The branch of physics that relates to the behavior of objects, typically particles, on small scales. Probability is very important in quantum mechanics.
Quantum theory was born in 1900, when Max Planck came up with the idea of treating light as tiny bundles, or "quanta". It entered its golden age within a few decades, and formed the basis for what became known as quantum field theory, which is the modern description of particle physics. It has thus far not been possible to formulate a quantum theory that describes gravity; it is thought that a theory of quantum gravity could lead to a theory of everything.