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Max Planck
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=== Black-body radiation === [[File:Max Planck 1901.GIF|thumb|Planck in 1901]] In 1894, Planck turned his attention to the problem of [[black-body radiation]]. The problem had been stated by Kirchhoff in 1859: "how does the intensity of the electromagnetic radiation emitted by a [[black body]] (a perfect absorber, also known as a cavity radiator) depend on the [[frequency]] of the radiation (i.e., the color of the light) and the temperature of the body?". The question had been explored experimentally, but no theoretical treatment had agreed with the experimentally observed evidence. [[Wilhelm Wien]] proposed [[Wien approximation|Wien's law]], which correctly predicted the behaviour at high frequencies, but failed at low frequencies. The [[Rayleigh–Jeans law]], another approach to the problem, agreed with experimental results at low frequencies, but created what was later known as the "[[ultraviolet catastrophe]]" at high frequencies, as predicted by [[classical physics]]. However, contrary to many textbooks, this was not a motivation for Planck.<ref name="Kragh">For a solid approach to the complexity of Planck's intellectual motivations for the quantum, for his reluctant acceptance of its implications, see Helge Kragh, [https://physicsworld.com/a/max-planck-the-reluctant-revolutionary/ Max Planck: the reluctant revolutionary] {{Webarchive|url=https://web.archive.org/web/20181105061830/https://physicsworld.com/a/max-planck-the-reluctant-revolutionary/ |date=5 November 2018 }}, ''Physics World''. December 2000.</ref> Planck's first proposed solution to the problem in 1899 followed from what he called the "principle of elementary disorder", which allowed him to derive Wien's law from a number of assumptions about the [[entropy]] of an ideal oscillator, creating what was referred to as the Wien–Planck law. Soon, however, it was found that experimental evidence did not confirm the new law at all, to Planck's frustration. He revised his approach and now derived the first version of the famous [[Planck's law of black-body radiation|Planck black-body radiation law]], which described clearly the experimentally observed black-body spectrum. It was first proposed in a meeting of the DPG on 19 October 1900 and published in 1901. (This first derivation did not include energy quantisation, and did not use [[statistical mechanics]], to which he held an aversion.) In November 1900 Planck revised this first version, now relying on [[Ludwig Boltzmann|Boltzmann]]'s statistical interpretation of the [[second law of thermodynamics]] as a way of gaining a more fundamental understanding of the principles behind his radiation law. Planck was deeply suspicious of the philosophical and physical implications of such an interpretation of Boltzmann's approach; thus his recourse to them was, as he later put it, "an act of despair ... I was ready to sacrifice any of my previous convictions about physics".<ref name="Kragh"/> The central assumption behind his new derivation, presented to the DPG on 14 December 1900, was the supposition, now known as the [[Planck postulate]], that electromagnetic energy could be emitted only in [[Quantization (physics)|quantized]] form, in other words, the energy could only be a multiple of an elementary unit: : <math>E = h\nu</math> where {{math|''h''}} is the [[Planck constant]], also known as Planck's action quantum (introduced already in 1899), and {{math|''ν''}} is the frequency of the radiation. Note that the elementary units of energy discussed here are represented by {{math|''hν''}} and not simply by {{math|''ν''}}. Physicists now call these quanta photons, and a photon of frequency {{math|''ν''}} will have its own specific and unique energy. The total energy at that frequency is then equal to {{math|''hν''}} multiplied by the number of photons at that frequency. [[File:Max Planck Nobel 1918.jpg|thumb|left|Planck in 1918, the year he was awarded the [[Nobel Prize in Physics]] for his work on [[Quantum mechanics|quantum theory]]]] At first Planck considered that quantisation was only "a purely formal assumption ... actually I did not think much about it ..."; nowadays this assumption, incompatible with [[classical physics]], is regarded as the birth of [[quantum physics]] and the greatest intellectual accomplishment of Planck's career. ([[Ludwig Boltzmann|Boltzmann]] had been discussing in a theoretical paper in 1877 the possibility that the energy states of a physical system could be discrete). The discovery of the Planck constant enabled him to define a new universal set of [[Planck units|physical units]] (such as the Planck length and the Planck mass), all based on fundamental physical constants, upon which much of quantum theory is based. In a discussion with his son in December 1918 Planck described his discovery as 'a discovery of the first rank, comparable perhaps only to the discoveries of Newton'.<ref>Egginton, William, ''[https://us5.campaign-archive.com/?e=01376ceb1d&u=6557fc90400ccd10e100a13f4&id=dc364d6f8b The Rigor of Angels: Max Planck unleashed a revolution in physics]'', pp. 52–54, Pantheon, Delancy Place, 2023</ref> In recognition of Planck's fundamental contribution to a new branch of physics, he was awarded the Nobel Prize in Physics for 1918; (he received the award in 1919).<ref>Kragh, Helge (1 December 2000), Max Planck: the reluctant revolutionary, PhysicsWorld.com</ref><ref>{{cite web|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1918/|title=The Nobel Prize in Physics 1918|website=www.nobelprize.org|access-date=11 June 2017|archive-url=https://web.archive.org/web/20170609233007/http://www.nobelprize.org/nobel_prizes/physics/laureates/1918/|archive-date=9 June 2017|url-status=live}}</ref> Subsequently, Planck tried to grasp the meaning of energy quanta, but to no avail. "My unavailing attempts to somehow reintegrate the action quantum into classical theory extended over several years and caused me much trouble." Even several years later, other physicists such as [[John William Strutt, 3rd Baron Rayleigh|Rayleigh]], [[James Jeans|Jeans]], and [[Hendrik Lorentz|Lorentz]] set the Planck constant to zero in order to align with classical physics, but Planck knew well that this constant had a precise nonzero value. "I am unable to understand Jeans' stubbornness – he is an example of a theoretician as should never be existing, the same as [[Hegel]] was for philosophy. So much the worse for the facts if they don't fit."<ref>Heilbron, 2000, [https://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA8 p. 8] {{Webarchive|url=https://web.archive.org/web/20180417074508/https://books.google.com/books?id=d5zKH2Bx2AwC&pg=PA8 |date= 2018 }}</ref> [[Max Born]] wrote about Planck: "He was, by nature, a conservative mind; he had nothing of the revolutionary and was thoroughly skeptical about speculations. Yet his belief in the compelling force of logical reasoning from facts was so strong that he did not flinch from announcing the most revolutionary idea which ever has shaken physics."<ref name="frs"/>
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