# 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)]quanta See quantum

# User Contributed Dictionary

## English

### Noun

quanta- Plural of quantum.

#### Antonyms

## Italian

### Adjective

quanta- feminine of quanto

# 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 black-body 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 red-hot, 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 black-body radiation formula, being
the very first piece of quantum mechanics, appeared Sunday evening
October 7, 1900, in a so-called back-of-the-envelope 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 Avogadro-Loschmidt 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, Springer-Verlag 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

quanta in Belarusian (Tarashkevitsa):
Квант

quanta in Bosnian: Kvant

quanta in Bulgarian: Квант

quanta in Catalan: Quàntum

quanta in German: Quant

quanta in Modern Greek (1453-): Κβάντο

quanta in Spanish: Cuanto

quanta in French: Quantum

quanta in Galician: Quanto

quanta in Croatian: Kvant

quanta in Italian: Quanto

quanta in Lithuanian: Kvantas

quanta in Latvian: Kvants

quanta in Hungarian: Kvantum

quanta in Dutch: Kwantum

quanta in Japanese: 量子

quanta in Norwegian: Kvant

quanta in Polish: Kwant

quanta in Russian: Квант

quanta in Albanian: Kuanti

quanta in Slovak: Kvant

quanta in Serbian: Квант

quanta in Finnish: Kvantti

quanta in Swedish: Kvantum

quanta in Ukrainian: Квант

quanta in Chinese: 量子