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**Quantum mechanics** is a theory in modern physics that models the
mechanics (physics of forces and actions) at the scale of atoms and
subatomic particles. Its mechanical principles have pronounced
differences from the mechanics of larger objects viewed in everyday
life, as modeled by Isaac Newton's physics theories
(**classical mechanics** aka **Newtonian mechanics**).
In particular, it models physical actions naturally thought of
as inevitable as merely having a certain probability.

Whereas in classical mechanics, certain quantities may have any
value, zero or greater, quantum mechanics in some cases imposes
minimal possible values (**quanta**), and it was developed precisely
for this feature when certain observed phenomena could be explained
by such a trait. The notion of quanta of light (photons) was
developed to explain observations about the photoelectric effect,
and quanta of angular
momentum to explain why an electron's orbit around a nucleus has a
minimal size that remains sufficiently stable so as not to immediately
decay into a merger with the nucleus.

Light quanta act as particles, resulting in two apparently-conflicting models of light: as waves and as particles, and consolidating these aspects resulted in some of the strangeness of quantum mechanics. Subsequent study of the mechanics of atomic and subatomic particles revealed an analogous particle/wave duality, i.e., the mechanics of particles in some cases can be modeled by considering them to be not particles, but waves.

Quantum mechanics' strangeness has been described as less well-known, yet stranger than the apparent paradoxes of relativity. An example is quantum mechanical phenomena's dependence on probability, e.g., quantum tunneling. Quantum mechanics has also been described as far more influential than relativity to current daily life, given it provides the successful explanations of the workings of transistors (thus virtually all current electronics), though it can be argued that workable transistors might be developed through trial and error.

The term **quantum system** refers to a bunch of things having a
quantum-mechanical interaction, i.e., under study in the science
of quantum mechanics. Examples might be an atom, or an electron
meeting a photon. Quantum mechanics predictions work best if
the system has some separation from other particles, e.g., a
nearby free electron can affect the calculations you would
carry out to model the behavior of an atom. A quantum system
has a **quantum state**, essentially the values of its
quantum numbers.

**Quantum fluctuations** are the results of the probability
aspect of quantum mechanics: activities have a
probability of happening at any time (a rare action
might be a decay that has a long half-life, and a physical
action that is an immediate consequence of previous
activity merely has a probability so high as to be
virtually inevitable), thus there are certain things
happening at random.

atomic excitation

black hole (BH)

Bohr model

Boltzmann equation

Bose-Einstein statistics

conservation law

continuum emission

cross section

dark energy

de Broglie wavelength

degeneracy

diffraction

eigenmode

eigenvalue (λ)

electron degeneracy

electron orbital

electron shell

false vacuum

fluorescence

Gamow peak

Hamiltonian

Kramers opacity law

mass shell

Maxwell-Boltzmann distribution

Monte Carlo method

metastable

Mikheyev-Smirnov-Wolfenstein effect (MSW effect)

neutrino (ν)

N-point function

oscillator strength

partition function (Z)

perturbation theory

quantum

quantum mixing

quantum number

quantum tunneling

Rayleigh-Jeans law

speed of light (c)

spin (m

statistical mechanics

superradiance