Uncertain Principles: What’s a Photon, and How Do We Know they Exist?

Even if the energy of a single photon is significantly greater than the energy required to trigger a “click” in your detector, for a given frequency of light, you will only ever find the energy arriving in single-photon amounts.

How do we know this is the case? There are three great historical experiments that mark the key steps on the way to full acceptance of the photon model: the photoelectric effect, the Compton effect, and photon anti-bunching.

Oops. Spoiler alert. Sorry.