In physics, a **quantum** (plural **quanta**) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantization".^{ [1] } 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.

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,^{ [2] } e.g. *quantum* was used in E.A. Poe's Loss of Breath. It was often used by physicians, such as in the term * quantum satis *, "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^{ [3] } 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^{ [4] } dated July 24, 1841.

In 1901, Max Planck used *quanta* to mean "quanta of matter and electricity",^{ [5] } gas, and heat.^{ [6] } 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 existed in spatially localized packets which he called "quanta of light" ("Lichtquanta").^{ [7] }

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. By assuming that energy can be absorbed or released only in tiny, differential, discrete packets (which he called "bundles", or "energy elements"),^{ [8] } Planck accounted for certain objects changing color when heated.^{ [9] } On December 14, 1900, Planck reported his findings to the German Physical Society, and introduced the idea of quantization for the first time as a part of his research on black-body radiation.^{ [10] } 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.

While quantization was first discovered in electromagnetic radiation, it describes a fundamental aspect of energy not just restricted to photons.^{ [11] } 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.^{ [12] }

- Particle
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- Introduction to quantum mechanics
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- Photon polarization
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**Max Karl Ernst Ludwig Planck** was a German theoretical physicist whose discovery of energy quanta won him the Nobel Prize in Physics in 1918.

A **photon** is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s. The photon belongs to the class of bosons.

The **photoelectric effect** is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid state and quantum chemistry to draw inferences about the properties of atoms, molecules and solids. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission.

**Wilhelm Carl Werner Otto Fritz Franz Wien** was a German physicist who, in 1893, used theories about heat and electromagnetism to deduce Wien's displacement law, which calculates the emission of a blackbody at any temperature from the emission at any one reference temperature.

In physics, **Planck's law** describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T, when there is no net flow of matter or energy between the body and its environment.

The **ultraviolet catastrophe**, also called the **Rayleigh–Jeans catastrophe**, was the prediction of late 19th century/early 20th century classical physics that an ideal black body at thermal equilibrium would emit an unbounded quantity of energy as wavelength decreased into the ultraviolet range.

The **emission spectrum** of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a transition from a high energy state to a lower energy state. The photon energy of the emitted photon is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum. Each element's emission spectrum is unique. Therefore, spectroscopy can be used to identify elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.

* Annalen der Physik* is one of the oldest scientific journals on physics; it has been published since 1799. The journal publishes original, peer-reviewed papers on experimental, theoretical, applied, and mathematical physics and related areas. The editor-in-chief is Stefan Hildebrandt. Prior to 2008, its ISO 4 abbreviation was

The **old quantum theory** is a collection of results from the years 1900–1925 which predate modern quantum mechanics. The theory was never complete or self-consistent, but was rather a set of heuristic corrections to classical mechanics. The theory is now understood as the semi-classical approximation to modern quantum mechanics. The main and final accomplishments of the old quantum theory were the determination of the modern form of the periodic table by Edmund Stoner and the Pauli Exclusion Principle which were both premised on the Arnold Sommerfeld enhancements to the Bohr model of the atom.

**Friedrich Hasenöhrl** was an Austrian physicist.

The ** annus mirabilis papers** are the four papers that Albert Einstein published in

A **first quantization** of a physical system is a possibly semiclassical treatment of quantum mechanics, in which particles or physical objects are treated using quantum wave functions but the surrounding environment is treated classically.

The **history of quantum mechanics** is a fundamental part of the history of modern physics. Quantum mechanics' history, as it interlaces with the history of quantum chemistry, began essentially with a number of different scientific discoveries: the 1838 discovery of cathode rays by Michael Faraday; the 1859–60 winter statement of the black-body radiation problem by Gustav Kirchhoff; the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system could be *discrete*; the discovery of the photoelectric effect by Heinrich Hertz in 1887; and the 1900 quantum hypothesis by Max Planck that any energy-radiating atomic system can theoretically be divided into a number of discrete "energy elements" *ε* such that each of these energy elements is proportional to the frequency *ν* with which each of them individually radiate energy, as defined by the following formula:

The **Bohr–Kramers–Slater theory** was perhaps the final attempt at understanding the interaction of matter and electromagnetic radiation on the basis of the so-called old quantum theory, in which quantum phenomena are treated by imposing quantum restrictions on classically describable behaviour. It was advanced in 1924, and sticks to a *classical* wave description of the electromagnetic field. It was perhaps more a research program than a full physical theory, the ideas that are developed not being worked out in a quantitative way. The purpose of BKS Theory was to disprove Einstein's hypothesis of the light quantum.

**Alfred Heinrich Bucherer** was a German physicist, who is known for his experiments on relativistic mass. He also was the first who used the phrase "theory of relativity" for Einstein's theory of special relativity.

