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Faster Than Light

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Maintained by suitti@uitti.net, Stephen Uitti
Is the speed of light ever exceeded?

The unexpected answer is yes.

While the speed of light in a vacuum is never exceeded, light moves slower through other mediums. The speed that light travels through things like air, glass, water, etc., is related to it's diffraction index. So, while light speed in a vacuum is never exceeded, other things may move faster than light does through the local medium.

When does this happen?

Cosmic rays are not really rays at all. They move slightly slower than the speed of light through a vacuum. That's because they are not light (massless photons), but rather particles (they have mass). Generally, they are protons (think hydrogen with the electron stripped off) that have been accelerated to high speeds - perhaps in a supernova or because they were caught in a really wild magnetic field. One might be moving at 99.999% of the speed of light in a vacuum. When they hit the Earth's atmosphere, they don't instantly slow down - they have to interact with something first - like air molecules. Until then, they have their initial high speed - which generally exceeds the speed of light through air. When they do this, they give off Cerenkov radiation. The charged particle excites the air molecules which then return to their normal state emitting photons of blue light.

In nuclear reactors, the fission products are high speed neutrons and protons. There is a blue glow in the water surrounding nuclear reactors, which is Cerenkov radiation.