Albert Einstein's theory of general relativity suggests that light travels at a constant speed of 299,792,458 metres per second in a vacuum.
Physicist James Franson of the University of Maryland claims to have found evidence that suggests the speed of light is actually slower than has been thought.
Franson's arguments are based on observations made of the supernova SN 1987A which exploded in February 1987.
Measurements on Earth picked up the arrival of both photons and neutrinos from the blast but the arrival of the photons was later than expected, by 4.7 hours.
Scientists at the time attributed it to a likelihood that the photons were actually from another source.
But what if that wasn't what it was, Franson wondered, what if light slows down as it travels due to a property of photons known as vacuum polarization - where a photon splits into a positron and an electron, for a very short time before recombining back into a photon,' phys.org' reported.
That should create a gravitational differential, he noted, between the pair of particles, which, he theorises, would have a tiny energy impact when they recombine - enough to cause a slight bit of a slowdown during travel.
If such splitting and rejoining occurred many times with many photons on a journey of 168,000 light years, the distance between us and SN 1987A, it could easily add up to the 4.7 hour delay, Franson said.