If you had to pick one idea that is emblematic of 20th century physics, a good candidate would be the universal constancy of the speed of light. When Einstein put forward this notion in the special theory of relativity, it solved the long-standing mystery of why measurements did not show any variation in the speed of light with Earth’s motion through space.

It also ushered in the idea of space-time as a single entity. This leads directly to the derivation of the laws of physics as spacetime symmetries, a process that has been crucial for both general relativity and the standard model of particle physics.

So to question the constancy of ‘c’ is to invite controversy. And yet there are reasons to think we should certainly investigate this hypothesis in more detail using the tools of experimental physics, say Farid Ahmed and pals at York University in Toronto Canada. Today, they give us an interesting review of the various ways in which the measurements have been made.

The experiments to measure the constancy of the speed of light fall in to two categories, say Ahmed and co. In the first are experiments like the famous Michelson Morely interferometer which measure the round trip speed of light along some closed loop. This is essentially its average speed over some distance and back again.

No variation has ever been found but these experiments leave open the possibility that the speed of light is different over each leg of the journey. So there is another category of experiments which attempt to measure the one-way speed of light.

One interesting question immediately arises: how do you measure the one way speed of light? It turns out there are various methods. One idea involves the emission and absorption of gamma rays by certain kinds of atoms in a solid. The process of absorption is very sensitive to the energy of the gamma rays. So if the speed of light (and therefore its energy) varies with direction, then the rate of absorption ought to change too.

In the 1960s and 70s, various physicists looked for a directional dependence by placing a gamma ray emitter at the edge of a rotating disc and an absorber at the centre. They then looked for any difference in the rate of absorption as the disc rotates but found none.

Neither have physicists using other techniques found any variation either. (The controversial Bulgarian Stefan Marinov claimed to have found evidence of a variation in the one way speed of light but his claims are not considered valid by most mainstream physicists.)

Ahmed and co say these experiments need to be refined and improved and are working on their own measurement of the one way speed of light.

The work may have important consequences. Ahmed and co point out that string theory predicts a violation of the constancy of the speed of light as does another idea which proposes that a variable speed of light would solve various problems in cosmology.