In 1894, physics seemed complete. Then Kelvin spotted 2 looming “clouds”

In 1894, Lord Kelvin, a British physicist, gave a lecture at the Institution of Electrical Engineers in which he highlighted two major unresolved issues in physics that he referred to as "two clouds" or "two dark bodies" that were looming over the otherwise clear sky of classical physics.


These "clouds" represented significant puzzles that the established theories of the time could not explain and pointed towards the need for new scientific frameworks that would eventually lead to the development of quantum mechanics and the theory of relativity.

The first "cloud" was the failure of classical thermodynamics to explain the discrepancies in the observed specific heats of solids at low temperatures, known as the "Ultraviolet Catastrophe." According to classical physics, the energy distribution of a black body (a perfect absorber and emitter of radiation) should become infinite at high frequencies, which contradicts the observed data that showed a peak in the emitted spectrum followed by a decline at shorter wavelengths. This problem was later addressed by Max Planck in 1900, who introduced the concept of quantized energy states, which laid the groundwork for quantum theory.

The second "cloud" was the Michelson-Morley experiment, which had shown that the speed of light was constant regardless of the motion of the observer. This was at odds with the predictions of the then-dominant luminiferous ether theory, which posited that light propagated through an invisible medium called the ether. The experiment's negative result suggested that the ether did not exist, leading to the question of what exactly light was and how it was able to move through space without a medium. This discrepancy was one of the factors that motivated Einstein to develop his theory of special relativity in 1905.

Kelvin's identification of these two issues was prescient, as they both played crucial roles in the development of modern physics. The resolution of these puzzles led to a profound transformation of our understanding of the universe, revealing that the classical framework was not complete and that new, more sophisticated theories were needed to describe the behavior of particles and energy at the atomic and subatomic scales, as well as the fabric of space and time itself.