**Quantum
Theory**, also quantum mechanics, in physics, a theory based on
using the concept of the quantum unit to describe the dynamic
properties of subatomic particles and the interactions of matter
and radiation. The foundation was laid by the German physicist
Max Planck, who postulated in 1900 that energy can be emitted or
absorbed by matter only in small, discrete units called quanta.
Also fundamental to the development of quantum mechanics was the
uncertainty principle, formulated by the German physicist Werner
Heisenberg in 1927, which states that the position and momentum
of a subatomic particle cannot be specified simultaneously.

**Early
History **

In the 18th and 19th centuries, Newtonian, or classical, mechanics appeared to provide a wholly accurate description of the motions of bodiesfor example, planetary motion. In the late 19th and early 20th centuries, however, experimental findings raised doubts about the completeness of Newtonian theory. Among the newer observations were the lines that appear in the spectra of light emitted by heated gases, or gases in which electric discharges take place. From the model of the atom developed in the early 20th century by the English physicist Ernest Rutherford, in which negatively charged electrons circle a positive nucleus in orbits prescribed by Newton's laws of motion, scientists had also expected that the electrons would emit light over a broad frequency range, rather than in the narrow frequency ranges that form the lines in a spectrum.

Another
puzzle for physicists was the coexistence of two theories of
light: the corpuscular theory, which explains light as a stream
of particles, and the wave theory, which views light as
electromagnetic waves. A third problem was the absence of a
molecular basis for thermodynamics. In his book *Elementary
Principles in Statistical Mechanics* (1902), the American
mathematical physicist J. Willard Gibbs conceded the
impossibility of framing a theory of molecular action that
embraced the phenomena of thermodynamics, radiation, and
electrical phenomena as they were then understood.

**Planck's
Introduction of the Quantum **

At
the turn of the century, physicists did not yet clearly recognize
that these and other difficulties in physics were in any way
related. The first development that led to the solution of these
difficulties was Planck's introduction of the concept of the
quantum, as a result of physicists' studies of blackbody
radiation during the closing years of the 19th century. (The term
*blackbody* refers to an ideal body or surface that absorbs
all radiant energy without any reflection.) A body at a
moderately high temperaturea "red
heat"gives off most of its radiation in the low
frequency (red and infrared) regions; a body at a higher
temperature"white heat"gives off
comparatively more radiation in higher frequencies (yellow,
green, or blue). During the 1890s physicists conducted detailed
quantitative studies of these phenomena and expressed their
results in a series of curves or graphs. The classical, or
prequantum, theory predicted an altogether different set of
curves from those actually observed. What Planck did was to
devise a mathematical formula that described the curves exactly;
he then deduced a physical hypothesis that could explain the
formula. His hypothesis was that energy is radiated only in
quanta of energy *h</i></font><font color="#000000" size="2"
face="Symbol">u**,
where u is the frequency and h
is the quantum action, now known as Planck's constant.*

**Einstein's
Contribution **

The next important developments in quantum mechanics were the work of German-born American physicist and Nobel laureate Albert Einstein. He used Planck's concept of the quantum to explain certain properties of the photoelectric effectan experimentally observed phenomenon in which electrons are emitted from metal surfaces when radiation falls on these surfaces.

According to classical theory, the energy, as measured by the voltage of the emitted electrons, should be proportional to the intensity of the radiation. The energy of the electrons, however, was found to be independent of the intensity of radiationwhich determined only the number of electrons emittedand to depend solely on the frequency of the radiation. The higher the frequency of the incident radiation, the greater is the electron energy; below a certain critical frequency no electrons are emitted. These facts were explained by Einstein by assuming that a single quantum of radiant energy ejects a single electron from the metal. The energy of the quantum is proportional to the frequency, and so the energy of the electron depends on the frequency.

**The
Bohr Atom **

In 1911 Rutherford established the existence of the atomic nucleus. He assumed, on the basis of experimental evidence obtained from the scattering of alpha particles by the nuclei of gold atoms, that every atom consists of a dense, positively charged nucleus, surrounded by negatively charged electrons revolving around the nucleus as planets revolve around the sun. The classical electromagnetic theory developed by the British physicist James Clerk Maxwell unequivocally predicted that an electron revolving around a nucleus will continuously radiate electromagnetic energy until it has lost all its energy, and eventually will fall into the nucleus. Thus, according to classical theory, an atom, as described by Rutherford, is unstable. This difficulty led the Danish physicist Niels Bohr, in 1913, to postulate that in an atom the classical theory does not hold, and that electrons move in fixed orbits. Every change in orbit by the electron corresponds to the absorption or emission of a quantum of radiation.

