Warning, Heavy Science Follows...
More than a century ago, Albert Michelson and Edward Morley discovered something extraordinary. Whether the Earth is travelling towards or away from the Sun, a beam of light from the Sun's surface will always travel with a constant speed.
Before they observed this, in 1887, it was thought that speed of light would change with direction and speed of the observer. This is how the rest of the world behaves. If a speed camera, as it measures the speed of a car, was travelling towards that car, then the vehicle's speed would appear far greater than it actually is.
But the Michelson-Morley experiment showed this is not the case with light. A hundred years ago, a humble patent-clerk, Albert Einstein, explained it. Einstein reasoned that for the speed of light to remain constant, a moving observer's notion of distance and time must change (since speed is distance over time, a change in either quantity will affect the light's speed).
As a person speeds up, his personal time slows down. Einstein had constructed an elaborate theory - his special theory of relativity - around the constancy of the speed of light.
It came as a shock, then, for astronomer John Webb, of the University of New South Wales in Sydney, when he discovered that light could be fickle. Webb analysed light from super-bright quasars and found that 12 billion years ago, the fine-structure constant or alpha - an amalgamation of constants, including the speed of light - seemed in fact to be inconstant, being lower long ago. "I was surprised and had expected a null result," says Webb.
Some, although not Webb, interpret the result as showing a faster speed of light in the past. At least one of the constants that make up alpha could have changed to cause this. While Webb and others maintain the validity of their results, French and Indian astronomers report they have detected no change in alpha. This could suggest errors in Webb's and other astronomers' results.
However, if the results are accurate, this would support current thinking suggesting that, of all the constants, the speed of light is the most likely to change. Einstein's theories may seem foolproof and proved, yet if there is a varying speed of light (VSL) something must be wrong. So is a VSL real or a delusion among theoretical physicists? Like detectives in pursuit of a criminal, VSL supporters are in search of proof.
Dr Joao Magueijo, of Imperial College London, has shown, with other scientists, that the speed of light might change either instantly or gradually. We still haven't any idea which one is likelier. "[It's] too early to know," Dr Magueijo admits.
"Theories may be set up that accommodate both. I prefer those that lead to gradual changes because they could, in principle, lead to observational confirmation or refutation of the theory. In such scenarios, there are clear predictions that, in the next five to 10 years, could lead to a clear answer."
The group at Imperial has discovered that a VSL can provide an alternative to the idea of inflation, a theory put forward to account for the evenness of temperature across the universe. The reasoning goes that if the early universe went through huge inflationary growth, then the temperature would level out.
But a faster speed of light in the past could have the same effect. Light is energy so super-fast it could have transferred the heat quickly. This is a step on the road to reliable proof.
More exciting still is the discovery that a VSL could unify gravity with quantum mechanics: the Holy Grail of modern physics, uniting a theory of the very big with that of the very small.
But no matter how many problems a VSL solves, it needs to be subjected to rigorous testing. At the moment, evidence is slim. The speed of light (as opposed to alpha) is a dimensional constant - it travels a distance over a set time.
"Most people agree that it is not possible to measure changes in dimensional constants," says Webb. An observer can equally well conclude that it is distance or time that has changed, rather than speed of light.
whoah... breaking the speed of light would be a huge, huge deal for humanity... seriously.