page_title: Branches of physics
section_title: Quantum mechanics, atomic physics, and molecular physics
hierarchical_section_title: Branches of physics / Quantum mechanics, atomic physics, and molecular physics
caption_attribution_description: English: First few hydrogen atom orbitals; cross section showing color-coded probability density for different n=1,2,3 and l="s","p","d"; note: m=0 The picture shows the first few hydrogen atom orbitals (energy eigenfunctions). These are cross-sections of the probability density that are color-coded (black=zero density, white=highest density). The angular momentum quantum number l is denoted in each column, using the usual spectroscopic letter code ("s" means l=0; "p": l=1; "d": l=2). The main quantum number n (=1,2,3,...) is marked to the right of each row. For all pictures the magnetic quantum number m has been set to 0, and the cross-sectional plane is the x-z plane (z is the vertical axis). The probability density in three-dimensional space is obtained by rotating the one shown here around the z-axis. Note the striking similarity of this picture to the diagrams of the normal modes of displacement of a soap film membrane oscillating on a disk bound by a wire frame. See, e.g., Vibrations and Waves, A.P. French, M.I.T. Introductory Physics Series, 1971, ISBN&#160;0393099369, page 186, Fig. 6-13. See also Normal vibration modes of a circular membrane. Slovenščina: Valovne funkcije elektrona v vodikovem atomu imajo določeno energijo (naraščajoče od zgoraj: n=1,2,3 ...) in vrtilno količino (naraščajoče prek: s, p, d ...). Svetlejša področja odgovarjajo višji verjetnostni gostoti za merjenje lege. Vrtilna količina in energija sta kvantizirani in zavzemata le nezvezdne vrednosti, kot jih kažejo slike.
context_page_description: Physics deals with the combination of matter and energy. It also deals with a wide variety of systems, about which theories have been developed that are used by physicists. In general, theories are experimentally tested numerous times before they are accepted as correct as a description of Nature. For instance, the theory of classical mechanics accurately describes the motion of objects, provided they are much larger than atoms and moving at much less than the speed of light. These "central theories" are important tools for research in more specialized topics, and any physicist, regardless of his or her specialization, is expected to be literate in them.
context_section_description: Quantum mechanics is the branch of physics treating atomic and subatomic systems and their interaction based on the observation that all forms of energy are released in discrete units or bundles called "quanta". Remarkably, quantum theory typically permits only probable or statistical calculation of the observed features of subatomic particles, understood in terms of wave functions. The Schrödinger equation plays the role in quantum mechanics that Newton's laws and conservation of energy serve in classical mechanics—i.e., it predicts the future behavior of a dynamic system—and is a wave equation that is used to solve for wavefunctions.
  For example, the light, or electromagnetic radiation emitted or absorbed by an atom has only certain frequencies (or wavelengths), as can be seen from the line spectrum associated with the chemical element represented by that atom. The quantum theory shows that those frequencies correspond to definite energies of the light quanta, or photons, and result from the fact that the electrons of the atom can have only certain allowed energy values, or levels; when an electron changes from one allowed level to another, a quantum of energy is emitted or absorbed whose frequency is directly proportional to the energy difference between the two levels. The photoelectric effect further confirmed the quantization of light.
  In 1924, Louis de Broglie proposed that not only do light waves sometimes exhibit particle-like properties, but particles may also exhibit wave-like properties. Two different formulations of quantum mechanics were presented following de Broglie's suggestion. The wave mechanics of Erwin Schrödinger (1926) involves the use of a mathematical entity, the wave function, which is related to the probability of finding a particle at a given point in space. The matrix mechanics of Werner Heisenberg (1925) makes no mention of wave functions or similar concepts but was shown to be mathematically equivalent to Schrödinger's theory. A particularly important discovery of the quantum theory is the uncertainty principle, enunciated by Heisenberg in 1927, which places an absolute theoretical limit on the accuracy of certain measurements; as a result, the assumption by earlier scientists that the physical state of a system could be measured exactly and used to predict future states had to be abandoned. Quantum mechanics was combined with the theory of relativity in the formulation of Paul Dirac. Other developments include quantum statistics, quantum electrodynamics, concerned with interactions between charged particles and electromagnetic fields; and its generalization, quantum field theory.
  String Theory
  A possible candidate  for the theory of everything, this theory combines the theory of general relativity and quantum mechanics to make a single theory. This theory can predict about properties of both small and big objects. This theory is currently under developmental stage.
caption: The first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
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