Turning and turning in the widening gyreRadioactivity, even more than X-rays, shook up physics at the end of the 19th century. Men and women of reason gradually realized that atoms do break down--that atoms were not indivisible as the name implied but that they could decompose and decay into other kinds of atoms. And when they do break down radiation and lots of mc2 follows.
The falcon cannot hear the falconer;
Things fall apart; the centre cannot hold;
Mere anarchy is loosed upon the world,
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.
~William Butler Yeats THE SECOND COMING (1919)