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Goldberg–Sachs theorem
The Goldberg–Sachs theorem is a result in Einstein's theory of general relativity about vacuum solutions of the Einstein field equations relating the existence of a certain type of congruence with algebraic properties of the Weyl tensor.
More precisely, the theorem states that a vacuum solution of the Einstein field equations will admit a shear-free null geodesic congruence if and only if the Weyl tensor is algebraically special.
The theorem is often used when searching for algebraically special vacuum solutions.
Linearised gravity
It has been shown by Dain and Moreschi (2000) that a corresponding theorem will not hold in linearized gravity, that is, given a solution of the linearised Einstein field equations admitting a shear-free null congruence, then this solution need not be algebraically special.
See also
Geodesic
Optical scalars
References
Dain, Sergio and Moreschi, Osvaldo, M. "The Goldberg Sachs theorem in linearized gravity". arXiv eprint server. Retrieved February 5, 2005.
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