Louis de Broglie · Matter waves
If light is also a particle, is a particle also a wave?
That time, that place
By 1924 the position was this: light was thought to be a wave, and turned out also to be a particle. The photoelectric effect had shown it.
De Broglie was a doctoral student. His question ran the other way.
Why this question
If light is also a particle, might the electron — thought to be a particle — also be a wave?
It was so symmetrical as to be suspicious. The examiners, unsure how to judge it, sent the thesis to Einstein.
Einstein's reply: he has lifted a corner of the great veil.
What was found
Three years later the experiment came. Davisson and Germer fired electrons at a nickel crystal and got a diffraction pattern. Only waves diffract.
We now use the property as a tool: the electron microscope. Because an electron's wavelength is far shorter than light's, it sees what light cannot. The shape of a virus was first seen with one.
And the idea passed to Schrödinger. If the electron is a wave, there must be an equation for that wave. Two years later there was.
Wave and particle were exclusive categories
Electrons diffracting from a crystal — something only waves do
The electron microscope followed, and Schrödinger's equation