  Due to the high energies and long distances to the sources, astrophysical
observations provide a unique opportunity to test possible signatures of
Lorentz invariance violation (LIV). Superluminal LIV enables the decay of
photons at high energy. The High Altitude Water Cherenkov (HAWC) Observatory is
among the most sensitive gamma-ray instruments currently operating above 10
TeV. HAWC finds evidence of 100 TeV photon emission from at least four
astrophysical sources. These observations exclude, for the strongest of the
limits set, the LIV energy scale to $2.2\times10^{31}$ eV, over 1800 times the
Planck energy and an improvement of 1 to 2 orders of magnitude over previous
limits.
