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

---

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

---

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

---

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.)

---

Other Word Forms

• quantum-mechanical adjective

Etymology

Origin of quantum mechanics

First recorded in 1920–25

Example Sentences

Because of this, defining memory in quantum mechanics has remained a challenge.

From Science Daily • Apr. 14, 2026

"Similar to conventional batteries, quantum batteries charge, store and discharge energy. But while everyday batteries rely on chemical reactions, quantum batteries leverage properties of quantum mechanics," Associate Professor Hutchison said.

From Science Daily • Mar. 22, 2026

Scientists hope the particle -- which is similar to a proton but four times heavier -- will reveal more about the strange behaviour of quantum mechanics.

From Barron's • Mar. 17, 2026

When the founders of quantum mechanics began exploring the strange behavior of matter and light, they realized Hamilton's framework was more than a simple analogy.

From Science Daily • Mar. 10, 2026

On the other hand, the other partial theories depend on quantum mechanics in an essential way.

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