Dictionary Definition
quantum
Noun
1 a discrete amount of something that is
analogous to the quantum in quantum theory
2 (physics) the smallest discrete quantity of
some physical property that a system can possess (according to
quantum theory) [also: quanta (pl)]
User Contributed Dictionary
English
Pronunciation
 /ˈkwan.təm/
 /"kwan.t@m/
Noun
 A property defineable as a number; a quantity.
 The smallest
possible, and therefore
indivisible, unit of
a given quantity or
quantifiable phenomenon.

 Planck's constant is the quantum of angular momentum.
 The photon is the quantum of light.

Related terms
Translations
indivisible unit of a given quantity
Adjective
 Of a change, significant
 Involving quanta
Derived terms
 quantum algorithm
 quantum bit
 quantum bogodynamics
 quantum brain dynamics
 quantum calculus
 quantum cascade laser
 quantum channel
 quantum chaos
 quantum chemistry
 quantum chromodynamics
 quantum circuit
 quantum computer
 quantum computing
 quantum cryptography
 quantum darwinism
 quantum decoherence
 quantum degeneracy
 quantum dense coding
 quantum dot
 quantum effect device
 quantum efficiency
 quantum electrochemistry
 quantum electrodynamics
 quantum electronics
 quantum entanglement
 quantum field theory
 quantum fingerprinting
 quantum flavordynamics
 quantum fluctuation
 quantum gate
 quantum gauge theory
 quantum geometry
 quantum gravity
 quantum group
 quantum gyroscope
 quantum Hall effect
 quantum harmonic oscillator
 quantum heterostructure
 quantum history
 quantum hydrodynamics
 quantum immortality
 quantum indeterminacy
 quantum inequality
 quantum information
 quantum jump
 quantum leap
 quantum level
 quantum libet
 quantum limit
 quantum link
 quantum mechanics
 quantum network
 quantum neural network
 quantum number
 quantum ontology
 quantum operation
 quantum optics
 quantum phase transition
 quantum physics
 quantum programming
 quantum psychology
 quantum randomness
 quantum register
 quantum scalar field
 quantum solvent
 quantum sort
 quantum state
 quantum statistical mechanics
 quantum suicide
 quantum superposition
 quantum teleportation
 quantum theory
 quantum tomography
 quantum valebant
 quantum vibration
 quantum virtual machine
 quantum waveform generator
 quantum well
 quantum wire
 quantum yield
 quantum Zeno effect
Translations
significant (of a change)
 Finnish: suuri, merkittävä, määrällinen
involving quanta
 Italian: quantico, quantistico
Italian
Noun
quantum (plural: quanta) quantum
Synonyms
Latin
Adjective
(quantus, quanta, quantum) how much, how many
Declension
First and second declensions.Extensive Definition
In physics, a quantum (plural:
quanta) is an indivisible entity of a quantity that has the
same units as the Planck
constant and is related to both energy and momentum of elementary
particles of matter
(called fermions) and
of photons and other
bosons. The word comes
from the Latin "quantus," for
"how much." Behind this, one finds the fundamental notion that a
physical property may be "quantized", referred to as "quantization".
This means that the magnitude can take on only certain discrete
numerical values,
rather than any value, at least within a range. There is a related
term of quantum
number.
A photon is often referred to as a
"light
quantum." The energy of an electron bound to an atom (at rest) is said to be
quantized, which results in the stability of atoms, and of matter in general. But these
terms can be a little misleading, because what is quantized is this
Planck's
constant quantity whose units can be viewed as either energy
times time or momentum times distance.
Usually referred to as quantum "mechanics," it is
regarded by virtually every professional physicist as the most
fundamental framework we have for understanding and describing
nature at the infinitesimal level, for the very practical reason
that it works. It is "in the nature of things", not a more or less
arbitrary human preference.
Development of quantum theory
Quantum
theory, the branch of physics which is based on quantization,
began in 1900
when Max
Planck published his theory explaining the emission
spectrum of black bodies.
In that paper Planck used the Natural
system of units he
invented the previous year. The
consequences of the differences between classical
and quantum
mechanics quickly became obvious. But it was not until 1926, by the work of
Werner
Heisenberg, Erwin
Schrödinger, and others, that quantum
mechanics became correctly formulated and understood
mathematically. Despite tremendous experimental success, the
philosophical interpretations of quantum theory are still widely
debated.
Planck was reluctant to accept the new idea of
quantization, as were many others. But, with no acceptable
alternative, he continued to work with the idea, and found his
efforts were well received. Eighteen years later, when he accepted
the Nobel
Prize in Physics for his contributions, he called it "a few
weeks of the most strenuous work" of his life. During those few
weeks, he even had to discard much of his own theoretical work from
the preceding years. Quantization turned out to be the only way to
describe the new and detailed experiments which were just then
being performed. He did this practically overnight, openly
reporting his change of mind to his scientific colleagues, in the
October, November, and December meetings of the German
Physical Society, in Berlin, where the
black body work was being intensely discussed. In this way, careful
experimentalists (including Friedrich
Paschen, O.R. Lummer,
Ernst
Pringsheim, Heinrich
Rubens, and F.
Kurlbaum), and a reluctant theorist, ushered in a momentous
scientific revolution.
