Chapter 1: What are cosmic rays and why are they significant?
From Earth, the night sky appears ethereal, peaceful. It's so far removed from human civilization. You'd be forgiven for thinking we're immune to anything going on up there. But that couldn't be further from the truth. We are under attack. Cosmic rays are bombarding us from every direction.
Tiny particles that collide with our planet's atmosphere, setting off a chain reaction of ionisation that can render our satellites and other electronic machinery useless. But where do they come from? For hundreds of years, astrophysicists have searched in vain to find the origin of these elusive attackers with little success. Even the type of source has evaded their searches.
But now, thanks to a whole new field of research, we're starting to find answers. Not only do we now know what to look for, but they are proving more powerful than we ever imagined. What in the cosmos is possibly capable of producing a quadrillion electronvolts of energy? I'm Alex McColgan, and you're watching Astrum.
Join me as we follow the trail of cosmic rays, leading us right to the limits of physics as we know it. We'll see how scientists detect the highest energy particles in the universe, and meet a new class of astronomical objects whose extreme behavior, until recently, seemed like the stuff of science fiction. The Milky Way is full of energy, but our eyes can only detect a tiny fraction of it.
Beyond the spectrum of visible light, charged particles can give off higher energy radiation in the form of X-rays and gamma rays, creating a mess of energetic fingerprints throughout space. Astronomers can forensically decode these cosmic clues to understand the flow of energy through our galaxy. But among these charged particles, there is one group in particular that remains clouded in mystery.
Back in 1912, the Austrian physicist Victor Hess made a historic air balloon ascent up to 5,300 meters, where he could measure the rate of ionization in the upper atmosphere, or how quickly atoms and molecules are becoming charged. He expected to find that it decreased at higher altitudes, confirming the prevailing theories at the time. However, this was not to be. Unexpectedly,
Hess recorded a rate of ionization that reached three times higher than at sea level. This led to the realization that the ionizing radiation he had dedicated his career to studying came not from Earth, but from space. He had discovered cosmic rays, and they did not come in peace. Earth is under constant barrage from them.
These high-energy particles, mostly protons, travel at nearly the speed of light and collide with our planet's atmosphere, sending a shower of secondary particles down onto its surface. These attacks can do some serious damage. The secondary particles produced in cosmic ray showers, the likes of muons, neutrons, electrons, positrons, and gamma rays, can interact with living organisms,
contributing to genetic mutations and radiation damage. And when cosmic rays interact with satellites or other orbiting electronics, they can trigger a bout of ionization that can cause the circuits to degrade or even fail catastrophically. And they are not only a nuisance to our best space equipment, cosmic rays have meddled with our best scientists too.
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Chapter 2: What challenges do scientists face in tracing the origin of cosmic rays?
Their physical properties make them almost impossible to track. Magnetic fields bend their path before we can locate their origin, and they break down into a shower of particles before we can learn their identity. These challenges are laid bare when scientists tried to measure the cosmic ray spectrum.
If you plot the number of incoming high-energy particles as a function of their energy, you notice a distinct steepening point known as the knee, beyond which the really high-energy cosmic rays above four petaelectronvolts are much less common. At first, some scientists assumed that this knee must mark the boundary between cosmic rays coming from inside our Milky Way and those coming from beyond.
Others simply didn't know. But just as scientists were about to accept defeat, one observatory stepped in to revolutionize the search for these sneaky interlopers.
Before I start revealing the mystery, want to have a crack at it yourself? Of course, you'll need the right equipment, but even if you don't have a full observatory in your backyard, you can still see the wonders of the universe, galaxies, or the remnants of dead stars like the Crab Nebula in incredible detail using the Dwarf Mini Telescope, who've kindly sponsored this video.
Check out this image of the Crab Nebula taken by one user, Julia Resch, using this book-sized ultra-portable telescope. Julia only had a small window to take this image, as it had been the first clear night in four weeks, and there was work in the morning, but just look at the results she achieved.
This was thanks to the Dwarf Mini's ability to auto-track stars, letting it quickly orient itself towards your preferred target and its live stacking function. By overlapping multiple photos, it produces these really clear images of beautiful deep space objects in our cosmos. I'm a big fan of this telescope and can't recommend this enough for someone who's looking to get into astronomy.
So scan our QR code or follow the link in the description below to see for yourself. Astronomy enthusiasts who use the code ASTROM5 at checkout get 5% off their purchase. Now, enough waiting, it's time to take a look at the observatory that's helping solve the mysteries of these incredibly high-energy cosmic rays.
The Large High Altitude Air Shower Observatory is a ground-based observatory located nearly 4,500 meters above sea level in the mountains of Sichuan Province, China. It has one main objective, to find the highest energy particles in the universe. The chief scientist on this mission is Professor Zhen Cao, who gave Astrum an exclusive interview about Lasso's work.
He said, to make good measurements for the knee of the cosmic ray spectrum, you need two things. The detector must be big enough, and it must be able to identify the original particle from the air shower it creates. Cosmic rays become more rare the higher the energy. For the highest energy particles, less than 1 per square kilometer per century is expected to hit Earth.
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Chapter 3: How did Victor Hess contribute to our understanding of cosmic rays?
Name the Banyardos-Silk-West effect after the scientists who came up with it. It describes a phenomenon where if two particles move towards a black hole and collide near the event horizon, they could reach near unlimited energies. However, this theory was thought to be effectively useless, since the particles would no doubt be sucked into the black hole and lost forever.
That was until more recent years, when newer models revealed that a fraction of the particles would more likely be ejected back out into space. Free of the black hole, these particles could travel through space as none other than cosmic rays. Now, we've discussed some of the theory behind cosmic rays and where they come from, but it begs the question, what have we actually found?
The main thing to understand about this search is that it's really hard. LASSO may have identified 12 candidate Hevertrons in our galaxy, but finding the actual object responsible is a whole other task. That aside, the search is still ongoing, and a few sources have been found, so without further ado, let's take a look. One of the first objects pinned down may be familiar to you, the Crab Nebula.
Known as Messier 1, it's a supernova remnant found around 6,500 light years away in the constellation Taurus. Although stunning to look at, Messier 1 is not just a pretty face. It's capable of accelerating electrons to a quadrillion electronvolts of energy.
And as one of the best studied objects in the known universe, observing Messier 1 gives astronomers a good insight into how nature's particle accelerators work, In the gamma ray domain, Messier 1 shows some extreme variability.
It produces intense flares which can last anywhere between a few hours to a few days, and with our new understanding of pevatrons, scientists realised that these flares were the photons resulting from some serious electron acceleration. Exactly how this happens has been debated.
It could be DSA at the boundary between the particle wind and the medium surrounding the pulsar, energy released by magnetic field lines breaking and reconnecting, or a more complex mechanism within the particle wind itself. For context, electrons at high energies transfer part of their energy to background photons, boosting them to gamma rays that scientists can detect.
However, accelerating electrons is really difficult because they lose energy very quickly. To produce gamma rays with energies of a quadrillion electron volts, the electrons themselves must have had several times that energy. This proves that Messier 1 is undoubtedly a pevatron, and an impressive one at that.
However, this has only been proven for electrons, making Messier 1 what scientists call a leptonic accelerator. But, pevatrons are capable of accelerating any charged particle. And if you remember, cosmic rays are mostly protons, so it's these pevatrons, otherwise known as hadronic accelerators, that scientists are most keen to find.
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