Petrov Classification in Numerical Relativity

The algebraic classification system of Petrov, Pirani, and Penrose provides a method to unambiguously characterize the gravitational degrees of freedom point-by-point throughout a spacetime. It is tempting to apply this system to the numerically-generated spacetimes that have recently proliferated, and some work has already gone in this direction. However, spacetimes of current physical interest --- such as binary black hole mergers --- raise subtleties in that they are generically, strictly speaking, Type I, but approximately, in some sense, Type D. To make any such claims about ``approximate Petrov class'' meaningful, one must introduce a ``degeneracy measure'' on the space of null rays. In this talk, I will describe some of the difficulties in this undertaking, and present results applying such degeneracy measures to binary black hole simulations from the Caltech/Cornell/CITA group.