Cosmic rays remain, for the most part, a cosmic mystery.

But then, what about the universe doesn’t still remain partially shrouded in mystery?

Cosmic rays are radiation, but they’re not electromagnetic. That is, they’re not on the electromagnetic spectrum.

So, what are they?

First, let’s run through the electromagnetic spectrum quickly. Because most of what we know about cosmic rays is more what they’re not, it’ll help to know more about electromagnetic radiation.

It’s a spectrum of most of the radiation in the universe. The wavy line at the top represents the wavelength; each one of these types of radiation manifests as a wavy ray. That wave might be longer or shorter depending on the type of radiation.

Gamma rays, for instance, have the shortest wavelength—and thus the highest energy. Radio waves have the longest wavelength and the shortest energy.

Everything we know about the universe, we know from this spectrum. Optical astronomers observe in visible light wavelengths. Other astronomers control radio telescopes or space telescopes.

We know that cosmic rays exist. But they’re the one type of radiation we don’t have telescopes for.

We measure cosmic rays using special detectors that are put into space. Why space?

Because cosmic rays, like most electromagnetic radiation, don’t make it through the atmosphere. But, like all electromagnetic radiation, they make contact with the atmosphere.

Unlike electromagnetic radiation, when they make contact, they don’t do so as a wavy ray.

They don’t have a wavelength. They’re subatomic particles, and they crash into the atmosphere from space at tremendous velocities, shattering gas atoms in the upper atmosphere.

Fragments from those atoms rain down on Earth’s surface every day and night. They’re passing through you right now.

So…where do they come from, anyway?

That’s the thing. We don’t know.

We can’t know—yet.

Cosmic rays are atomic particles with electric charges. That’s a problem because the universe is full of magnetic fields. Every star has one, and every planet that’s large enough has one.

What does that have to do with anything?

Well…electric charges have a bad habit of interacting with magnetic fields.

Okay, maybe it’s a good habit—if it weren’t true, we wouldn’t be protected from the solar wind, and Earth’s atmosphere might never have formed.

But still. This interaction between cosmic rays and magnetic fields mean that cosmic rays bounce off of every magnetic field they hit. Every single one.

What happens if you shine a light into a room full of mirrors?

Before you know it, the light has zapped across every mirror it hits. If you weren’t standing there holding the light, how could you possibly figure out the original source?

Cosmic rays are more complicated than tracking down a single source of light. For one thing, we’re not talking about a single cosmic ray—we’re talking about a hazy, glowing fog of cosmic rays.

Yeah. When we try to detect cosmic rays, that’s what the universe looks like. A hazy, glowing fog.

How the heck do you track down the source of that?

I don’t have the answers. At least not yet. Later on, we’ll revisit cosmic rays—and hopefully learn a bit more about them.

In the meantime, however, we’re moving on to a different kind of particle—the atom.