The quantum blackbody radiation formula
When a body is heated, it emits radiant heat, a form of electromagnetic radiation in the infrared region of the EM spectrum. All of this was well understood at the time, and of considerable practical importance. When the body becomes redhot, the red wavelength parts start to become visible. This had been studied over the previous years, as the instruments were being developed. However, most of the heat radiation remains infrared, until the body becomes as hot as the surface of the Sun (about 6000 °C, where most of the light is green in color). This was not achievable in the laboratory at that time. What is more, measuring specific infrared wavelengths was only then becoming feasible, due to newly developed experimental techniques. Until then, most of the electromagnetic spectrum was not measurable, and therefore blackbody emission had not been mapped out in detail.The quantum blackbody radiation formula, being
the very first piece of quantum mechanics, appeared Sunday evening
October 7, 1900, in a socalled backoftheenvelope calculation by
Planck. It was based on a report by Rubens
(visiting with his wife) of the very latest experimental findings
in the infrared. Later that evening, Planck sent the formula on a
postcard, which Rubens received the following morning. A couple of
days later, he informed Planck that it worked perfectly. At first,
it was just a fit to the data; only later did it turn out to
enforce quantization.
This second step was only possible due to a
certain amount of luck (or skill, even though Planck himself called
it "a fortuitous guess at an interpolation formula"). It was during
the course of polishing the mathematics of his formula that Planck
stumbled upon the beginnings of Quantum Theory. Briefly stated, he
had two mathematical expressions:
 (i) from the previous work on the red parts of the spectrum, he had x;
 (ii) now, from the new infrared data, he got x².
Combining these as x(a+x), he still has x,
approximately, when x is much smaller than a (the red end of the
spectrum); but now also x² (again approximately) when x is much
larger than a (in the infrared). The formula for the energy E, in a
single mode of radiation at frequency λ, and temperature T, can be
written
 E = \frac
This is (essentially) what is being compared with
the experimental measurements. There are two parameters to
determine from the data, written in the present form by the symbols
used today: h is the new Planck's
constant, and k is Boltzmann's
constant. Both have now become fundamental in physics, but that
was by no means the case at the time. The "elementary quantum of
energy" is hλ. But such a unit does not normally exist, and is not
required for quantization.
Beyond electromagnetic radiation
While quantization was first discovered in electromagnetic radiation, it describes a fundamental aspect of energy not just restricted to photons.The birthday of quantum mechanics
From the experiments, Planck deduced the numerical values of h and k. Thus he could report, in the German Physical Society meeting on December 14, 1900, where quantization (of energy) was revealed for the first time, values of the AvogadroLoschmidt number, the number of real molecules in a mole, and the unit of electrical charge, which were more accurate than those known until then. This event has been referred to as "the birth of quantum mechanics".See also
 Quantum mechanics
 Quantum state
 Quantum number
 Quantum cryptography
 Quantum electronics
 Quantum computer
 Quantum entanglement
 Quantum coherence
 Quantum immortality
 Quantum lithography
 Quantum metrology
 Quantum sensor
 Quantum dot
 Magnetic flux quantum
 Quantum cellular automata
 Quantization
 Subatomic particle
 Elementary particle
 Photon polarization
References
 J. Mehra and H. Rechenberg, The Historical Development of Quantum Theory, Vol.1, Part 1, SpringerVerlag New York Inc., New York 1982.
 Lucretius, "On the Nature of the Universe", transl. from the Latin by R.E. Latham, Penguin Books Ltd., Harmondsworth 1951. There are, of course, many translations, and the translation's title varies. Some put emphasis on how things work, others on what things are found in nature.
 M. Planck, A Survey of Physical Theory, transl. by R. Jones and D.H. Williams, Methuen & Co., Ltd., London 1925 (Dover editions 1960 and 1993) including the Nobel lecture.
Notes
quantum in Belarusian (Tarashkevitsa):
Квант
quantum in Bosnian: Kvant
quantum in Bulgarian: Квант
quantum in Catalan: Quàntum
quantum in German: Quant
quantum in Modern Greek (1453): Κβάντο
quantum in Spanish: Cuanto
quantum in French: Quantum
quantum in Galician: Quanto
quantum in Croatian: Kvant
quantum in Italian: Quanto
quantum in Lithuanian: Kvantas
quantum in Latvian: Kvants
quantum in Hungarian: Kvantum
quantum in Dutch: Kwantum
quantum in Japanese: 量子
quantum in Norwegian: Kvant
quantum in Polish: Kwant
quantum in Russian: Квант
quantum in Albanian: Kuanti
quantum in Slovak: Kvant
quantum in Serbian: Квант
quantum in Finnish: Kvantti
quantum in Swedish: Kvantum
quantum in Ukrainian: Квант
quantum in Chinese: 量子
Synonyms, Antonyms and Related Words
ASA scale, British candle, Hefner candle,
Scheiner scale, aggregate, allotment, allowance, amount, amplitude, apportionment, atomerg, big end, bigger half,
bit, bite, bougie decimale, budget, bulk, calorie, candle, candle lumen, candle
power, candlefoot, candlehour, candlemeter, chunk, commission, contingent, cut, deal, decimal candle, destiny, dinamode, dividend, dole, dyne, end, energid, equal share, erg, exposure meter, extent, fate, flux, footcandle, footpound,
force, half, halver, helping, horsepowerhour,
horsepoweryear, intensity, interest, international candle,
joule, kilogrammeter,
kilowatthour, lamphour, light, light meter, light quantum,
lot, lumen, lumen meter, lumenhour,
lumeter, luminous flux,
luminous intensity, luminous power, lux, magnitude, mass, matter, measure, measurement, meed, mess, modicum, moiety, numbers, part, percentage, photon, piece, portion, proportion, quantity, quota, rakeoff, ration, segment, share, slice, small share, stake, stock, strength, substance, sum, total, unit of flux, unit of
light, whole