Null-Energy Bounds on Photon-Orbit Instability in Einstein-Gauss-Bonnet Gravity
We derive a local upper bound on the coordinate-time Lyapunov exponent of unstable circular null orbits in static, spherically symmetric, asymptotically flat Einstein--Gauss--Bonnet gravity in $n\geq5$ dimensions with nonnegative Gauss--Bonnet coupling. On the regular Einstein branch, the tangential null energy condition suffices; no additional density-pressure inequality is required for the principal result. The bound retains the local matter-density contribution dictated by the field equations. Using the spherical mass relation, we express the departure of its dimensionless coefficient from the Einstein value $n-3$ in terms of the difference between the local density and the enclosed generalized Misner--Sharp mass per areal volume. This gives a sufficient density-contrast criterion for the Einstein optical bound to remain valid at nonzero coupling. Optical Gaussian curvature and tangential null convergence provide geometrical interpretations that distinguish radial tidal instability from the focusing of a null congruence. Vacuum solutions saturate the bound, while a Maxwell field with nonzero charge produces a strictly positive gap. The smooth Einstein limit and explicit vacuum and charged solutions provide consistency checks. A separate matter condition yields a density-free corollary. We also discuss consequences for strong-deflection lensing and test-field eikonal damping within their respective regimes of validity.