Quantum mechanics is the branch of physics that describes matter and energy at the atomic and subatomic scales. It is one of the most successful theories in science, and one of the strangest. It describes a world where particles can be in two places at once, where measurement changes the outcome, and where uncertainty is fundamental. The theory was developed in the 1920s by Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and others. It has passed every experimental test, and it underpins modern technology, from semiconductors to lasers to MRI.

Quantum mechanics is built on a few key principles. Wave-particle duality says that particles behave as waves and waves behave as particles. Superposition says that a quantum system can be in multiple states at once until measured. Entanglement says that particles can be linked in ways that defy classical intuition. The uncertainty principle says that certain pairs of properties, like position and momentum, cannot both be known precisely. Quantum mechanics is not intuitive. It does not match our everyday experience. But it works. It predicts the behavior of atoms, molecules, and subatomic particles with extraordinary accuracy. It is the foundation of modern physics.

Quantum mechanics principles

• Wave-particle duality. Particles and waves are two aspects of the same thing.
• Superposition. A system can be in multiple states at once.
• Entanglement. Particles can be correlated across distance.
• Uncertainty principle. Limits on knowing position and momentum.
• Quantization. Properties take discrete values.

Quantum mechanics is not the same as classical mechanics. Classical mechanics describes everyday objects. Quantum mechanics describes the atomic and subatomic world.