Light

# Cherenkov Radiation Is a Sonic Boom for Light, and It's Beautiful

When something goes faster than the speed of sound, you hear a loud sonic boom. When something goes faster than the speed of light, likewise, there's a "luminal boom." It appears as a pretty blue glow known as Cherenkov radiation, and despite what the laws of the universe may tell you, yes — it actually does happen.

## Yes, Officer, I Know Just How Fast I Was Going

If you're scratching your head over that last fact, then you know that nothing can go the speed of light, much less go faster. Einstein's special theory of relativity says that mass and energy are intimately linked, so anything with mass that goes the speed of light would require an infinite amount of energy, leading its mass to become infinite as well. That, unfortunately, is impossible.

But here's the catch: Nothing can go the speed of light in a vacuum. In another medium, like water or glass, light slows down since it interacts with the atoms it comes in contact with. You can see it happen in the way images are warped when you view them through a magnifying glass or an aquarium — that's a result of light's refraction, or the way it changes speed and direction when it moves from one medium to another. The refractive index of a medium describes the difference between the speed of light in a vacuum (299,792,458 meters per second, for those playing along at home) and the speed of light in that medium. For air, it's just slightly slower, at 1.0003. Water is a bit slower at 1.3; glass is even slower at about 1.5; and don't get us started on diamond — that has a refractive index of 2.4.

## Big Blue Boom

Photons are particles of light, and they're what's slowing down in air and water and glass. But they're not the only particles out there. Electrons, for example, don't interact with their environment the same way photons do. That means that if you have electrons traveling in the right medium, they can exceed the speed of light.

That's just what happens in the Advanced Test Reactor at the Idaho National Laboratory. The reactor is submerged in water to help keep it cool, and while light travels through water 30 percent slower than it does in a vacuum, electrons travel a bit faster than that. When electrons win this photonic footrace, it results in a "luminal boom" called Cherenkov radiation, named for the Soviet scientist who discovered it in 1934 (and won a Nobel Prize for it in 1958). It shows up as a beautiful blue glow — way more attractive than the deafening thunder of a sonic boom if you ask us.

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Learn more about our curious cosmos in "The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory" by physicist Brian Greene. The audiobook is free with an Audible trial. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Ashley Hamer

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