Quantum mechanics
The physics of the very small, where energy is quantised and outcomes are probabilities.
Quantum mechanics describes nature at the scale of atoms and particles. Energy comes in discrete packets, particles behave as waves, and predictions are inherently probabilistic.
Einstein helped found it but never accepted its probabilistic core — “God does not play dice” (in a 1926 letter to Max Born: “I, at any rate, am convinced that He does not throw dice”) — sparking decades of debate with Niels Bohr that ultimately deepened the theory.
Source: Wikipedia: Quantum mechanics — Read the full article on Wikipedia.