Why is it interesting?

The field equations and the boundary conditions are inextricably connected and the latter can in no way be considered less important than the formerV. Fock, The theory of space, time and gravitation

Now, Nature is described by fields, and this elegant and powerful formulation of classical and quantum mechanics based on the action needs to be supplemented with a careful treatment of boundary conditions at infinity. The issue of boundary conditions is particularly important and interesting in the case of gauge theories where the assumption ‘all fields decay sufficiently rapidly at infinity’ is not justified. https://arxiv.org/pdf/1601.03616.pdf

[I]t is natural to regulate infinite sized systems by imposing boundary conditions at finite distance, often described as placing the system in a box. This idea has a long history in the gravitational context (see e.g. [15–27]) where it is common to impose a Dirichlet boundary condition, fixing the induced metric at the walls of the box1 .https://arxiv.org/abs/1508.02515

Layman

Student

Important Boundary Conditions:

Dirichlet boundary condition

Dirichlet = Data on boundary

- The value of the dependent variable is specified on the boundary.

- Needed for elliptic or parabolic partial differential equations. Other boundary conditions are insufficient to determine a unique solution, overly restrictive, or lead to instabilities.

- "In thermodynamics, Dirichlet boundary conditions consist of surfaces (in 3D problems) held at a fixed temperatures." (Source)

Neumann boundary condition

Neumann = Normal derivative on boundary

- The normal derivative of the dependent variable is specified on the boundary.

- Needed for elliptic or parabolic partial differential equations. Other boundary conditions are insufficient to determine a unique solution, overly restrictive, or lead to instabilities.

- "In thermodynamics, the Neumann boundary condition represents the heat flux across the boundaries." (Source)

Cauchy boundary condition

- Both the value and the normal derivative of the dependent variable are specified on the boundary.

- Schematically: Cauchy = Dirichlet $\oplus$ Neumann.

- Cauchy boundary conditions are analogous to the initial conditions for a second-order ordinary differential equation.

- Needed for Hyperbolic equations on an open surface. Other boundary conditions are either too restrictive for a solution to exist, or insufficient to determine a unique solution.

- In physics needed for classical and quantum field theory.

Robin boundary condition

- The the value of a linear combination of the dependent variable and the normal derivative of the dependent variable is specified on the boundary.

- Robin = only a condition on linear combination of Dirichlet and Neumann. Not Dirichlet + Neumann!

Recommended Resources:

• https://www.simscale.com/docs/content/simwiki/numerics/what-are-boundary-conditions.html
• For boundary conditions for waves, see chapter 9 in Georgi's "THE PHYSICS OF WAVES"
• For boundary conditions in gauge field theory, see section 4.5 "Cauchy problem and gauge conditions" in Rubakov's "Classical Theory of Gauge Fields".

Researcher

Examples

Example1

Example2:

History