The University of Maryland physicist believes the delay could have been because the light was in fact slowed as it travelled due to something known as 'vacuum polarisation'. During this phenomenon, photons break down to something known as ‘positrons’ and electrons for a split second. before combining together again. When they split, quantum mechanics creates a gravitational potential between the pair of ‘virtual’ particles. Dr Franson argues that the process might have a gradual impact on the speed of the photon, meaning that over 168,000 light years, the photons may have suffered a near five-hour delay. If the physicist is...