Is the speed of light slower than we thought?
spaceSupernova 1987A, seen in light of different wavelengths.                                                          Image: ALMA/NASA

A new peer-reviewed paper by physicist James Franson from the University of Maryland, Baltimore County in the US has caused a stir among physics community. Published in the New Journal of Physics, the paper points to evidence suggesting that the speed of light as described by the theory of general relativity, is slower than originally thought.

According to the theory of general relativity, light travels in a vacuum at a constant speed of 299,792,458 metres per second. The speed of light - or number of light years - is what we measure practically everything in the universe by, so it’s pretty important we get it right. 

Franson’s paper is based on measurements taken of the supernova SN 1987A, which collapsed and exploded in February 1987. Physicists watching the supernova collapse picked up on the presence of both photons and neutrinos in the blast, but as Bob Yirka reports at, there was a problem.

The physicists recorded an odd time for the arrival of the photons. According to their calculations, the photons were supposed to arrive three hours after the neutrinos and keep the same pace as they travelled through space. But they arrived 4.7 hours late. Maybe the photons were emitted slower than expected, some scientists suggested, or maybe the neutrinos' travelling speed was slower than expected. The most popular theory was that the photons came from some other source entirely.

But what if they came from the supernova explosion, says Franson, and their late arrival is explained by light slowing down as it travels due to a property of photons known as 'vacuum polarisation’. Vacuum polarisation describes a process where an electromagnetic field causes a photon to be split into a positron and an electron for a few moments, change the current and charge of the electromagnetic field, and then snap back together into a photon.

Yirka explains why this is important:

That should create a gravitational differential, [Franson] notes, 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] suggests.

If Fransons’s theory is correct, every distance measured by light years is wrong, including how far away the Sun and distant galaxies are from the Earth. In some cases, says Yirka, astrophysicists would have to start all over from scratch.