The **Planck constant**, or **Planck's constant**, is a fundamental physical constant of foundational importance in quantum mechanics. The constant gives the relationship between the energy of a photon and its frequency, and by the mass-energy equivalence, the relationship between mass and frequency. Specifically, a photon's energy is equal to its frequency multiplied by the Planck constant. The constant is generally denoted by . The **reduced Planck constant**, or **Dirac constant**, equal to the constant divided by , is denoted by .

Physics is a scientific discipline that seeks to construct and experimentally test theories of the physical universe. These theories vary in their scope and can be organized into several distinct branches, which are outlined in this article.

In 1922, American physicist William Duane presented a discrete momentum-exchange model of the reflection of X-ray photons by a crystal lattice. Duane showed that such a model gives the same scattering angles as the ones calculated via a wave diffraction model, see Bragg's Law.

**Jun Ishiwara** or **Atsushi Ishihara** was a Japanese theoretical physicist, known for his works on the electronic theory of metals, the theory of relativity and quantum theory. Being the only Japanese scientist who made an original contribution to the old quantum theory, in 1915, independently of other scientists, he formulated quantization rules for systems with several degrees of freedom.

A hallmark of Albert Einstein's career was his use of visualized thought experiments as a fundamental tool for understanding physical issues and for elucidating his concepts to others. **Einstein's thought experiments** took diverse forms. In his youth, he mentally chased beams of light. For special relativity, he employed moving trains and flashes of lightning to explain his most penetrating insights. For general relativity, he considered a person falling off a roof, accelerating elevators, blind beetles crawling on curved surfaces and the like. In his debates with Niels Bohr on the nature of reality, he proposed imaginary devices intended to show, at least in concept, how the Heisenberg uncertainty principle might be evaded. In a profound contribution to the literature on quantum mechanics, Einstein considered two particles briefly interacting and then flying apart so that their states are correlated, anticipating the phenomenon known as quantum entanglement.

- ↑ Wiener, N. (1966).
*Differential Space, Quantum Systems, and Prediction*. Cambridge: The Massachusetts Institute of Technology Press - ↑ E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
- ↑ E. Helmholtz, Robert Mayer's Priorität Archived 2015-09-29 at the Wayback Machine (in German)
- ↑ Herrmann, Armin (1991). "Heimatseite von Robert J. Mayer" (in German). Weltreich der Physik, GNT-Verlag. Archived from the original on 1998-02-09.
`{{cite web}}`

: CS1 maint: bot: original URL status unknown (link) - ↑ Planck, M. (1901). "Ueber die Elementarquanta der Materie und der Elektricität".
*Annalen der Physik*(in German).**309**(3): 564–566. Bibcode:1901AnP...309..564P. doi:10.1002/andp.19013090311. - ↑ Planck, Max (1883). "Ueber das thermodynamische Gleichgewicht von Gasgemengen".
*Annalen der Physik*(in German).**255**(6): 358–378. Bibcode:1883AnP...255..358P. doi:10.1002/andp.18832550612. - ↑ Einstein, A. (1905). "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt" (PDF).
*Annalen der Physik*(in German).**17**(6): 132–148. Bibcode:1905AnP...322..132E. doi: 10.1002/andp.19053220607 .. A partial English translation is available from Wikisource. - ↑ Max Planck (1901). "Ueber das Gesetz der Energieverteilung im Normalspectrum (On the Law of Distribution of Energy in the Normal Spectrum)".
*Annalen der Physik*.**309**(3): 553. Bibcode:1901AnP...309..553P. doi: 10.1002/andp.19013090310 . Archived from the original on 2008-04-18. - ↑ Brown, T., LeMay, H., Bursten, B. (2008).
*Chemistry: The Central Science*Upper Saddle River, NJ: Pearson Education ISBN 0-13-600617-5 - ↑ Klein, Martin J. (1961). "Max Planck and the beginnings of the quantum theory".
*Archive for History of Exact Sciences*.**1**(5): 459–479. doi:10.1007/BF00327765. S2CID 121189755. - ↑ Melville, K. (2005, February 11). Real-World Quantum Effects Demonstrated
- ↑ Modern Applied Physics-Tippens third edition; McGraw-Hill.

- B. Hoffmann,
*The Strange Story of the Quantum*, Pelican 1963.^{[ ISBN missing ]} - Lucretius,
*On the Nature of the Universe*, transl. from the Latin by R.E. Latham, Penguin Books Ltd., Harmondsworth 1951. - J. Mehra and H. Rechenberg,
*The Historical Development of Quantum Theory*, Vol.1, Part 1, Springer-Verlag, New York 1982.^{[ ISBN missing ]} - 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.^{[ ISBN missing ]} - Rodney, Brooks (2011)
*Fields of Color: The theory that escaped Einstein*. Allegra Print & Imaging.^{[ ISBN missing ]}

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