Many-body problem

The quantum many-body problem is a general name for a vast category of physical problems pertaining to deriving the behavior of multi-particle systems using fundamental quantum-mechanical principles.^[1]

This article is about the many-body problem in quantum mechanics. For the n-body problem in classical mechanics, see n-body problem.

Terminology

Many can be anywhere from three to infinity, although three- and four-body systems can be treated by specific means (respectively the Faddeev^[2] and Faddeev–Yakubovsky^[3] equations) and are thus sometimes separately classified as few-body systems.

Body, in this case, is referring to a particle (electron, nuclei, atom, etc.).^[1]

Explanation of the problem

The study of the collection of particles can be extremely complex. In such a quantum system, the repeated interactions between particles create quantum correlations.

All information about such a system can be extracted from its wave function. As a consequence of the complexity of the system, the wave function is a complicated object holding a large amount of information, making exact calculations impractical or even impossible.^[4]

This becomes especially clear by a comparison to classical mechanics. Imagine a single particle that can be described with ${\displaystyle k}$ numbers (take for example a free particle described by its position and velocity vector, resulting in ${\displaystyle k=6}$). In classical mechanics, ${\displaystyle n}$ such particles can simply be described by ${\displaystyle k\cdot n}$ numbers. The dimension of the classical many-body system scales linearly with the number of particles, ${\displaystyle n}$.

In quantum mechanics, however, the dimension of the many-body system scales exponentially with ${\displaystyle n}$, much faster than in classical mechanics.

Because the required numerical expense grows so quickly, simulating the dynamics of more than three quantum-mechanical particles is already infeasible for many physical systems.^[5] Thus, many-body theoretical physics most often relies on a set of approximations specific to the problem at hand, and ranks among the most computationally intensive fields of science.

In many cases, emergent phenomena may arise which bear little resemblance to the underlying elementary laws.

Many-body problems play a central role in condensed matter physics.

Examples

• Condensed matter physics (solid-state physics, nanoscience, superconductivity)
• Bose–Einstein condensation and Superfluids
• Quantum chemistry (computational chemistry, molecular physics)
• Atomic physics
• Molecular physics
• Nuclear physics (Nuclear structure, nuclear reactions, nuclear matter)
• Quantum chromodynamics (Lattice QCD, hadron spectroscopy, QCD matter, quark–gluon plasma)

Approaches

• Mean-field theory and extensions (e.g. Hartree–Fock, Random phase approximation)
• Dynamical mean field theory
• Many-body perturbation theory and Green's function-based methods
• Configuration interaction
• Coupled cluster
• Various Monte-Carlo approaches
• Density functional theory
• Lattice gauge theory
• Matrix product state
• Neural network quantum states
• Numerical renormalization group

Further reading

• Jenkins, Stephen. "The Many Body Problem and Density Functional Theory".
• Thouless, D. J. (1972). The quantum mechanics of many-body systems. New York: Academic Press. ISBN 0-12-691560-1.
• Fetter, A. L.; Walecka, J. D. (2003). Quantum Theory of Many-Particle Systems. New York: Dover. ISBN 0-486-42827-3.
• Nozières, P. (1997). Theory of Interacting Fermi Systems. Addison-Wesley. ISBN 0-201-32824-0.
• Mattuck, R. D. (1976). A guide to Feynman diagrams in the many-body problem. New York: McGraw-Hill. ISBN 0-07-040954-4.