In 1905, Albert Einstein calculated that the speed of light remains at a constant 186,282 miles per second (299,792 km per second) when travelling through a vacuum. While this theory has been accepted for over a century, a controversial new study suggests Einstein was in fact wrong, and that the speed of light is slower than we think. The study was conducted by Baltimore-based physicist, James Franson, who looked at why light particles of supernova SN 1987A arrived 4.7 hours later than expected. The star’s collapse, which was seen from Earth in 1987, triggered a burst of neutrinos - an electrically neutral, weakly interacting elementary subatomic particle. According to Einstein, this should have happened roughly three hours before a burst of optical light - and from that moment on, the pulses should have kept pace, both travelling at the speed of light. However, the optical light arrived roughly 7.7 hours after the neutrinos - or 4.7 hours late. 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 correct, it means scientists have to recalculate everything from our distance to the sun to some of the most distant objects seen in other galaxies. Dr Franson’s paper has been submitted to the New Journal of Physics and is currently undergoing peer review.