  The Stokes formalism of polarization physics has astounding structural
parallels with the formalism used for relativity theory in Minkowski spacetime.
The structure and symmetry properties of the Mueller matrices are the same as
those for the matrix representations of the electromagnetic tensor and the
Lorentz transformation operator. The absorption terms $\eta_k$ in the Mueller
matrix correspond to the electric field components $E_k$ in the electromagnetic
tensor and the Lorentz boost terms $\gamma_k$ in the Lorentz transformation
matrix, while the anomalous dispersion terms $\rho_k$ correspond to the
magnetic field components $B_k$ and the spatial rotation angles $\phi_k$. In a
Minkowski-type space spanned by the Stokes $I,Q,U,V$ parameters, the Stokes
vector for 100 % polarized light is a null vector living on the surface of null
cones, like the energy-momentum vector of massless particles in ordinary
Minkowski space. Stokes vectors for partially polarized light live inside the
null cones like the momentum vectors for massive particles. In this description
the depolarization of Stokes vectors appears as a "mass'' term, which has its
origin in a symmetry breaking caused by the incoherent superposition of
uncorrelated fields or wave packets, without the need to refer to a ubiquitous
Higgs field as is done in particle physics. The rotational symmetry of Stokes
vectors and Mueller matrices is that of spin-2 objects, in contrast to the
spin-1 nature of the electromagnetic field. The reason for this difference is
that the Stokes objects have substructure: they are formed from bilinear tensor
products between spin-1 objects, the Jones vectors and Jones matrices. The
governing physics takes place at the substructure level.
