 Quantum Mechanics, Quantum Mechanics QM, also known as Quantum Physics. Quantum theory, the wave-mechanical model, or matrix mechanics 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. Classical physics, the physics existing before quantum mechanics, 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, angular momentum and other quantities of a system are 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 measured uncertainty principle. Quantum mechanics gradually arose from theories to explain observations which could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black body radiation problem, and from the correspondence between energy and frequency in Albert Einstein's 1905 paper which explained the photoelectric effect. Early quantum theory was profoundly reconceived in the mid-1920s by Erwin Schrodinger, Werner Heitseberg, Max Born and others. The modern theory is formulated in various specially developed mathematical formalisms. In one of them, the mathematical function, the wave function, provides information about the probability amplitude of position, momentum, and other physical properties of a particle. Important applications of quantum theory include quantum chemistry, quantum optics, quantum computing, superconducting magnets, light emitting piods, and the laser, the transistor and semiconductors such as a 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 macromolecule DNA.