Spacetime structure and asymmetric metric from the premetric formulation   of electromagnetism
  After reviewing the construction of metric from premetric electrodynamics with empirical improvement of accuracy in the skewonless case, we explore the role of skewons in the construction of spacetime metric in the full premetric electrodynamics. For metric principal part plus skewon part, we have shown that the Type I skewon part is constrained to < a few x 10^(-35) in the weak field/weak EEP violation limit. Type II skewon part is not constrained in the first order. However, in the second order it induces birefringence; the nonbirefringence observations constrain the Type II skewon part to 10^(-19). Nevertheless, an additional nonmetric induced second-order contribution to the principal part constitutive tensor compensates the Type II skewon birefringence and makes it nonbirefringent. This second-order contribution is just the extra piece to the core-metric principal constitutive tensor induced by the antisymmetric tensor. From the Tamm-Rubilar tensor density and the dispersion relation for the spacetime medium with the asymmetric-metric-induced constitutive tensor, we know that it is nonbirefringent. The antisymmetric metric induced constitutive tensor has a pseudoscalar part in the decomposition. The variation of his part is constrained by observation on cosmic polarization rotation to < 0.03, and gives one constraint on the 6-degree-of-freedom antisymmetric metric. Further studies of these 6 degrees of freedom experimental and theoretically are desired. 
Wei-Tou Ni
gr-qc