Space is filled with cosmic rays (high-energy protons) and the cosmic microwave background radiation. These can Compton scatter off one another. Suppose a photon of energy hν = 2 × 10−4 eV scatters off a proton of energy 109mP = 1018 eV, energies measured in the Sun's rest frame. Use Eq. (2.43) in the proton's initial rest frame to calculate the maximum final energy the photon can have in the solar rest frame after the scattering. What energy range is this (X-ray, visible, etc.)?