In the realm of particles, many phenomena are profoundly different from anything we experience in our everyday world. For instance, it seems obvious to us that a tennis ball and an ocean wave are two entirely different things. We’re also absolutely sure that ocean waves will never turn into tennis balls, and vice versa.

This kind of reasoning, which we consider logical, makes no sense when it comes to particles. No one has ever seen electrons directly, so we can’t know their true nature. However, many experiments have shown that sometimes they behave like waves rather than like particles (Book 1 and Book 2).

Photons also have this dual nature (Book 10), which physicists call wave-particle duality.

Because of this dual behavior, electrons and photons are referred to as quantum objects.