  At very high energies, the Landau-Pomeranchuk-Migdal effect reduces the cross
sections for electron bremsstrahlung and photon $e^+e^-$ pair production. The
fractional electron energy loss and pair production cross sections drop as the
energy increases. In contrast, the cross sections for photonuclear interactions
grow with energy. In solids and liquids, at energies above $10^{20}$ eV,
photonuclear reactions dominate, and showers that originate as photons or
electrons quickly become hadronic showers. These electron-initiated hadronic
showers are much shorter (due to the absence of the LPM effect), but wider than
purely electromagnetic showers would be. This change in shape alters the
spectrum of the electromagnetic and acoustic radiation emitted from the shower.
These alterations have important implications for existing and planned searches
for radiation from $\nu_e$ induced showers above $10^{20}$ eV, and some
existing limits should be reevaluated.
