  The observation of gravitational waves from the Laser Interferometer
Gravitational-Wave Observatory (LIGO) event GW150914 may be used to constrain
the possibility of Lorentz violation in graviton propagation, and the
observation by the Fermi Gamma-Ray Burst Monitor of a transient source in
apparent coincidence may be used to constrain the difference between the
velocities of light and gravitational waves: $c_g - c_\gamma < 10^{-17}$.
