I recently finished reading The Orphan Master’s Son, a (Pulitzer-winning, apparently) novel set in 2000’s-era North Korea. In one plot point, Kim Jong Il has agents steal a Japanese telescope designed to measure the cosmic microwave background radiation, under the mistaken impression that it will help him find uranium.

The novel plays it for (horrified) laughs, but I’ve seen this kind of misunderstanding crop up in the real world too. Sure, most people would realize that a telescope probably won’t help you find something buried under a mountain of rock. But there’s a deeper misunderstanding here. Ask yourself: what does “radiation” mean?

We talk about radioactive elements like uranium releasing radiation. We talk about electromagnetic radiation, including everything from gamma rays to visible light to the 5G of your cell phone. We talk about cosmic radiation coming in from space, and about the cosmic background radiation that originated in the early universe. For someone who doesn’t know much about physics, it probably sounds like all of these are the same kind of thing.

But they’re not!

It’s helpful to break things down in terms of particles. Radioactive elements release three main types of radiation: alpha, beta, and gamma. Alpha radiation consists of helium nuclei: two protons stuck together with two neutrons. Beta radiation consists of electrons. Gamma radiation is a type of electromagnetic radiation, and consists of photons: particles of light.

Anything we call electromagnetic radiation is a wave in the electromagnetic field, a ripple that moves through space. That’s different from other shapes of electromagnetic fields, like a magnetic field that stays in place. From a particle perspective, an electromagnetic wave is made up of photons, and physicists will often describe all such waves as light. Some of that light is the familiar rainbow of visible light, while some has lower-energy photons, like microwaves and radio waves, or higher-energy photons, like gamma rays or X-rays.

Cosmic radiation (more often called cosmic rays), like radiation from radioactive elements, can be many types of particles again. Most of it consists of protons, while some consist of various nuclei, or electrons. A smaller fraction are antimatter, like antiprotons or positrons. Sometimes, physicists include neutrinos when they talk about cosmic rays, while sometimes they include gamma rays.

The cosmic background radiation is once again different. This is an overall hum of microwaves, electromagnetic radiation from the early universe that has gotten fainter and more diffuse over time. Cosmologists will sometimes talk about when the universe was “radiation-dominated” versus “matter-dominated”. They’re referring to times when most of the energy of the universe was in electromagnetic radiation, versus when it was mostly in other particles.

The only thing that ties all of these meanings together is the word’s literal meaning: radiation radiates. It starts in one place and travels outwards, having an effect at a distance. For the first scientists to observe phenomena like X-rays, this was almost all they knew about them, so they tossed them together in one category. Now, we know much more, but the names stuck.

So if you hear a physicist use the word “radiation”, try to avoid making any assumptions. You can’t know, just from that word, what they mean.

And please, don’t steal any Japanese space telescopes.