The MAGIC Telescope Collaboration (J.Albert, et al) has reported that the gamma ray photons coming from a distant blazar did not all seem to travel here at the same speed.  They found that lower energy photons arrived here at Earth about 4 minutes before higher energy photons.  If it is true that the photons did not all travel at the same speed and the speed difference was caused by differing energies then Einstein’s theory of relativity may have a problem.  However, these are very preliminary results and the experiment needs to be repeated with other sources.  In addition, there is likely a more mundane answer to this question.

The authors have also proposed that perhaps the photons were interacting with quantum foam.