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Quantum Mechanics

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Wikipedia:

"The Davisson–Germer experiment was a 1923-27 experiment by Clinton Davisson and Lester Germer at Western Electric (later Bell Labs), in which electrons, scattered by the surface of a crystal of nickel metal, displayed a diffraction pattern. This confirmed the hypothesis, advanced by Louis de Broglie in 1924, of wave-particle duality, and was an experimental milestone in the creation of quantum mechanics."

a very brief overview

Davisson and Germer were not attempting to confirm the work of DeBroglie. Instead, their purpose was to measure the energies of electrons scattered from a metal surface.

Alan Holden offers a summary of the famous experiment. In their research, the two scientists used an apparatus which...

“shoots electrons out of an electron gun (a heated filament)”

“… they (the electrons) go through a tube (accelerated by a voltage)”

“… bounce off a (nickel-metal) crystal”

“… and come back, hit a florescent screen, and make spots on it.”

There was also an electron detector off to the side, which was moveable, could be rotated, in order to “observe” the electrons from different angles.

The researchers expected to find the “spots” as spots, points, on the screen, but that’s not what they found.

Instead, to their surprise, the electrons, from certain angles, gathered themselves into “peaks of intensity.”

This is not what random particles do.

Recall from the work of Thomas Young, who conducted the original Double-Slit Experiment, light was discovered to hit the measurement screen with “peaks of intensity.”

The bands of light, the “peaks of intensity” which Davisson and Germer observed, were not ordered in the classic diffraction pattern that Young had observed.

Why the difference? The atoms in the nickel crystal were arranged something like this, very symmetrically placed (as is the case with all crystals).

This caused the “peaks of intensity” to present themselves in ways not found with ordinary light.

Be that as it may, “peaks of intensity” were, in fact, discovered by Davisson and Germer on the screen, not ordinary spots. With high probability, this meant that the electrons, thought to be solely matter, were acting like waves.

the first experimental evidence of quantum mechanics

The Davisson-Germer findings are considered to be the first direct experimental evidence confirming DeBroglie's hypothesis that particles, matter, can also manifest as waves.

Why is this the first experimental evidence? Did not Einstein and DeBroglie do this? No, because Einstein and DeBroglie were theoreticians and did not work in a lab.