Particles from Space
Cosmic Rays travel at speeds beyond the reach of any accelerator on Earth.
Hey Friends!
I wanted to share a recent essay I wrote on the excess of positrons in cosmic rays, but without some background, I worried it wouldn’t make sense to many readers! To compensate, I’m sending out some transcripts from a podcast series I did a while back on Cosmic Rays. In case it helps, I’m also including the audio recording here.
Sean
Particle From Space!
Well before the gigantic particle accelerators like the Large Hadron Collider (LHC) at CERN or the Tevatron at Fermilab, Particle Physics was studied with weather balloons.
Well, I suppose it still is. I don’t want to give an overly simplified take on the history of Particle Ohysics - but it’s fair to say that physicists will study high energy particles wherever they can find them. And it just so happens that a large number of really high energy particles are constantly bombarding us from space.
A while back we did a podcast series on the alpha particle, were we learned about the solar wind and how the Earth’s magnetic field catches much of that ionizing radiation. Much of which we occasionally see displayed as the aurora.
Our Earth’s geomagnetic field is no match for these cosmic rays, which come flying at us from space with much higher velocities than anything in the solar wind. These particles smash right though the magnetosphere and into the molecules of the upper atmosphere.
From our perspective on Earth, these cosmic rays appear as showers of debris left over from those high altitude collisions. And they’re happening all the time.
There’s so much debris out there that hundreds of particles - particles of that debris - have passed through you since you started reading this essay.
Cosmic Rays
On the 15th of October, 1991, a particle with enormous energy entered our upper atmosphere.
A particle with this much energy had never been seen before on Earth. At least by humans. It had tens of millions of times more energy than anything produced at the Large Hadron Collider or FermiLab.
All told, it had the kinetic energy of a baseball moving at around 60 miles per hour. All packed into a single particle. And it was heading right for us.
The first thing it found upon arrival at Earth was the magnetic field. Traveling at such a high speed, it barely noticed the deflecting force and smashed through through into the atmosphere.
Even in the rarified air of our upper atmosphere, tens of kilometers above the Earth’s surface, there are plenty of molecules to go around. Way more than you’d find in outer space. And what do particles from space with a lot of energy do when suddenly surrounded by a bunch of molecules? They spend it.
That poor first molecule it encountered didn’t stand a chance. Whether it simply lost an electron or got completely blown apart is hard to say, but that incident cosmic ray quickly broke apart into a shower of particles high above the Earth.
Pions were certainly created, but with that much energy - 50ish Joules of energy - all kinds of particles - from Lambda Zero to Kaons to the Cascades and Sigma baryons - could have been present.
And all of them decayed as they usually do.
The resulting shower of decays grew wider and wider, until the final, resultant charged pions decayed into muons. And the neutral pions decayed into photons. There was so much energy to go around that electron-positron pairs were probably brought to life who would in turn radiate the rest of the energy away.
That final blast of radiation filled a circle kilometers wide that slammed into the US Army’s Dugway Proving Ground in Utah desert and - as luck would have it - right into the detectors of a physics experiment.
The detectors measured resultant spray and were able to back-calculate the energy of the original, impinging particle from space.
This aptly-named Oh-my-god particle was far and away the highest energy particle ever detected. To date, it’s not entirely clear what could accelerate a particle to such outrageous speeds.
And the current candidates are, frankly, terrifying.
The Power Law
In some respects, cosmic rays are kind of like earthquakes. There’s a lot more little ones than there are big ones. For earthquakes, big ones with lots of shaking - magnitude 7 or 8 - are, mercifully, fairly uncommon around the world. Small ones, like magnitude 1’s or 2’s happen almost every day.
Cosmic rays follow a similar law.
Cosmic rays with an energies around 1000 MeV arrive almost continuously, from every direction, where as cosmic rays with energies a million times that might strike near you a few times per year. Those ultra high energy cosmic rays, a bit like the oh my god particle, whose energies can be measured macroscopic units like Joules? They might strike your whole town maybe once a year, if that.
As scientists would say, the frequency depends inversely on energy.
More precisely, the frequency is derived from a power law1. A power law for cosmic rays implies that the relative likelihood of two events, with energy E1 and E2 is proportional to their ratio, raised to some exponent.
Power laws are simple enough to understand, but difficult to explain. Bigger cosmic rays are less common, sure. But what’s difficult to explain about a power laws is that little number. The scaling exponent. Where does it come from?
Sometimes there’s a simple explanation. A mass tied to a spring has an energy that scales with the length of the spring, squared2:
For other phenomena like earthquakes and cosmic rays, it’s a collective effect. All the things inside and outside our galaxy that throw cosmic rays at us contribute to that effect. Other stars and planets sometimes get in the way and capture some of them. Cosmic rays in interstellar space sometimes decay or interact en route, turning into other particles with somewhat lower energy, muddying things a bit. Magnetic fields in space - like the one surrounding our Earth - deflect cosmic rays ever so slightly.
It’s a bit like studying earthquakes. There are just so many moving parts and we don’t know all the details.
The Ankle
Despite all this complexity, the shape of the cosmic ray power law can tell us a lot about the nature of cosmic rays. For example:
For particles coming in with the highest energies, the so-called UHECR3s /YOOheckers/ the power law for cosmic rays actually changes a bit. It flattens out. It becomes slightly less sensitive to Energy.
Aficionados call this change the “ankle”.
The ankle represents a cut in the cosmic ray spectrum at about half a joule of kinetic energy.
It’s a small effect on a small fraction of the total number cosmic rays we see on Earth. But to astrophysicists who study these things, it says a lot.
Namely, this “kink” or “ankle” in the power law suggests a possible change in where those cosmic rays are coming form. The leading explanation is that those ultra high energy cosmic rays - those cosmic rays with kinetic energy greater than half a Joule or so - are coming from outside our galaxy.
Which is just as well. It’s hard to imagine anything inside our own galaxy capable of accelerating individual, microscopic particles up to macroscopic energies. The possibilities are just… too frightening.
Next time, we’ll explore what kinds of objects in space serve as particle accelerators millions of times more powerful than anything humans ever built on Earth.
Often in nature, phenomena described by such power laws which occur when there is no characteristic length or energy scale governing the physics of that system. Earthquakes, tornados and cosmic rays occur across vast ranges of energies. In particle physics, this also occurs with high energy collisions inside some composite particles like the proton. In any case, these phenomena are both approximate and valid only across a finite range of values.
As with the last note, this relationship, sometimes known as Hooke’s Law, is only valid for a finite range of x. When x gets too large, the spring eventually stretches too far and permanently deforms.
Ultra High Energy Cosmic Rays!
Good narration dude, a lot of info.