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**Quantum mechanics** (**QM**) is a modern physics theory 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 theories
(**classical mechanics** aka **Newtonian mechanics**).
In particular, QM 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, QM 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 (photon) 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 like particles, resulting in two apparently-conflicting models of light: as waves and as particles, and consolidating these aspects results in some of the strangeness of QM. 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.

Quantum mechanics has a traditional interpretation that is at odds with our everyday experience and our notions of logic, and debates as well as perhaps a whole branch of philosophy attempt to address the problem of that interpretation's meaning and consequences, as well as other possible interpretations. In any case, QM calculations do produce numbers that match observation whereas other methods fail despite years of expert efforts to come up with something different, and an attitude of QM users has been "shut up and calculate".

The term **quantum system** refers to a bunch of things having a
quantum-mechanical interaction, i.e., in a manner modeled by
QM. Examples might be the workings of an atom,
or the incident of an electron
meeting a photon. QM predictions work best if
the system has some separation from other particles, e.g., a
nearby free electron can affect the calculations that
model the behavior of an atom. A quantum system
has a **quantum state**, essentially the values of its
quantum numbers.

**Quantum fluctuations** are the result of the probability
aspect of QM: there are activities with a
probability of happening at any time, thus there are certain
things that happen at random.

http://en.wikipedia.org/wiki/Quantum_mechanics

atomic excitation

Bose-Einstein condensate (BEC)

black hole (BH)

Bohr model

Boltzmann equation

Bose-Einstein statistics

Compton wavelength

conservation law

continuum emission

cosmological simulation

cross section

dark energy

dark matter

de Broglie wavelength

degeneracy

diffraction

eigenmode

eigenvalue (λ)

electron degeneracy

electron orbital

electron shell

false vacuum

fluorescence

fuzzy dark matter (FDM)

Gamow peak

Hamiltonian

initial fluctuations

Kramers opacity law

Madelung equations

mass shell

Maxwell-Boltzmann distribution

Monte Carlo method

metastable

Mikheyev-Smirnov-Wolfenstein effect (MSW effect)

N-body problem

neutrino (ν)

N-point function

oscillator strength

partition function (Z)

perturbation theory

quantum Monte Carlo (QMC)

quantum

quantum mixing

quantum number

quantum tunneling

Rayleigh-Jeans law

speed of light (c)

spin (m

statistical mechanics

superradiance

Heisenberg uncertainty principle