The application of Bohr's theory to atoms with more than one electron proved difficult. The mathematical equations for the next simplest atom, the helium atom, were solved during the 1910s and 1920s, but the results were not entirely in accordance with experiment. For more complex atoms, only approximate solutions of the equations are possible, and these are only partly concordant with observations.

**Wave
Mechanics **

The French physicist Louis Victor de Broglie suggested in 1924 that because electromagnetic waves show particle characteristics, particles should, in some cases, also exhibit wave properties. This prediction was verified experimentally within a few years by the American physicists Clinton Joseph Davisson and Lester Halbert Germer and the British physicist George Paget Thomson. They showed that a beam of electrons scattered by a crystal produces a diffraction pattern characteristic of a wave. The wave concept of a particle led the Austrian physicist Erwin Schrödinger to develop a so-called wave equation to describe the wave properties of a particle and, more specifically, the wave behavior of the electron in the hydrogen atom.

Although
this differential equation was continuous and gave solutions for
all points in space, the permissible solutions of the equation
were restricted by certain conditions expressed by mathematical
equations called eigenfunctions (German *eigen,*
"own"). The Schrödinger wave equation thus had only
certain discrete solutions; these solutions were mathematical
expressions in which quantum numbers appeared as parameters.
(Quantum numbers are integers developed in particle physics to
give the magnitudes of certain characteristic quantities of
particles or systems.) The Schrödinger equation was solved for
the hydrogen atom and gave conclusions in substantial agreement
with earlier quantum theory. Moreover, it was solvable for the
helium atom, which earlier theory had failed to explain
adequately, and here also it was in agreement with experimental
evidence. The solutions of the Schrödinger equation also
indicated that no two electrons could have the same four quantum
numbersthat is, be in the same energy state. This rule,
which had already been established empirically by Austro-American
physicist and Nobel laureate Wolfgang Pauli in 1925, is called
the exclusion principle.

**Matrix
Mechanics **

Simultaneously with the development of wave mechanics, Heisenberg evolved a different mathematical analysis known as matrix mechanics. According to Heisenberg's theory, which was developed in collaboration with the German physicists Max Born and Ernst Pascual Jordan, the formula was not a differential equation but a matrix: an array consisting of an infinite number of rows, each row consisting of an infinite number of quantities. Matrix mechanics introduced infinite matrices to represent the position and momentum of an electron inside an atom. Also, different matrices exist, one for each observable physical property associated with the motion of an electron, such as energy, position, momentum, and angular momentum. These matrices, like Schrödinger's differential equations, could be solved; in other words, they could be manipulated to produce predictions as to the frequencies of the lines in the hydrogen spectrum and other observable quantities. Like wave mechanics, matrix mechanics was in agreement with the earlier quantum theory for processes in which the earlier quantum theory agreed with experiment; it was also useful in explaining phenomena that earlier quantum theory could not explain.

**The
Meaning of Quantum Mechanics **

Schrödinger subsequently succeeded in showing that wave mechanics and matrix mechanics are different mathematical versions of the same theory, now called quantum mechanics. Even for the simple hydrogen atom, which consists of two particles, both mathematical interpretations are extremely complex. The next simplest atom, helium, has three particles, and even in the relatively simple mathematics of classical dynamics, the three-body problem (that of describing the mutual interactions of three separate bodies) is not entirely soluble. The energy levels can be calculated accurately, however, even if not exactly. In applying quantum-mechanics mathematics to relatively complex situations, a physicist can use one of a number of mathematical formulations. The choice depends on the convenience of the formulation for obtaining suitable approximate solutions.

Although quantum mechanics describes the atom purely in terms of mathematical interpretations of observed phenomena, a rough verbal description can be given of what the atom is now thought to be like. Surrounding the nucleus is a series of stationary waves; these waves have crests at certain points, each complete standing wave representing an orbit. The absolute square of the amplitude of the wave at any point is a measure of the probability that an electron will be found at that point at any given time. Thus, an electron can no longer be said to be at any precise point at any given time.

