Ask a Physicist

I have heard that there are particles that travel faster than light emitted in nuclear reactors. Is this true? How can things travel faster than the speed of light?
Submitted by Vinayak from India

Yes, it's true. In fact it's why radioactive stuff glows.

But it doesn't violate special relativity, which famously states that nothing can travel faster than the speed of light. Actually the words "in vacuum" should be added to the end of that statement.

Light travels through vacuum at a certain speed, usually called c, and that speed is the absolute limit. But light slows down when it travels through other things. For example, in water the speed is about 3/4 of c. In glass the speed can be a lot less, and in the last few years people have come up with materials that slow light down to almost nothing. Basically light involves a vibrating electric field, and when that field has to push around the electrons in a material its life gets more complicated. It may even die out altogether, as in opaque materials.

Many nuclear reactor cores consist of radioactive fuel rods dunked in water. The fuel rods emit electrons, all of which have to obey the ultimate speed limit of c. But in water they don't have to obey the local speed limit of 3/4 of c, and some don't. Those fast electrons cause the water to emit light, which comes out more or less along the electron's direction of travel. This effect is called Cherenkov radiation, and it's what makes that eerie blue glow you see in a reactor core (preferably from a safe distance). The electrons eventually slow down, which is why the glow isn't seen too far from the fuel rods. The glow occurs in air, too, but it's a lot fainter because the speed of light in air is almost c and fewer electrons are emitted that fast.

Cherenkov radiation isn't just pretty, it's useful: Many physics experiments involve looking for charged particles traveling almost as fast as c. It can be hard to track those particles directly, but you can set up a tank of water and look for the Cherenkov radiation. You can even use it to work out how energetic the particles are and which direction they are traveling.

Of course, the glow is also useful as a "hands off" warning.