|(functioning as singular) the branch of mechanics, based on the quantum theory used for interpreting the behaviour of elementary particles and atoms, which do not obey Newtonian mechanics|
|a scrap or morsel of food left at a meal.|
|an arrangement of five objects, as trees, in a square or rectangle, one at each corner and one in the middle.|
A fundamental theory of matter and energy that explains facts that previous physical theories were unable to account for, in particular the fact that energy is absorbed and released in small, discrete quantities (quanta), and that all matter displays both wavelike and particlelike properties, especially when viewed at atomic and subatomic scales. Quantum mechanics suggests that the behavior of matter and energy is inherently probabilistic and that the effect of the observer on the physical system being observed must be understood as a part of that system. Also called quantum physics, quantum theory. Compare classical physics. See also probability wave, quantum, uncertainty principle, wave-particle duality.
The branch of physics that deals with the behavior of matter at the level of the atom, the nucleus, and the elementary particle. At this level, energy, mass, momentum, and other quantities do not vary continuously, as they do in the large-scale world, but come in discrete units, or quanta. (See Bohr atom and photon.)