  If Lorentz invariance is broken at an energy scale Eq, as has recently been
suggested in the context of attempts to quantize gravity, the kinematics of
photon-photon collisions would be profoundly affected at lower energies.
Specifically, electron-positron pair creation on soft photons may be forbidden
at photon energies as low as 30 TeV times square root of (Eq/10**17 GeV) and
the Universe would then be transparent to high energy photons.
  The proposition that Lorentz invariance is broken may be falsified by the
techniques of TeV astronomy.
