Take 10^{23} atoms, cool them until they lock together, and something remarkable happens: the crowd develops behaviours no single atom possesses. Electrons flow without resistance; sound travels in discrete packets; a material decides, all on its own, to become a magnet. Condensed matter physics is the study of these emergent phenomena — the physics of the solid and liquid stuff the world is actually built from.

This branch covers crystal structure and the reciprocal lattice, lattice vibrations (phonons), the free-electron and band theories that explain metals, insulators and semiconductors, magnetism in solids, and the quantum spectacle of superconductivity. It is quantum mechanics and statistical mechanics working together on matter in bulk.