In physics, a quantum (plural quanta) is the minimum amount of any physical entity (

physical property A physical property is any property that is measurable, whose value describes a state of a physical system. The changes in the physical properties of a system can be used to describe its changes between momentary states. Physical properties are ...

) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantization". This means that the magnitude of the physical property can take on only discrete values consisting of integer multiples of one quantum. For example, a photon is a single quantum of light (or of any other form of electromagnetic radiation). Similarly, the energy of an electron bound within an atom is quantized and can exist only in certain discrete values. (Atoms and matter in general are stable because electrons can exist only at discrete energy levels within an atom.) Quantization is one of the foundations of the much broader physics of quantum mechanics. Quantization of energy and its influence on how energy and matter interact ( quantum electrodynamics) is part of the fundamental framework for understanding and describing nature.

Etymology and discovery

The word ''quantum'' is the neuter singular of the Latin interrogative adjective ''quantus'', meaning "how much". "Quanta", the neuter plural, short for "quanta of electricity" (electrons), was used in a 1902 article on the photoelectric effect by Philipp Lenard, who credited Hermann von Helmholtz for using the word in the area of electricity. However, the word ''quantum'' in general was well known before 1900, e.g. ''quantum'' was used in E.A. Poe's

Loss of Breath "Loss of Breath", also known as "Loss of Breath: A Tale Neither In Nor Out of 'Blackwood' ", is a short story written by American author Edgar Allan Poe under the pseudonym "Littleton Barry". A satirical tale, the story is narrated by Mr ...

. It was often used by

physicians A physician (American English), medical practitioner (Commonwealth English), medical doctor, or simply doctor, is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through th ...

, such as in the term ''

quantum satis Quantum satis (abbreviation q.s. or Q.S.) is a Latin term meaning the amount which is enough. It has its origins as a quantity specification in medicine and pharmacology, where a similar term ''quantum sufficit'' ("as much as is sufficient") has b ...

'', "the amount which is enough". Both Helmholtz and Julius von Mayer were physicians as well as physicists. Helmholtz used ''quantum'' with reference to heat in his article on Mayer's work, and the word ''quantum'' can be found in the formulation of the first law of thermodynamics by Mayer in his letter dated July 24, 1841. In 1901, Max Planck used ''quanta'' to mean "quanta of matter and electricity", gas, and heat. In 1905, in response to Planck's work and the experimental work of Lenard (who explained his results by using the term ''quanta of electricity''), Albert Einstein suggested that

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

existed in spatially localized packets which he called "quanta of light" ("Lichtquanta").. A partia
English translation
is available from Wikisource. The concept of quantization of radiation was discovered in 1900 by Max Planck, who had been trying to understand the emission of radiation from heated objects, known as

black-body radiation Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spect ...

. By assuming that energy can be absorbed or released only in tiny, differential, discrete packets (which he called "bundles", or "energy elements"), Planck accounted for certain objects changing color when heated. On December 14, 1900, Planck reported his findings to the

German Physical Society The German Physical Society (German: , DPG) is the oldest organisation of physicists. The DPG's worldwide membership is cited as 60,547, as of 2019, making it the largest physics society in the world. It holds an annual conference () and multiple ...

, and introduced the idea of quantization for the first time as a part of his research on black-body radiation. As a result of his experiments, Planck deduced the numerical value of ''h'', known as the Planck constant, and reported more precise values for the unit of electrical charge and the Avogadro–Loschmidt number, the number of real molecules in a mole, to the German Physical Society. After his theory was validated, Planck was awarded the Nobel Prize in Physics for his discovery in 1918.

Quantization

While quantization was first discovered in electromagnetic radiation, it describes a fundamental aspect of energy not just restricted to photons. In the attempt to bring theory into agreement with experiment, Max Planck postulated that electromagnetic energy is absorbed or emitted in discrete packets, or quanta.Modern Applied Physics-Tippens third edition; McGraw-Hill.

Etymology and discovery

Quantization

See also

References

Further reading