Cern's Large Hadron Collider routinely collides  particles at energies equivalent to a fraction   of a second after the Big Bang.

If this  worries you, then the following fact will   either put you at ease or scare the hell out  of you.

And that's that a particle with the   energy of an LHC collision hits every square  kilometer of the Earth every single second.

And we only relatively recently figured out  where these cosmic rays are coming from.

—- In 1912, Victor Hess took a hot-air balloon ride  and got a Nobel prize.

Hess had equipped the   balloon with devices sensitive to electrically  charged particles.

As the balloon rose,   particle detections increased, quadrupling by the  maximum altitude of 5 kilometers.

Hess wondering   if these might be charged particles coming  from the Sun, and so repeated the experiment   at night and during a solar eclipse.

The  results were the same.

He concluded that   the vastness of space itself must be pelting  us with energetic charged particles.

In this   way cosmic rays were discovered.

Hess’s  Nobel followed a couple of decades later.

Now we routinely measure cosmic rays with a  wide range of advanced observatories—enormous   arrays of ground detectors like the  Pierre Auger Observatory in Argentina,   or orbiting detectors on satellites and the  ISS.

But for a long time the mystery only   intensified.

We learned that the energies  of cosmic rays span 12 orders of magnitude,   up to energies 10 million times that of the  Large Hadron Collider.

The most energetic   of all—the so-called oh-my-god particle—had  the kinetic energy of a well-thrown baseball.

We did an episode on the oh-my-god particle  a while back, but didn’t get into the awesome   details of how cosmic rays are really produced.

And we should probably cover that, because the   origin of cosmic rays remained a mystery for  a century after they were discovered.

It was   only a decade or so ago that scientists came to a  general agreement about the origin of most cosmic   rays—the ones without the most ridiculous energies  mostly come from supernovae.

From exploding stars.

We still don’t know about the more energetic ones,  but it turns out that by understanding the complex   process behind the supernova-generated rays we  can come up with possible mechanisms for the rest.

So let’s see how exploding stars can act  like particle accelerators.

For that we   need two things: shockwaves, and magnetic fields.

Shockwaves are a big part of what makes an  explosion an explosion, versus say a bunch of   hot gas expanding.

They’re also a key ingredient  in making cosmic rays.

We’ll start with sound   waves.

These are pressure waves—oscillating  patterns of high and low pressure and density.

We have some initial event that compresses  the air, creating a high density region.

That region pushes away surrounding air molecules,  creating a new high density region down the path,   which pushes on the following region, and  so on.

The wave propagates at the speed   of sound while individual air molecules  just move back and forth on the spot.

A shockwave is what happens when the initial event  causes the particles to move faster than the speed   of sound in whatever medium they’re in.

That  means the particles will outpace any sound waves,   and instead of bouncing backwards will continue  on, sweeping up the material they pass through.

Familiar sources of shockwaves include supersonic  vehicles or meteors, which push the air ahead   faster than the speed of sound.

Or any explosion  in which the expanding material is supersonic.

Shockwaves in space are pretty common, both from  explosions and from objects moving faster than the   local speed of sound.

Although it’s generally  true that in space no one can hear you scream,   that doesn’t mean there isn’t sound in space.

The  gas between the stars may be incredibly diffuse,   but it’s still there, which means it can support  sound waves and so can also experience shockwaves.

Perhaps the most famous type of space  explosion is the supernova.

When the   central core of a massive star has fused  itself into a giant ball of iron and nickel,   it collapses into a neutron star and releases a  prodigious amount of energy in neutrinos.

These   sweep up the outer layers of the collapsing core,  sending them out a few percent the speed of light.

This is much faster than the speed of sound in  the interstellar medium, so we have a shockwave.

But a few percent lightspeed is not the  99.999 etc % light speed of some of the   cosmic rays we’ve seen.

It was Enrico  Fermi who figured out how supernova   shockwaves can accelerate particles to  vastly higher speeds than they themselves   traveled.

This is the same Fermi who was  one of the founders of quantum mechanics,   built the first nuclear reactor, and  wondered where all the aliens are.

At around the same time that we were building  our first particle accelerators on Earth using   powerful magnetic fields, Fermi realized  that magnetic fields might also do the   trick in supernovae.

We all know that the  Sun has a magnetic field.

When that field   gets tangled up, like it is right now, we  get magnetic storms.

These frequently erupt,   spewing out knots of magnetic field and  energetic plasma into the solar system.

So if the Sun can do that over and over  .. imagine what an exploding star can do.

Let’s think about magnetism for a minute.

Moving  electric charges make magnetic fields with field   lines that loop around the direction of  motion.

And magnetic fields generate a   force on moving charged particles that cause  those particles to circle around the field   lines.

The net result is that magnetic  fields and the currents they generate   are self-reinforcing.

In a plasma, magnetic  fields sort of act like rebar in concrete,   reinforcing the plasma, giving  it inertia, and also shaping it.

So when a star explodes,  its magnetic field doesn’t   just vanish.

Some gets locked in the  resulting neutron star or black hole,   but the rest is dragged outwards with  the rest of the explosion.

The expanding   shockwave is a tsunami of both charged  particles and of intense magnetic field.

