One hundred years ago, in September 1923, a young aristocrat named Louis de Broglie presented the French Academy of Sciences with a note called Waves and Quanta. In total it was about three pages long. It contained a few brief thought experiments about the motion of microscopic particles. The accompanying equations were certainly advanced from a layman’s perspective, but they hardly would have fazed an expert audience like the members of the Academy. There was nothing too elaborate or far-fetched in de Broglie’s argument. But the implications of what he had to say would transform the most rudimentary assumptions of modern science about how reality is structured and what the universe is made of. He was simply upending the foundations of the world.

De Broglie’s note was the first published defense of what is now called “wave-particle duality”: the idea that the minute components of the physical world are not simply very small lumps of matter (“particles”), but also patterns of cyclical change (“waves”). This perplexing notion has become fundamental to the set of theories known as quantum physics. By 1923, it had already been acknowledged as a possibility in certain special cases. But de Broglie proposed that wave-particle duality could apply even to subatomic particles like electrons—that is, even to particles with mass.


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