**The
Uncertainty Principle **

The impossibility of pinpointing an electron at any precise time was analyzed by Heisenberg, who in 1927 formulated the uncertainty principle. This principle states the impossibility of simultaneously specifying the precise position and momentum of any particle. In other words, the more accurately a particle's momentum is measured and known, the less accuracy there can be in the measurement and knowledge of its position. This principle is also fundamental to the understanding of quantum mechanics as it is generally accepted today: The wave and particle character of electromagnetic radiation can be understood as two complementary properties of radiation.

Another
way of expressing the uncertainty principle is that the
wavelength of a quantum mechanical principle is inversely
proportional to its momentum. As atoms are cooled they slow down
and their corresponding wavelength grows larger. At a low enough
temperature this wavelength is predicted to exceed the spacing
between particles, causing atoms to overlap, becoming
indistinguishable, and melding into a single quantum state. In
1996 a team of Colorado scientists, led by National Institutes of
Standards and Technology physicist Eric Cornell and University of
Colorado physicist Carl Weiman, cooled rubidium atoms to a
temperature so low that the particles entered this merged state,
known as a *Bose-Einstein condensate.* The condensate
essentially behaves like one atom even though it is made up of
thousands.

**Results
of Quantum Theory **

Quantum mechanics solved all of the great difficulties that troubled physicists in the early years of the 20th century. It gradually enhanced the understanding of the structure of matter, and it provided a theoretical basis for the understanding of atomic structure and the phenomenon of spectral lines: Each spectral line corresponds to the energy of a photon transmitted or absorbed when an electron makes a transition from one energy level to another. The understanding of chemical bonding was fundamentally transformed by quantum mechanics and came to be based on Schrödinger's wave equations. New fields in physics emergedsolid-state physics, condensed matter physics, superconductivity, nuclear physics, and elementary particle physics that all found a consistent basis in quantum mechanics.

**Further
Developments **

In the years since 1925, no fundamental deficiencies have been found in quantum mechanics, although the question of whether the theory should be accepted as complete has come under discussion. In the 1930s the application of quantum mechanics and special relativity to the theory of the electron allowed the British physicist Paul Dirac to formulate an equation that referred to the existence of the spin of the electron. It further led to the prediction of the existence of the positron, which was experimentally verified by the American physicist Carl David Anderson.

The
application of quantum mechanics to the subject of
electromagnetic radiation led to explanations of many phenomena,
such as bremsstrahlung (German, "braking radiation,"
the radiation emitted by electrons slowed down in matter) and
pair production (the formation of a positron and an electron when
electromagnetic energy interacts with matter). It also led to a
grave problem, however, called the divergence difficulty: Certain
parameters, such as the so-called bare mass and bare charge of
electrons, appear to be infinite in Dirac's equations. (The terms
*bare mass* and *bare charge* refer to hypothetical
electrons that do not interact with any matter or radiation; in
reality, electrons interact with their own electric field.) This
difficulty was partly resolved in 1947-49 in a program called
renormalization, developed by the Japanese physicist Shin'ichir</font><font
color="#000000" size="2" face="MS Reference 1">o Tomonaga, the American
physicists Julian S. Schwinger and Richard Feynman, and the
British physicist Freeman Dyson. In this program, the bare mass
and charge of the electron are chosen to be infinite in such a
way that other infinite physical quantities are canceled out in
the equations. Renormalization greatly increased the accuracy
with which the structure of atoms could be calculated from first
principles.

**Future
Prospects **

Quantum mechanics underlies current attempts to account for the strong nuclear force and to develop a unified theory for all the fundamental interactions of matter . Nevertheless, doubts exist about the completeness of quantum theory. The divergence difficulty, for example, is only partly resolved. Just as Newtonian mechanics was eventually amended by quantum mechanics and relativity, many scientistsand Einstein was among themare convinced that quantum theory will also undergo profound changes in the future. Great theoretical difficulties exist, for example, between quantum mechanics and chaos theory, which began to develop rapidly in the 1980s. Ongoing efforts are being made by theorists such as the British physicist Stephen Hawking, to develop a system that encompasses both relativity and quantum mechanics.