  The dynamics of the magnetic field in a superconducting phase is described by
an effective massive bosonic field theory. If the superconductor is confined in
a domain M with boundary \partial M, the boundary conditions of the
electromagnetic fields are predetermined by physics. They are time-reversal and
also parity invariant for adapted geometry. They lead to edge excitations while
in comparison, the bulk energies have a large gap. A similar phenomenon occurs
for topological insulators where appropriate boundary conditions for the Dirac
Hamiltonian also lead to similar edge states and an 'incompressible bulk'. They
give spin-momentum locking as well. In addition time-reversal and parity
invariance emerge for adapted geometry. Similar edge states appear in QCD bag
models with chiral boundary conditions.
