Light-Cone Broadening and TeV Scale Extra Dimensions

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.