quantum mechanics

American  

noun

Physics.

1. a theory of the mechanics of atoms, molecules, and other physical systems that are subject to the uncertainty principle. QM

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quantum mechanics British  

noun

1. (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

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quantum mechanics Scientific  

1. 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.

2. Also called quantum physics quantum theory

3. Compare classical physics See also probability wave quantum uncertainty principle wave-particle duality

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quantum mechanics Cultural  

1. 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.)

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Other Word Forms

• quantum-mechanical
  adjective

Etymology

Origin of quantum mechanics

First recorded in 1920–25

Example Sentences

Among the most important concepts in quantum mechanics is quantum coherence.

From Science Daily • Jul. 5, 2026

Scientists working on concepts such as string theory and quantum gravity are attempting to reconcile quantum mechanics with Einstein's theory of relativity, one of the biggest unsolved problems in modern physics.

From Science Daily • Jul. 5, 2026

Trump also directed the Commerce and Defense departments to deploy quantum sensors, which use quantum mechanics to offer alternatives to traditional global-positioning systems, in the next five years.

From The Wall Street Journal • Jun. 22, 2026

One of the most surprising features of quantum mechanics is that objects can exist in multiple states simultaneously.

From Science Daily • Jun. 15, 2026

Interference can also occur for particles, because of the duality introduced by quantum mechanics.

From "A Brief History of Time: And Other Essays" by Stephen Hawking