In 1900, Kelvin noted that "two clouds" were hovering on the horizon, one to do with properties of light's motion, and the other with aspects of the radiation objects emit when heated, but there was a general feeling that these were mere details, which, no doubt, would soon be addressed.

However, within a decade, everything changed. As anticipated, the two problems Kelvin had raised were promptly addressed, but they proved anything but minor. Each ignited a revolution, and each requires a fundamental rewriting of nature's laws.