Max Planck

:This article is about Planck, the German physicist. For the proposed European Space Agency artificial satellite, see "Planck (satellite)".

Black-body radiation

In 1894 Planck turned his attention to the problem of black-body radiation. He had been commissioned by electric companies to discover how to create the most light from light-bulbs with the minimum energy. The problem had already 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 (e.g., the colour of the light) and the temperature of the body? The question had been explored experimentally, but the Rayleigh-Jeans law, derived from classical physics, failed to explain the observed behavior at high frequencies. Wilhelm Wien proposed Wien's law, which correctly predicted the behavior at high frequencies, but failed at low frequencies.

Related Topics:
Black-body radiation - Black body - Frequency - Rayleigh-Jeans law - Wilhelm Wien - Wien's law

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Planck interpolated between the laws of Wein and Rayleigh Jeans but found that he could only derive a satisfactory equation using what he thought was merely a mathematical trick, namely that light is only emitted in "packets". He did not for many years believe that these packets, known as quanta, corresponded with reality.

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His famous Planck black-body radiation law described the experimentally observed black-body spectrum very well; it was first proposed in a meeting of the DPG on October 19, 1900 and published in 1901.

Related Topics:
Planck black-body radiation law - October 19 - 1900

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By December 14, 1900 he was already able to present a theoretical derivation of the law, but this required him to use ideas from statistical mechanics, as introduced by Boltzmann. So far he had been holding a strong aversion against any statistical interpretation of the Second law of thermodynamics which he regarded as of axiomatic nature: "... an act of despair ... I was ready to sacrifice any of my previous convictions about physics ..." The central assumption behind his derivation was the supposition that the electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit E = h u, where h is Planck's constant, also known as Planck's action quantum (introduced already in 1899), and u is the frequency of the radiation.

Related Topics:
December 14 - 1900 - Statistical mechanics - Boltzmann - Second law of thermodynamics - Quantize - Planck's constant

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At first Planck considered that the quantisation was only as "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 (however, Ludwig Boltzmann had already in 1877 in a theoretical paper been discussing the possibility that the energy states of a physical system could be discrete). It was in recognition of this accomplishement that Planck was awarded the Nobel prize for physics in 1918.

Related Topics:
Classical physics - Quantum physics - Ludwig Boltzmann - Nobel prize

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The discovery of Planck's constant enabled him to define a new universal set of physical units (such as the Planck length and the Planck mass), all based on fundamental physical constants.

Related Topics:
A new universal set of physical units - Planck length - Planck mass - Physical constant

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Subsequently, Planck tried to grasp the meaning of the 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 like Rayleigh, Jeans, and Lorentz set Planck's constant to zero in order to align with classical physics, but Planck knew well that this constant had a precise nonzero value.

Related Topics:
Rayleigh - Jeans - Lorentz

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"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 are wrong."

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Max Born wrote about Planck: "He was by nature and by the tradition of his family conservative, averse to revolutionary novelties and sceptical towards speculations. But his belief in the imperative power of logical thinking based on facts was so strong that he did not hesitate to express a claim contradicting to all tradition, because he had convinced himself that no other resort was possible."

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