Practical Introduction to Action-Dependent Field Theories
Abstract
Action-dependent field theories are systems where the Lagrangian or Hamiltonian depends on new variables that encode the action. They model a larger class of field theories, including non-conservative behavior, while maintaining a well-defined notion of symmetries and a Noether theorem. This makes them especially suited for open systems. After a conceptual introduction, we make a quick presentation of a new mathematical framework for action-dependent field theory: multicontact geometry. The formalism is illustrated with a variety of action-dependent Lagrangians, some of which are regular and others singular, derived from well-known theories whose Lagrangians have been modified to incorporate action-dependent terms. Detailed computations are provided, including the constraint algorithm for the singular cases, in both the Lagrangian and Hamiltonian formalisms. These are the one-dimensional wave equation, the Klein-Gordon equation and the telegrapher equation, Maxwell's electromagnetism, Metric-affine gravity, the heat equation and Burguers' equation, the Bosonic string theory, and (2+1)-dimensional gravity and Chern-Simons equation.
Cite
@article{arxiv.2409.08340,
title = {Practical Introduction to Action-Dependent Field Theories},
author = {Manuel de León and Jordi Gaset Rifà and Miguel C. Muñoz-Lecanda and Xavier Rivas and Narciso Román-Roy},
journal= {arXiv preprint arXiv:2409.08340},
year = {2025}
}
Comments
34 pages, plus appendix and references. The paper has been substantially improved. A better explanation of the results is presented. New subdivisions, a list notation and diagrams have been added to add clarity and make the text more readable. Some results have been improved. Notational errors and misprints have been corrected along the paper