Knotted solutions to electromagnetism are investigated as an independent subsector of the theory. We write down a Lagrangian and a Hamiltonian formulation of Bateman's construction for the knotted electromagnetic solutions. We introduce a general definition of the null condition and generalize the construction of Maxwell's theory to massless free complex scalar, its dual two form field, and to a massless DBI scalar. We set up the framework for quantizing the theory both in a path integral approach, as well as the canonical Dirac method for a constrained system. We make several observations about the semi-classical quantization of systems of null configurations.
IntroductionElectromagnetism is a free (non-selfinteracting) theory so, according to standard lore, we wouldn't expect topologically nontrivial solutions. Indeed, solitons are usually found in interacting theories, as in the case of water solitons, which started the field, with John Scott Russel's observation of a solitonic wave in a canal in Scotland. Sometimes there is a topological reason for the existence and stability of a soliton, which is the case for "kinks" in 1+1 dimensional scalar theories, vortices in 2+1 dimensional gauge theories, or monopoles in 3+1 dimesional gauge theories, for instance.But the existence of a topological constraint, of a fixed topological number, turns out to be possible even in a free theory like Maxwell electromagnetism without sources. Thus it was realized rather late that there exist solutions with a nonzero Hopf index, or "Hopfions," and the explicit solutions were written only in [1,2] by Rañada, after the early work by Trautman in [3]. The standard Hopfion solution is null in the sense of the Riemann-Silberstein (RS) vector F = E + i B, i.e. F 2 = 0, corresponding to E 2 = B 2 and E · B = 0, but there are also partially null solutions, as we will explain in the following.