The Classical Double-Slit Experiment
If you want to figure out whether something is a wave or a particle, you look for interference.
The history of science is often presented as a series of revolutions where one person comes up with a new idea that utterly destroys the old understanding. This presentation could not be further from the truth. Instead, many people come up with small ideas that slowly extend and reshape the old understanding. For this reason, every new theory must be able to not only predict new results but also predict everything the old theory could.
During the 17th and 18th centuries, we considered light to be a Classical particle because light acted like a Classical particle in all experiments involving reflection and refraction. More specifically, both light and particles follow the Principle of Stationary Action. There’s just one small problem: we showed that waves also follow the Principle of Stationary Action in the previous article. Since waves, particles, and light follow the Principle of Stationary Action, the reflection and refraction experiments cannot tell us if light is a wave or a particle. We need to come up with a new experiment that will settle this debate once and for all.
