </P> <P> Important applications of quantum theory include quantum chemistry, superconducting magnets, light - emitting diodes, and the laser, the transistor and semiconductors such as the microprocessor, medical and research imaging such as magnetic resonance imaging and electron microscopy . Explanations for many biological and physical phenomena are rooted in the nature of the chemical bond, most notably the macro-molecule DNA . </P>

Who created the quantum mechanical model of the atom