Action of the conformal group on steady state solutions to Maxwell's   equations and background radiation
  The representation of the conformal group (PSU(2,2)) on the space of solutions to Maxwell's equations on the conformal compactification of Minkowski space is shown to break up into four irreducible unitarizable smooth Fr\'echet representations of moderate growth. An explicit inner product is defined on each representation. The frequency spectrum of each of these representations is analyzed. These representations have notable properties; in particular they have positive or negative energy, they are of type $A_{\frak q}(\lambda)$ and are quaternionic. Physical implications of the results are explained. 
Bertram Kostant and Nolan Wallach
math-ph math.MP math.RT