The term “the speed of light”, while accurate, is misleading. Light in a vacuum does travel at “the speed of light”, but that’s because the fundamental quantum constituents of light, photons, have no mass. Any zero-mass particle must travel at this speed.

When Einstein worked out Special Relativity, he proved that there was a maximum speed at which energy, matter, and information in the universe can travel. He showed that it takes infinite energy to accelerate particles with mass to that velocity, but that zero-mass particles naturally travel at this speed.

Though, to be honest, there are only two types of particle currently known to be massless: photons and gluons, and gluons can only exist inside subatomic particles, sticking them together. Maybe “the speed of light” is a fair enough term after all.

Except the actual speed at which light travels depends on the medium that it’s passing through. As I said, light in a vacuum travels at “the speed of light”, but light passing through a solid, liquid or gas is slower. For example, light in glass is slowed wayyyyyy down to about 2/3 of “the speed of light”. In water, it’s about ¾. Even air slows down light by about 3%.

reactorAlthough particles with mass can’t ever reach the speed of light, it’s common enough to get them to a respectable fraction of it. The Large Hadron Collider at CERN can accelerate protons to 99.999999% of light speed, the highest ever achieved artificially (on Earth), but the particles from space called cosmic rays can be ten million times more energetic, nudging them even closer to that ultimate speed limit.

Even radioactive decay can produce very fast-moving particles as the atoms spontaneously break up and fire out shrapnel. Nuclear fuel rods and waste for reprocessing are often kept under water to contain the particles emitted by the radioactivity — the particles repeatedly collide with water molecules and lose energy as they bounce off until they no longer are dangerous. Though initially, they can have enough energy to be travelling close to “the speed of light”. But remember that light in water can only travel at ¾ of its maximum speed. The radioactive particles can actually travel faster than light does.

Just to be clear, nothing is travelling faster than “the speed of light” because that’s impossible, but radioactive substances can emit particles that travel faster than light in water. And you can tell. In a process analagous to the way a faster-than-sound aircraft creates a shock wave that can be heard as a sonic boom, faster-than-light particles create a shock wave that can be seen as a flash of light. With lots of speeding particles, their flashes merge into a steady glow.

It’s called Cherenkov radiation, after the Russian physicist who first explained it, and it’s a beautiful blue colour. Ever since I first saw it, I’ve loved it. Not just the pretty colour, but what it repesents: a billion little sonic booms in light.


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