In an explosion in the atmosphere, the  shockfront high-density of air smashes into   the lower-density air ahead of it, dragging it  into the propagating front.

But the interstellar   medium is so diffuse that we don’t expect a lot  of collisions with an oncoming supernova front.

That would kill the shockwave pretty quickly  if it weren’t for magnetic fields—but not just   those in the shockfront.

Weak tendrils of  magnetic field suffuse the entire galaxy,   weaving through the interstellar medium.

Our  supernova shockfront first grabs this ISM field   and drags it forward, and this in turn drags the  ISM plasma.

We call these collisionless shocks.

Let’s think about the two components of the  shockfront.

We have the matter.

Material from   the explosion is coming in fast.

It slows down  in the shockfront because this is where we’re   colliding with the interstellar medium.

Meanwhile  material in the interstellar medium is being swept   up and accelerated as it enters the front.

So if  you’re a proton traveling with the shockfront,   in your reference frame you actually see  material rushing in towards you from both sides.

Next we have the magnetic fields.

Behind you  there’s the magnetic field from the explosion,   which piles up as it slows down approaching  the front, and ahead you have the weaker   interstellar field which is also piling up.

These result in two walls of magnetic turbulence.

So our proton is chilling in the shockfront  and bounces its way out, forwards or backwards,   it doesn’t matter.

There it finds itself in  a magnetic tangle that’s effectively moving   towards it.

It gets deflected back into  the front and is likely to pick up a bit   of energy in the process due to the relative  motion of the deflecting field.

And then it   goes out the other side of the front and gets  turned around again, gaining a little more   energy.

This can happen many, many times,  accelerating the proton to enormous speeds.

This is the Fermi Acceleration Mechanism,  also known as diffusive acceleration because   the particles diffuse or bounce randomly through  the shock and tangled fields This is how we turn   a supernova into a particle accelerator.

In fact the resemblance to an artificial   particle accelerator is stronger than just  the fact that they both use magnetic fields.

Why is the Large Hadron Collider so … what’s the  word … Large?

It’s because larger loops can keep   hold of faster charged particles.

The turn radius  of a particle in a magnetic field is called the   Larmor radius—or gyroradius or synchrotron  radius—same thing.

This radius increases with   the speed of the particle and decreases  with the strength of the magnetic field.

Supernova remnants have magnetic fields not that  different to the Earth’s—really weak compared   to those in a typical particle accelerator.

But  they can still accelerate particles to a large   fraction of the speed of light.

The combination of  weak fields and high speeds requires an extremely   large Larmor radius.

And this is why supernova  remnants are good particles accelerators—they   don’t have strong fields, they are just  ridiculously enormous.

The turbulent regions can   extend many times the width of our solar system,  while the overall radii of these expanding shells   reach several light years.

Plenty of space for  cosmic rays to bounce their way to extreme speeds.

Supernovae can energize a proton up to 10^17  electron volts.

A supernova-accelerated iron   nuclear can reach about 10 times that  energy.

But the maximum energy of a   supernova-accelerated particle is  still around 100 times lower than   the highest energy cosmic rays.

So where do the rest come from?

It’s not so easy to tell where any  given cosmic ray comes from because   we usually don’t see them directly.

They tend to collide with molecules   in the upper atmosphere and send showers  of particles towards the ground.

But by   collecting those particles we can get a  rough sense of the origin of a cosmic ray.

The Pierre Auger Observatory is a giant cosmic  ray detector that spans 3000 square kilometers   in western Argentina.

It has found that  the origins of ultra high energy cosmic   rays are scattered across the sky.

If they  were from sources within the Milky Way then   we’d expect a concentration in the plane of  the Milky Way’s disk.

We can’t see exactly   where these particles are coming from, however  their origin does appear to be correlated with   the large-scale structure of the universe.

They’re likely coming from other galaxies.

There are many potential origins for the highest  energy cosmic rays—perhaps in the extreme magnetic   fields around magnetars or around the supermassive  black holes in active galactic nuclei.

These   cosmic particle accelerators look a bit more like  traditional synchrotron accelerators like the LHC.

But there’s also a strong possibility that these  cosmic rays also come from scaled-up versions of   the supernova shock front.

Shocks are observed  everywhere in the universe, and on all scales.

Enormous shockwaves result from collisions between  galaxies, from the most extreme stellar explosions   like hypernovae or gamma-ray bursts, and from  the jets produced by active galactic nuclei.

This last one is especially promising.

When  a feeding and magnetized supermassive black   hole sends jets of material punching through its  surrounding galaxies, shockwaves are inevitable,   and the Fermi mechanism will give us cosmic rays.

And when those jets punch into the intergalactic   medium beyond the galaxy, they can bloom into  giant lobes of diffuse plasma and magnetic field.

Although magnetic fields are weak in these lobes,  they span hundreds of thousands of light years,   and so are able to accelerate cosmic  rays to the highest possible energies.

It took a balloon ride and a century  of sciencing to be fairly sure that   exploding stars are bombarding the earth with  cosmic rays.

It’s going to take a bit longer   to fully understand the most energetic of  these particles.

Whether they’ll turn out   to be from magnetars or supermassive black  holes or galaxy-scale magnetized shockwaves,   the search is now on for the largest  particle accelerators in all of space time.