Editing Lord Kelvin (section)

=== Limits of classical physics ===
In 1884, Lord Kelvin led a [[master class]] on "Molecular Dynamics and the Wave Theory of Light" at [[Johns Hopkins University]].<ref>Kargon, Robert and Achinstein, Peter (1987) ''Kelvin's Baltimore Lectures and Modern Theoretical Physics: historical and philosophical perspectives''. [[MIT Press]]. {{ISBN|0-262-11117-9}}</ref> Kelvin referred to the [[acoustic wave equation]] describing sound as waves of pressure in air and attempted to describe also an [[electromagnetic wave equation]], presuming a [[luminiferous aether]] susceptible to vibration. The study group included [[Albert A. Michelson]] and [[Edward W. Morley]] who subsequently performed the [[Michelson–Morley experiment]], which found no luminiferous aether. Kelvin did not provide a text, but [[A. S. Hathaway]] took notes and duplicated them with a [[Mimeograph#Papyrograph|papyrograph]]. As the subject matter was under active development, Kelvin amended that text and in 1904 it was typeset and published. Kelvin's attempts to provide mechanical models ultimately failed in the electromagnetic regime. Starting from his lecture in 1884, he was the first scientist to formulate the hypothetical concept of [[dark matter]]; he then attempted to define and locate some "dark bodies" in the [[Milky Way]].<ref>{{cite web |url=https://cerncourier.com/a/how-dark-matter-became-a-particle/ |title=How dark matter became a particle |publisher=CERN Courier |date=13 April 2017 |access-date=16 March 2022}}</ref><ref>{{cite web|url=https://ned.ipac.caltech.edu/level5/Sept16/Bertone/Bertone2.html|title=A History of Dark Matter- Gianfranco Bertone & Dan Hooper|website=ned.ipac.caltech.edu}}</ref>

He was skeptical about Maxwell's prediction of [[radiation pressure]], but admitted that it did exist after seeing [[Pyotr Lebedev]]'s experimental proof of radiation pressure.<ref>{{Cite journal |last=Khramov |first=Yu A |date=31 December 1986 |title=Petr Nikolaevich Lebedev and his school (On the 120th anniversary of the year of his birth) |url=https://iopscience.iop.org/article/10.1070/PU1986v029n12ABEH003609 |journal=Soviet Physics Uspekhi |volume=29 |issue=12 |pages=1127–1134 |doi=10.1070/PU1986v029n12ABEH003609 |issn=0038-5670}}</ref>

On 27 April 1900, he gave a widely reported lecture titled "Nineteenth-Century Clouds over the Dynamical Theory of Heat and Light" to the Royal Institution.<ref>"Lord Kelvin, Nineteenth Century Clouds over the Dynamical Theory of Heat and Light", reproduced in ''Notices of the Proceedings at the Meetings of the Members of the Royal Institution of Great Britain with Abstracts of the Discourses'', Volume 16, [https://books.google.com/books?id=YvoAAAAAYAAJ&pg=PA363 p. 363–397]</ref><ref>''[[w:Philosophical Magazine|The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science]]'', Series 6, volume 2, pages 1–40 (1901)</ref> The two "dark clouds" he was alluding to were confusion surrounding how matter moves through the aether (including the puzzling results of the Michelson–Morley experiment) and indications that the [[equipartition theorem]] in [[statistical mechanics]] might break down. Two major physical theories were developed during the 20th century starting from these issues: for the former, the [[theory of relativity]]; for the second, [[quantum mechanics]]. In 1905, [[Albert Einstein]] published the so-called ''annus mirabilis'' papers, one of which explained the [[photoelectric effect]] based on [[Max Planck]]'s discovery of energy quanta which was the foundation of quantum mechanics, another of which described [[special relativity]], and the last of which explained [[Brownian motion]] in terms of statistical mechanics, providing a strong argument for the existence of atoms.