The speed of light is constant, or so textbooks say. But some scientists are exploring the possibility that this cosmic speed limit changes, a consequence of the nature of the vacuum of space.
The definition of the speed of light has some broader implications for fields such as cosmology and astronomy, which assume a stable velocity for light over time. For instance, the speed of light comes up when measuring the fine structure constant (alpha), which defines the strength of the electromagnetic force. And a varying light speed would change the strengths of molecular bonds and the density of nuclear matter itself.
A non-constant speed of light could mean that estimates of the size of the universe might be off. (Unfortunately, it won't necessarily mean we can travel faster than light, because the effects of physics theories such as relativity are a consequence of light's velocity). (10 Implications of Faster-Than-Light Travel)
Two papers, published in the European Physics Journal D in March, attempt to derive the speed of light from the quantum properties of space itself. Both propose somewhat different mechanisms, but the idea is that the speed of light might change as one alters assumptions about how elementary particles interact with radiation. Both treat space as something that isn't empty, but a great big soup of virtual particles that wink in and out of existence in tiny fractions of a second.
The first, by lead author Marcel Urban of the Université du Paris-Sud, looks at the cosmic vacuum, which is often assumed to be empty space. The laws of quantum physics, which govern subatomic particles and all things very small, say that the vacuum of space is actually full of fundamental particles like quarks, called "virtual" particles. These matter particles, which are always paired up with their appropriate antiparticle counterpart, pop into existence and almost immediately collide. When matter and antimatter particles touch, they annihilate each other.
Photons of light, as they fly through space, are captured and re-emitted by these virtual particles. Urban and his colleagues propose that the energies of these particles -- specifically the amount of charge they carry -- affect the speed of light. Since the amount of energy a particle will have at the time a photon hits it will be essentially random, the effect on how fast photons move should vary too.
As such, the amount of time the light takes to cross a given distance should vary as the square root of that distance, though the effect would be very tiny -- on the order of 0.05 femtoseconds for every square meter of vacuum. A femtosecond is a millionth of a billionth of a second. The speed of light has been measured over the last century to high precision, on the order of parts per billion, so it is pretty clear that the effect has to be small.
To find this tiny fluctuation, the researchers say, one could measure how light disperses at long distances. Some astronomical phenomena, such as gamma-ray bursts, produce pulses of radiation from far enough away that the fluctuations could be detected. The authors also propose using lasers bounced between mirrors placed about 100 yards apart, with a light beam bouncing between them multiple times, to seek those small changes.