Integrable system

Integrable system refers to a system in mathematical and physical theory characterized by the presence of a maximal number of independent conserved quantities. In classical mechanics, these are often quantities like energy, momentum, and angular momentum that remain constant over time for a closed system. In the context of quantum mechanics, integrable systems are those in which the wave functions can be solved exactly for the energy levels of the system. The concept plays a crucial role in various areas of physics and mathematics, including statistical mechanics, quantum field theory, and the theory of differential equations.

Definition

An integrable system in the realm of Hamiltonian mechanics is defined by the existence of as many independent constants of motion as there are degrees of freedom. These constants of motion are associated with symmetries of the system through Noether's theorem. In the context of partial differential equations (PDEs), an integrable system is one that can be solved by the method of Inverse Scattering Transform or has a Lax pair representation, indicating a deep geometric and algebraic structure.

Examples

• The Korteweg-de Vries equation (KdV equation), which models waves on shallow water surfaces.
• The nonlinear Schrödinger equation, relevant in optics and quantum mechanics.
• The Sine-Gordon equation, which appears in various areas of theoretical physics including field theory and condensed matter physics.

Mathematical Framework

The study of integrable systems often involves the use of Lie algebras, Poisson brackets, and symplectic geometry. These mathematical structures help in understanding the underlying symmetries and conservation laws of the systems. The Lax pair, a pair of matrices or operators depending on the dynamical variables of the system, is a common tool used to demonstrate the integrability of a system.

Quantum Integrable Systems

In quantum mechanics, an integrable system is one where the Schrödinger equation can be solved exactly for the energy spectrum. This often involves the use of Bethe Ansatz or Quantum Inverse Scattering Method. Quantum integrable systems are important in the study of exactly solvable models in statistical mechanics and quantum field theory.

Applications

Integrable systems have wide-ranging applications across physics and mathematics. They are used to model phenomena in fluid dynamics, nonlinear optics, and quantum field theory. The integrability property allows for the exact solution of models that can describe physical phenomena such as solitons, wave propagation, and quantum phase transitions.

Challenges and Open Problems

Despite the significant progress in the theory of integrable systems, there are numerous challenges and open problems. These include the classification of integrable systems, understanding the role of integrability in chaotic systems, and extending the concepts to higher-dimensional and non-Hamiltonian systems.

See Also

• Hamiltonian (quantum mechanics)
• Symmetry in physics
• Conservation law
• Soliton
• Chaos theory