perturbation theory (quantum mechanics)

Perturbation theory is a way of looking at how we understand very small things in quantum mechanics. It helps us understand the behavior of things that are too small for us to see, like atoms and particles. It works by looking at small changes in a particular system and understanding how those changes affect what happens. For example, we can look at how a tiny change in the amount of energy of an atom will affect the way it behaves. Perturbation theory helps us figure out what would happen if we made bigger or smaller changes, or changed something else about the atom. This helps us better understand how atoms and particles work, and how they interact with each other.