Abstract
The quasi-local energy-momentum of Dougan and Mason(1991), associated with a spacelike topological two-sphere, is examined. It is is shown that the energy-momentum four-vector is zero iff the Cauchy development of the 3-surfaces spanned by the two-sphere is flat, and is null (i.e. the quasi-local mass is zero) if and only if the Cauchy development is a pure radiative pp-wave spacetime geometry with a common principle null direction of the Weyl and Ricci tensors.

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