Abstract
We present a new method for finding principal null directions (PNDs). Because our method assumes as an input the intrinsic metric and extrinsic curvature of a space-like hypersurface, we expect it will be useful to numerical relativists. We illustrate our method by finding the PNDs of the Kastor--Traschen spacetimes, which contain arbitrarily many Q=M black holes in a de Sitter background.
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