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It is shown that in the Born approximation the helicity amplitudes for graviton-graviton scattering are completely determined by kinematical considerations alone, without detailed Feynman graph calculations, reflecting thus the uniqueness of pure Einstein theory. We show that the nonuniqueness of the matter-graviton amplitudes is related to the existence of nonminimal interactions. In the presence of such interactions, helicity is not conserved in elastic scattering of massless particles. A simplified three-graviton vertex with two on-shell gravitons is presented.
Grisaru et al. (Tue,) studied this question.