In 1900, Lord Kelvin, the preeminent scientist of the time, was 76 years old.

He was kind of the Neil deGrasse Tyson of his day.

He gave science lectures and was the go-to guy for journalists who wanted to write pieces about science.

He gave a lecture that year entitled

19th century clouds over the dynamical theory of heat and light,

in which he identified two dark clouds, as he called them,

over modern science,

that were spoiling what was otherwise

an elegant and virtually complete picture of the world around us.

Solving these two mysteries, he suggested, would pretty much complete our understanding of the world around us.

Lord Kelvin's two dark clouds, and others he hadn't even suspected, would, upon investigation, turn out not to complete science's understanding of

the physical world, but would instead open up a Pandora's box of new scientific puzzles, some of

which we are still working through to this very day. Welcome to the history of the 20th century.

Music Welcome to the history of the 20th century. Episode 9. Dark clouds.

In the realms of physics and chemistry, the great accomplishments of the 19th century

did indeed revolve around heat and light.

The 19th century saw the rise of the steam engine, which converts heat into useful work,

which led physicists like Lord Kelvin to investigate the physics of heat.

Lord Kelvin, while still in his 20s, became one of the first physicists to

understand that what we call heat is the vibration of molecules in an object. One of the

consequences of this understanding is that there is such a thing as the coldest possible temperature,

the temperature when molecular motion stops altogether.

...