Some of the effects predicted by the theory [of loop quantum gravity] appear to be in conflict with one of the principles of Einstein's special theory of relativity... that the speed of light is a universal constant.... Photons of higher energy travel slightly slower than low-energy photons.... the principle of [general] relativity is preserved but Einstein's special theory of relativity requires modification.... A photon can have an energy-dependent speed without violating the principle of [general] relativity!

"Loop Quantum Gravity," The New Humanists: Science at the Edge (2003)