  Astronomical observations of distant quasars may be important to test models
for quantum gravity, which posit Planck-scale spatial uncertainties ('spacetime
foam') that would produce phase fluctuations in the wavefront of radiation
emitted by a source, which may accumulate over large path lengths. We show
explicitly how wavefront distortions cause the image intensity to decay to the
point where distant objects become undetectable if the accumulated path-length
fluctuations become comparable to the wavelength of the radiation. We also
reassess previous efforts in this area. We use X-ray and gamma-ray observations
to rule out several models of spacetime foam, including the interesting
random-walk and holographic models.
