  The structure of quantum interactions with fields of helicity two
("gravitons") is strongly constrained by three principles: positivity (Hilbert
space), covariance, and locality of observables. To fulfil them simultaneously,
some (non-observable) fields need to be non-local. We work with
string-localized fields. The results then follow from the condition that
entities closely related to observables, like the S-matrix, be local and
string-independent. They in particular reproduce the interactions dictated by
general covariance in classical field theory. Graviton-matter couplings are
consistent only when the graviton self-interaction is taken into account as
well.
