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Planck described blackbody radiation as quantized resonances in a cavity.

It was not until five years after Planck made his heuristic assumption of abstract elements of energy or of action that Albert Einstein conceived of really existing quanta of light in 1905 as a revolutionary explanation of black-body radiation, of photoluminescence, of the photoelectric effect, and of the ionization of gases by ultraviolet light. … Contrary to Planck's beliefs of the time, Einstein proposed a model and formula whereby light was emitted, absorbed, and propagated in free space in energy quanta localized in points of space.

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The German Wikipedia concerning the quanta of blackbody radiation:

A blackbody (also: blackbody radiator, Planckian radiator, ideal blackbody) is an idealized thermal radiation source. … At the same time, it emits electromagnetic radiation as thermal radiation …

The attempt to describe the blackbody radiation theoretically has contributed essentially to the emergence of quantum physics. ... Only the assumption of Max Planck in 1900 that matter can absorb and emit the radiation energy only in the form of certain energy quanta could solve this riddle.

What in a blackbody emits the quanta?

HolgerFiedler
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    A blackbody is defined in a way that what emits or absorbs the quanta doesn’t matter. It could be atoms, it could be plasma, it could be a black hole, etc. – Ghoster Nov 27 '22 at 08:03
  • @Ghoster „ Even when spectral lines are excited in the molecules and the relaxation releases a photon, this photon can interact in a continuum with Compton etc scattering which will destroy most coherence and spectral lines, due to the huge number of molecules involved.“ https://physics.stackexchange.com/a/59873/46708 – HolgerFiedler Nov 28 '22 at 04:15
  • Interaction of matter and energy is given in terms of Planck law, so obviously it is matter in blackbody whether in solid or fluid form. Here energy is considered as in radiation form, so only heat and light are form of energy. This also means that energy in form of non-physical mode of transmission, can only be hold by matter. It is based on concept of kirchoff's law, so idea was amount of energy stored in cavity is emitted through radiation, this is for all cavity oscillators including laser, which is an example of this law. Although the way, law was derived is incorrect and still the whole – Neil Libertine Nov 27 '22 at 11:26

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