The speed of light has been in the news recently - or at least the new sources I rely on - and it has left some people confused. For over a century we have been told that the speed of light is constant. No matter how fast you are traveling, or how bright or energetic the light is, it has a constant speed. That is the basis of the special theory of relativity which is one of the most experimentally verified theories in all of science. And now a team of physicists is claiming it might be wrong. How can this be?

As with so many topics in scientific research, the answer is in the extremes. Every experiment that has been conducted has confirmed a constant speed of light, but none of these experiments has been conducted a fraction of a second after the Big Bang, or billions of years into the future, or at energies orders of magnitude beyond the reach of the LHC, or ten billion lightyears away from our solar system. In these extreme realms and at extreme ranges of time, space and energy, it is still possible that the speed of light can vary. That is where variable speed of light theories come in.

Before I go on, it is important here to state clearly what VSL theories are not. We know that the speed of light in different materials is different - but that is due to the indirect path taken by photons as they move between atoms. That is not a variation in the speed of light at a fundamental level, but only in the apparent speed of light at a macroscopic scale. We also know that the speed of light in vacuum can be different in a gravitational field, and this effect has been measured in experiments already. However this is due to changes in the shape and size of spacetime itself, rather than a variation in the properties of the photons of light. It is still a variation in the speed of light, but one that is explained by general relativity and requires no new physics to explain it.

Now back to the original topic...

One possibility is that at very high energies, the speed of light could have a different value. Terrestrial experiments can measure the speed of light up to a few TeV of energy, and astrophysics experiments can indirectly probe a few orders of magnitude higher. (For comparison, visible light has energies measured in tens of electron volts, while these probes are into the trillions of electron volts and measurements of cosmic rays and other violent events can explore photon energies that are billions of times more energetic still.). But at some energy scale between 10 TeV and 1016 TeV quantum gravity becomes important, and we simply do not know how quantum gravity behaves or how it affects spacetime. Many models have been studied in which space and time become discrete at higher energies, in which case it is meaningless to talk about photon wavelengths that are smaller than the smallest unit of space. You also cannot consider a photon moving continuously between two points in discrete spacetime, and so speed cannot be defined. A photon disappears from one point and reappears at another point instantly. If these models are correct, then the speed of light will clearly change at very high energies.

Another possibility is that the speed of light could be different in different parts of the Universe. The best telescopes and experiments we have can make measurements of what the Universe looked like a few years after the Big Bang, but we cannot look back much further. Which also means that since no signal can travel faster than light, no experiment can measure the speed of light at distances greater than about fourteen billion light years. But the Universe is believed to be far bigger, and it is possible that the speed of light varies over these scales. Perhaps at the edge of our visible Universe the speed of light is a little different than what we have measured it to be, and yet it is forever outside of our range and so we will never know. But if the speed of light can vary over large distances, there may be a way to make it vary in other extreme systems - such as near neutron stars or black holes, or in the interactions of subatomic particles at extremely high energies.

And since space and time are two aspects of the same four-dimensional spacetime, it is also possible that the speed of light could change over time. We know for example that the still unexplained phenomena of dark energy has caused the rate of expansion of the Universe to start accelerating in recent years (which in cosmology means the last few million years), so it is possible that some as yet unobserved effect could make the speed of light start to change in the future. As with spatial variations, it is always possible that extreme energy events could allow such temporal variations to be measured now. 

And then there are the possibilities that there is a very small variation of the speed of light that is too minuscule to be measured by even the most precise experiments. Regardless of how good any experiment is, there will always be the possibility of an effect that is too small for it to detect. There are models in which the Universe has small hidden dimensions, and a photon that travels through these dimensions will appear to us to move very slightly slower. There is also a very negligible interaction between the photon and the Higgs field that permeates the Universe, and this effect would also slightly slow down photons travelling through it. Such an effect would introduce both a subtle time variation, and random delay on photon speeds, and would also reflect spatial variations in the Higgs field. That is why we always need bigger and better experiments - to constantly search for even smaller and more subtle effects.

So why were these theories in the news recently? Because of a couple of new papers in the academic literature. Astrophysics measurements of the early Universe and of high energy cosmic rays are now precise enough that some of the more popular variable speed of light models are testable. The data isn't quite good enough yet, but it is expected to be in the next few years. Personally I am not sure that the effects of VSL models will be strong enough to be detected in the current generation of experiments, but I am quite happy to be proven wrong.

So is the speed of light a fundamental constant of nature, or is it variable due to one or more of these effects? Only time and more experiments will be able to tell. Either way it will be an important step forward for science, and I cannot wait to see the results!