Quantum theory evolved as a new branch of theoretical
physics during the first few decades of the 20th century in an endeavour to
understand the fundamental properties of matter. It began with the study of the
interactions of matter and radiation. Certain radiation effects could neither
be explained by classical mechanics, nor by the theory of electromagnetism. In
particular, physicists were puzzled by the nature of light. Peculiar lines in
the spectrum of sunlight had been discovered earlier by Joseph von Fraunhofer
(1787-1826). These spectral lines were then systematically catalogued for
various substances, yet nobody could explain why the spectral lines are there
and why they would differ for each substance. It took about one hundred years,
until a plausible explanation was supplied by quantum theory.
Quantum theory is
about the nature of matter.
Quantum mechanics (QM – also known as quantum physics, or
quantum theory) is a branch of physics dealing with physical phenomena at
microscopic scales, where the action is on the order of the Planck constant.
Quantum mechanics departs from classical mechanics primarily at the quantum
realm of atomic and subatomic length scales. Quantum mechanics provides a
mathematical description of much of the dual particle-like and wave-like behavior
and interactions of energy and matter.
In advanced topics of quantum mechanics, some of these
behaviors are macroscopic and only emerge at extreme (i.e., very low or very
high) energies or temperatures. The name quantum mechanics derives from the
observation that some physical quantities can change only in discrete amounts
(Latin quanta), and not in a continuous (cf. analog) way. For example, the
angular momentum of an electron bound to an atom or molecule is quantized. In
the context of quantum mechanics, the wave–particle duality of energy and
matter and the uncertainty principle provide a unified view of the behavior of
photons, electrons, and other atomic-scale objects.
Applications
Quantum mechanics had enormous success in explaining many of
the features of our world. The individual behaviors of the subatomic particles
that make up all forms of matter (electrons, protons, neutrons, photons, and
others) can often only be satisfactorily described using quantum mechanics.
Quantum mechanics has strongly influenced string theories, candidates for a
Theory of Everything and the multiverse hypotheses.
Quantum mechanics is also critically important for
understanding how individual atoms combine covalently to form molecules. The
application of quantum mechanics to chemistry is known as quantum chemistry.
Relativistic quantum mechanics can, in principle, mathematically describe most
of chemistry. Quantum mechanics can also provide quantitative insight into
ionic and covalent bonding processes by explicitly showing which molecules are
energetically favorable to which others, and the magnitudes of the energies
involved. Furthermore, most of the calculations performed in modern
computational chemistry rely on quantum mechanics.
