  It is postulated in Einstein's relativity that the speed of light in vacuum
is a constant for all observers. However, the effect of quantum gravity could
bring an energy dependence of light speed. Even a tiny speed variation, when
amplified by the cosmological distance, may be revealed by the observed time
lags between photons with different energies from astrophysical sources. From
the newly detected long gamma ray burst GRB~160509A, we find evidence to
support the prediction for a linear form modification of light speed in
cosmological space.
