Lord Kelvin's Second Cloud

TL;DR

This paper revisits Lord Kelvin's second scientific cloud, clarifying its historical context and its relation to the development of quantum mechanics, emphasizing that it concerned the specific heat of polyatomic molecules rather than black-body radiation.

Contribution

It clarifies Kelvin's second cloud as related to molecular specific heat, not black-body radiation, and situates it within the historical development leading to quantum mechanics.

Findings

Kelvin's second cloud was about molecular specific heat.

The problem was not initially linked to black-body radiation.

Historical context shows the development of quantum theory was influenced by this issue.

Abstract

On April 27, 1900, William Thomson, better known as Lord Kelvin, delivered a visionary speech before the Royal Institution of Great Britain. In it, he presented two unresolved problems which, to him, appeared fundamental and unavoidable at the turn of the 20th century. He compared them to two clouds obscuring our understanding of physics. Dissipating these two clouds would eventually require the development of special relativity and quantum mechanics. This article revisits the second cloud which, contrary to what is often claimed in the literature, did not concern black-body radiation, but rather the specific heat of polyatomic molecules. To clarify this, the article aims to place Kelvin's speech within the historical context of the time and to situate it within the sequence of developments, from Kirchhoff to the first Solvay Conference in 1911, that marked the path of the extraordinary intellectual adventure that led to the birth of quantum mechanics. It will also be shown that Max Planck's initial motivation was not to solve the problem of the so-called "ultraviolet catastrophe."