Many-body physics studies systems with large $N$ interacting degrees of freedom, where collective behavior can be qualitatively different from the microscopic laws. Examples include magnets, superconductors, and strongly correlated electrons.

Low-energy behavior is often captured by effective Hamiltonians and quasiparticles. Renormalization explains why many macroscopic observables are insensitive to microscopic details.

Quantum phase transitions occur at zero temperature as a nonthermal control parameter changes. Near criticality, correlation lengths diverge and scaling behavior becomes universal.

Tensor network methods such as matrix product states, projected entangled pair states, and MERA exploit entanglement structure to approximate ground states and low-lying excitations efficiently.