the idea that matter has a wave nature turns 100 years old

Modern quantum mechanics began in 1925.

But one of its most solid intellectual pillars rests in a speculation that was advanced by Louis de Broglie one year earlier: exactly 100 years ago.

Let us summarise his reasoning:

• de Broglie acknowledges that light has a dual nature, that of wave and particle, as had been pointed out by Maxwell and Einstein respectively. This seemed to many little less than an intellectual impossibility!
• But rather than seeing this dualism as an absurdity - or a philosophical limitation to be amended - he turns this acquisition into a motif of inspiration.
• de Broglie was guided by the vision, that there should be a fundamental correspondence between light and matter; this led him to think that also the electron (discovered a few decades earlier) could be conceived along similar lines.
• Thus, he boldly proposed the idea that the electron could behave like a wave. On this basis, he concludes that it can be subject to diffraction.

Recall that electron where discovered at the end of 1800, thanks to the study of the cathode rays, shown in the figure.

It is curious that the debate on their nature had seen supporters of the corpuscular nature arrayed against those who argued for the wave nature.

The corpuscular view had a momentary triumph with JJ Thomson's Nobel Prize (1906); but then GP Thomson (Nobel 1937), his son, proved that electrons could indeed be diffracted, just as de Broglie had hypothesised.

de Broglie's ideas would receive countless supporting evidence and acclaim, earning him the Nobel Prize in Physics in 1929.

They would be developed by many people, notably by Schrödinger, Dirac, Majorana.

Today, they form the basis of our understanding of the world of the infinitely small:

in suitable conditions, all material particles are deemed to behave as waves.