  We examine the effect of light-cone broadening induced by quantum-gravity
foam in the context of theories with ``large'' extra dimensions stretching
between two parallel brane worlds. We consider the propagation of photon probes
on one of the branes, including the response to graviton fluctuations, from
both field- and string-theoretical viewpoints. In the latter approach, the
dominant source of light-cone broadening may be the recoil of the D-brane,
which scales linearly with the string coupling. Astrophysical constraints then
place strong restrictions on consistent string models of macroscopic extra
dimensions. The broadening we find in the field-theoretical picture seems to be
close to the current sensitivity of gravity-wave interferometers, and therefore
could perhaps be tested experimentally in the foreseeable future.
