Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

by Anil Ananthaswamy

Hertz, however, in just a few years solved the puzzle. He designed an experiment that proved Maxwell correct. The experiment involved building a transmitter of electromagnetic waves, and a receiver—and showing that these invisible waves did indeed exist and could propagate through air. Hertz had inadvertently discovered radio waves.

When asked about the usefulness of such waves, Hertz reportedly said, “It is of no use whatsoever. This is just an experiment that proves Maestro Maxwell was right. We just have these mysterious electromagnetic waves that we cannot see with the naked eye. But they are there.”

But while doing this experiment, Hertz stumbled upon another curious phenomenon that would, within a decade, challenge the light-is-a-wave argument. The phenomenon is now called the photoelectric effect. When light falls on certain metals, it can eject electrons. Most important, for a given metal, the electrons are ejected only when the light is above a threshold frequency unique to that metal.

The odd thing, of course, is that terms like frequency and wavelength refer to the wave nature of light, and yet these were getting tied to the idea of light as particles. A disturbing duality was beginning to raise its head. Things were getting confusing.