What is quantum theory in simple terms? Quantum theory is the branch of physics theory that seeks to explain phenomena occurring at an atomic, and even smaller, scale. It provides a mathematical framework to study the behavior of subatomic particles, explaining phenomena such as entanglement and quantum tunneling.

Quantum mechanics is a description of reality. It is a very precise description of reality that allows us to make predictions because it is so precise.

THE TWO GOLDEN RULES OF QUANTUM MECHANICS Rule 1: A particle can be in quantum superposition where it behaves as though it is both here and there. Rule 2: When measured, the particle will be found either here or there. Many phenomena predicted by quantum mechanics can be understood using these two simple rules.

These early attempts to understand microscopic phenomena, now known as the "old quantum theory", led to the full development of quantum mechanics in the mid-1920s by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born, Paul Dirac and others.