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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.
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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 exploding stars. From supernovae.
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 before we get to shockwaves.
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 mean there is no shockwave 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 magnetic 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 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 so for supernova remnants to be good particles accelerators—
without strong fields, they just need to be 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 eneregies.
Supernovae can energize a proton up to 10^17
electron volts. A supernova-accelerated iron nucleus 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 shockwave. 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.
How do you do fellow simulations? Before we end the episode, I just wanted to let you know that
we are adding a couple new permanent items to the merch store. Back by popular demand
is the return of the Be Quiet The Devs Will Notice T-shirt, which now also available for
the first time as a hoodie. And we also have our new How Do You Do fellow simulations,
T-shirt and sweatshirt. For those of you who'd like to blend more seamlessly into the simulation.
Whatever your stance is on your virtual existence,
we've got you covered at our very virtual marketplace over at pbsspacetime.com/shop.