According to string theory, perhaps the top contender in the struggle to create a quantum theory of gravity, the vacuum of space is frothing. According to Dimitri V. Nanopoulos, a professor at Texas A&M University in College Station, and his colleagues, John Ellis of CERN in Geneva and Nikolaos E. Mavromatos of King's College in London, these Planck-scale defects in the space-time manifold should produce macroscopic effects over cosmic distances. Namely, the vacuum should be dispersive, so that the speed of light will depend on its frequency.

Research seems to be supporting this position. Physicists interpreting data from the High Energy Gamma Ray Astronomy facility in La Palma, Canary Islands, found that a frequency-dependent speed of light explains the lack of observable electron-positron pairs that should be produced by energetic photons from the galaxy Markarian 501.