Cosmogenic Muons. Part One.

Bombardment from Space

On the trail. In your car. At your desk. Hundreds of particles are zipping by us every second. They’re raining down upon us from the sky. Most zip right through us. Sometimes we get hit, but you can’t really feel them. They’re just too small.

Unlike the alpha particles which launched from heavy nucleus deep underground, these little atmospheric beasts are moving with extremely high energy. They fly right through our skin. They fly straight through glass. Through bricks. Through hundreds of feet of rock. Physicists have set up laboratories in deep underground caves just to keep them from messing up their experiments.

But just what are these particles flying at us from the sky? And where are they coming from? What exactly is going on in the upper atmosphere?

Cosmic Rays

From our perspective on Earth, the stars are fixed in the night sky. Constellations like Cassiopeia, Orion, Taurus and Scorpius move long in a reasonably predictable pattern. Night after night. Planets move through this backdrop with a familiar regularity. You might see the occasional satellite or even meteor shower, but little else changes.

Outer space - the literal space between things like planets and stars - might seem sterile, cold or even barren. But nothing could be further from the truth.

The Earth is constantly being bombarded by particles of outrageously high energy which come from outer space. Oddly, these are not the particles that bombard us. They are much heavier and much faster, and they are usually destroyed upon contact with molecules in the upper atmosphere.

We call these particles from space cosmic rays, and they don’t penetrate much further than our atmosphere. What we experience on Earth amounts to the shrapnel from the explosions tens of thousands of feet above the ground.

And there’s a lot of it. Typical averages report about 144 particles per square meter per second at sea level. That’s about how many fly through you! quite a bit!

Before we discuss those “shrapnel” particles - and the associated explosions - we should probably describe these cosmic rays.

What are Cosmic Rays?

Cosmic rays themselves are a fairly diverse lot, made up of both atomic nuclei and electrons. What they all share in common is an immense velocity.

90% of those particles are simply protons. They’d be simple and mostly harmless as hydrogen atoms if not for their outrageous energy.

Of the rest of the cosmic rays, about 9% are alpha particles, you know, helium nuclei. An even smaller fraction includes heavier stuff, like Oxygen or iron nuclei. They’ve been stripped of electrons in the process of accelerating up to such intense speeds.

There’s also electrons, which comprise about 1% of the full bombardment.

The energy of the individual particles - these so-called cosmic rays - falls on a spectrum. A spectrum from faster, to almost impossibly fast. The faster the cosmic ray, the higher the energy. The higher the energy, the rarer they are.

The precise distribution is often used to identify where they come from. You can see a plot of that distribution with some these descriptions on our website which we’ll link in the show notes.

A big challenge in sending humans to Mars, is making sure they don’t get killed or harmed by all that radiation during the years of travel in the space between Earth and Mars.

Suffice it to say, our atmosphere protects us from a lot!

Accelerators in Space

Big stars, stars bigger than our sun, end their lives in dramatic explosions. For a brief moment, these run of the mill stars become the brightest objects in the universe. The result of those explosions are a remnant of hot, expanding dust and particles, expanding into space.

Particles fly at us from these explosions, sure. But what remains behind after a supernova can be far more powerful.

Astrophysical models tell us that supernovae remnants - like neutron stars - can generate large magnetic fields, and particles get trapped in them, just like those particles responsible for the aurora.

These particles bounce around, collide with other particles, and sometimes, every once in a while, a particle gets a kick so large that it can escape the magnetic trap and flies out into deep space.

A cosmic ray is born.

Now, these cosmic rays are statistical flukes. Particles in those gas clouds trapped in magnetic fields have all kinds of energies. Some are low, but most are average. Average depends on the size and strength of the accelerating magnetic field.

Of course, some velocities are higher than average, and some are super high. Very few are super duper high. The magnetic fields of these massive supernova remnants can only bind particles of a certain energy level before they get kicked out.

In a sense, those particles boil off into cosmic rays, and that boiling point is fixed by the size and strength of the supernovae remnant. We only get to see those particles that amassed enough energy to escape.

There are bigger and scary things out there with even larger magnetic fields that can accelerate particles even faster, perhaps arbitrarily fast. The supermassive black holes at the center of galaxies comes to mind. But the physics of acceleration is essentially the same.

Despite the fact that cosmic rays are statistical flukes - very rare, ultra high energy escapees from their magnetic field traps - there are a lot of them. Because there are a LOT of stars in each galaxy and a LOT of galaxies out there.

Outer space is very much alive with magnetic fields and radiation.

Cosmogenic Muons

These cosmic rays - these extremely high energy particles coming at us from all directions - collide with molecules in the upper atmosphere. The cosmic rays are traveling with such high energy that these collisions are rather dramatic explosions, creating a spray of different particles.

Pions are a common part of that spray. The upper atmosphere is a great place to find pions in the wild. Of course, the charged pions decay rapidly into muons and neutrinos.

We can’t really see neutrinos - remember they don’t interact much. Muons, on the other hand, are far more tactile. They have an electric charge and quite a bit of mass. We know how they interact and see them all the time in particle accelerators. They eventual decay into electrons and positrons.

So muons - hundreds of muons per square meter per second - are smashing into to us.

Long Lived Muons

So let’s review what we’ve discussed today. The upper atmosphere protects us from the constant bombardment of high energy cosmic rays coming from monstrous accelerators in outer space. It filters out much of the radiation and energy, and all that remains are muons. The Cosmogenic Muons. Muons you can detect here on the surface of the Earth with a cloud chamber or a Geiger counter.

But there’s a problem. Born in the upper atmosphere, these muons have to travel 30 miles or so - almost 50 kilometers - before they strike the ground. They move very, very fast of course. But muons are unstable.

As you might recall from our earlier podcast, their lifetime is 2.2 microseconds, an eternity by particle physics standards. But it’s not really that long. Do you know far light travels in 2.2 microseconds? About 660 meters. Less than half a mile. Planes fly higher than that!

The thing is, muons are definitely the particles that we see on the ground. You can measure their mass by watching them collide. You can see them decay into electrons if you try hard enough.

But nothing. Nothing can travel faster than the speed of light. So how do those muons make it the other 29 and a half miles - the other 49.4 kilometers - before they impact us? Why don’t they decay? Or do they? Or maybe, are they traveling faster than the speed of light?

Find out next time, in part two of our discussion of Cosmogenic Muons.