A US scientist claims that he has found evidence which suggests that the speed of light is slower than thought as described by Einstein's theory of relativity. According to the theory of general relativity, light travels at 299,792,458 metres/second in a vacuum.

The claim by physicist James Franson of the University of Maryland is based on observations that were made of the supernova SN 1987A which exploded in 1987. Scientists on the Earth picked up the arrival of photons as well as neutrinos from the blast but the photons arrived later than expected by 4.7 hours. Scientists said that there was a likelihood that this happened because the photons were from another source.

Franson wondered what if that was not the case and the fact that light slows down due to vacuum polarization when it travels. This creates a gravitational differential between those particles.

Franson proposed that this may be because of the fact that light slows down when it travels due to a property of photons called vacuum polarization in which a photon splits into a positron and an electron for a very short duration before recombining back into a photon. That should create a gravitational differential according to Franson between the pair of particles, which would in turn have a tiny energy impact when they recombine. It is enough to cause a slight bit of a slowdown during travel.

This splitting and rejoining would have happened many times with many photons on a journey of 168,000 light years which is the distance between us and SN 1987A and therefore, could easily add up to the 4.7 hour delay, according to Franson.