By the 1920s, modern science had not only proved the existence of atoms, but had begun to pick them apart into smaller particles.

But it turned out that the world of the atom was quite different from the familiar world on the human scale.

Not only different, but unsettling,

even to those who studied it most closely.

Welcome to the history of the 20th century. Episode 246, The Great Debate, Part 2.

Back in episode 221, which I called the Great Debate Part 1, we looked at the question of things very large.

The scale of the universe, and our place in it.

A subject human beings have pondered, probably since the first time one of our ancestors

looked up at the night sky.

In the decade of the 1920s, human science finally uncovered the full and humbling reality

of just how large the universe really is.

For almost as long as humans have wondered how big was big, they also wondered how small was small. If you take a small quantity of, say,

water and divide it in half, and then divide it in half again, can you continue this process

endlessly and produce infinitely small quantities of water? Or would you reach some fundamental stopping point,

where you had the smallest amount of water there could possibly be,

a quantity that either could not be divided any further, or perhaps it could, but then it wouldn't be

water anymore.

This latter hypothesis that there was such a thing as a fundamental particle of water

came to be known as the atomic theory and the particles themselves as atoms, from a Greek

term that means indivisible.

Atomic theory was little more than a philosophical debating point until 19th century chemists

worked out that chemical elements always reacted with each other in certain fixed proportions.

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