  The Stokes parameters form a Minkowskian four-vector under various optical
transformations. As a consequence, the resulting two-by-two density matrix
constitutes a representation of the Lorentz group. The associated Poincare
sphere is a geometric representation of the Lorentz group. Since the Lorentz
group preserves the determinant of the density matrix, it cannot accommodate
the decoherence process through the decaying off-diagonal elements of the
density matrix, which yields to an incerese in the value of the determinant. It
is noted that the O(3,2) deSitter group contains two Lorentz subgroups. The
change in the determinant in one Lorentz group can be compensated by the other.
It is thus possible to describe the decoherence process as a symmetry
transformation in the O(3,2) space. It is shown also that these two coupled
Lorentz groups can serve as a concrete example of Feynman's rest of the
universe.
