  Point particles fall freely along geodesics; strings do not. In string theory
all probes of spacetime structure, including photons, are extended objects and
therefore always subject to tidal forces. We illustrate how string theory
modifies the behavior of light in weak gravitational fields and limits the
applicability of the principle of equivalence. This gives in principle a window
on the short-distance structure of geometry in quantum gravity where one can
see in a model-independent way how some of its predictions differ from those of
classical gravity. We compare this with the lessons of high-energy string
scattering.
