  The momentum transfer $\Delta k$ required for a photon to scatter from a
target and emerge as a $\rho^0$ decreases as the photon energy $k$ rises. For
$k>3\times10^{14}$ eV, $\Delta k$ is small enough that the interaction cannot
be localized to a single nucleus. At still higher energies, photons may
coherently scatter elastically from bulk matter and emerge as a $\rho^0$, in a
manner akin to kaon regeneration. Constructive interference from the different
nuclei coherently raises the cross section and the interaction probability
rises linearly with energy. At energies above $10^{23}$ eV, coherent conversion
is the dominant process; photons interact predominantly as $\rho^0$. We compute
the coherent scattering probabilities in slabs of lead, water and rock, and
discuss the implications of the increased hadronic interaction probabilities
for photons on ultra-high energy shower development.

Nuclear Theory;Cosmology and Nongalactic Astrophysics;High Energy Physics - Phenomenology