Lord Kelvin
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. 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 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.

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.

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. 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.
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