Two component spinor techniques, similar to those used by Witten (1981) are used to express the Bondi momentum of an asymptotically flat space-time in the form of an integral over an asymptotically null, space-like hypersurface Sigma . It is then shown that the Bondi mass is positive, given the existence of a Green function for the 'Witten equation' on Sigma .
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Ludvigsen et al. (1981) studied this question.
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