The Oh My God Particle - Script (Patreon)

Long before the God Particle, there was the Oh My God particle - a cosmic ray vastly more energetic than had ever been seen - or was even thought possible. These ultra-high energy cosmic rays still perplex scientists. Where are these extragalactic death rays coming from?

On October 15th 1991, a single atomic nucleus traveling at 99.999,999,999,999,999,999,999,510% of the speed of light crashed through our atmosphere and streaked across the Utah sky.  The nucleus quickly disintegrated into a shower of subatomic particles and light. That light was seen by the Fly’s Eye observatory – a collection of oversized tin cans that was an early experiment by the University of Utah to spot the highest energy cosmic rays in the universe. Scientists analyzing the Fly’s Eye data calculated that the cosmic ray responsible for this particular flash must have had a kinetic energy of 300 Exa-electron volts. That’s 48 Joules - an amount of energy we associate with macroscopic, not subatomic objects. That single atomic nucleus carried as much kinetic energy as a a good-sized stone thrown at your head at 50 miles/hour. The particle was dubbed the Oh-My-God particle. Nothing like it had ever been seen before. In fact, cosmic rays of that energy were supposed to be impossible.

Let’s talk about cosmic rays for a second. Radioactivity was discovered by Marie Curie and Henri Becquerel at the end of the 1800s. High-energy particles – electrons and small atomic nuclei – are ejected when heavier radioactive elements decay. We’re bathed in a very low level of this radiation due to naturally occurring radioactive elements in the Earth. Soon after its discovery, this ambient radioactive flux was found to weaken with height above the ground as the radiation loses energy to air molecules. But weirdly, above a certain height this radiation starts to increase again. Theodor Wulf first noticed this in 1909 when he took a detector to the top of the Eiffel tower, but the real proof came a few years later in 1912 when Victor Hess took some of Wulf’s detectors on a hot air balloon ride. Radiation levels increased with height. That meant there had to be a source of these high energy particles somewhere above. It turns out they were coming from space. Cosmic rays had been discovered.

In the years following Hess’s balloon ride, and even following the Fly’s Eye detection of the Oh-My-God particle, we’ve come a long way in the art of catching cosmic rays. They’re elusive little critters. You can’t focus them into a camera like you can with light. Also, as Hess discovered, most don’t make it to the ground anyway. For lower energy cosmic rays, one approach is to look for their Cherenkov radiation. All cosmic rays are traveling at pretty close to the speed of light. That’s the speed of light in a vacuum. Light travels slower in a medium, like air. So when cosmic rays enter the atmosphere they’re actually traveling faster than the new, lower speed of light. This results in a burst of gamma rays – Cherenkov radiation – that is detectable from the ground.

High-energy cosmic rays tend to obliterate themselves several kilometers above the ground in massive collisions with the nuclei of air molecules. The result is a cascade of subatomic particles  - the debris of the collision - that can spread out over several kilometers. These cascades are called air showers. Streams of charged particles cause the air to fluoresce; a glow that can be seen by specialized telescopes. Many of the debris particles do reach the ground and can be detected there. By analyzing the energies and trajectories of the debris, the collision that produced them and the nature of the original cosmic ray can be reconstructed.

Several facilities around the world are devoted catching cosmic rays. For example, the Pierre Auger Observatory in Argentina monitors a region of around 3000 square kilometers for high-energy cosmic rays. It including 1660 giant tanks of water designed see Cherenkov radiation when air shower particles pass through them, as well as telescopes to spot fluorescence in the air above, just like the old Fly’s Eye. The Fly’s Eye itself has evolved into the Telescope Array Project. It still uses upgraded fluorescence telescopes, and has added scintillation detectors on the ground. These are simple slabs of acryllic between metal slabs designed to top air shower particles and detect the light produced as they smack into nuclei within the slab.

By now we have a pretty good census of the types of cosmic rays that tend to hit the Earth. Most of them are single protons – the nuclei of hydrogen atoms – and a fair number are helium nuclei, but about 1% of cosmic rays are heavier nuclei – as heavy as iron. We also see gamma rays and even anti-matter particles. They come in at all energies, from a sickly billion electron volts at the low end, to the crazy 10^20 eV or higher like the Oh-My-God particle. The higher the energy, the rarer they are. At the lowest energies, the cosmos flings one particle every second per square meter of the Earth’s surface. At energies up near that of the Oh-My-God particle they are incredibly rare – only a handful have been spotted since the first, giving a rate estimate of one per square kilometer every couple of centuries.

So where do these things come from anyway? To accelerate a particle to the energies of cosmic rays you need a particle accelerator. We build artificial ones on Earth using giant rings and powerful magnetic fields. It turns out the universe is full of natural particle accelerators. For lower energy cosmic rays, it’s believed that many - perhaps most - come from supernova explosions within our galaxy. When a star explodes, the expanding shockwave carries a strong magnetic field. It can trap particles and accelerate them until they’re energetic enough to escape the shock. The higher the energy of the cosmic ray, the more likely it is to have originated from outside our galaxy. The exact sources of these so-called extra-galactic cosmic rays are more mysterious, but they may come from magnetic acceleration in quasars or perhaps they’re blasted out in gamma ray bursts. 

The most ridiculously energetic cosmic rays like the Oh-My-God particle shouldn’t exist at all. See, the Universe is basically opaque to particles with such high energies. Empty space isn’t really empty, it’s full of low-energy, microwave photons left over from the heat-glow of the very earliest of times. This is the cosmic microwave background radiation. We talk about it in this episode. Cosmic rays with energies over 5x10^19 electron volts - about 8 Joules - can’t travel far before smacking into these photons and giving up some of their energy. This is the so-called Greisen-Zatsepin-Kuz’min, or GZK limit. For years it was thought that no cosmic ray could exceed it, Except that the Oh-My-God particle was six times more energetic.

Only a very small number of extreme energy cosmic rays have been seen since the OMG particle. But those are perplexing. See, they must have come from nearby. From close enough to our galaxy to not be wiped out by the CMB. Now I’m talking “close by” on cosmic scales, so within 1-2 hundred million light years. But at that distance, sources like quasars and gamma ray bursts should be very obvious. Yet they seem to come from … nowhere in particular. We do see more than the average number coming from the direction of the Ursa Major Cluster, but there’s no obvious source there. It’s still a mystery exactly what produces these extreme cosmic rays and how close to us those sources are. For cosmic ray astrophysicists, there’s a giant, invisible, particle-accelerating elephant in the room. 

Part if the challenge in studying cosmic rays is that atmosphere and magnetic field shield the surface of the Earth so well. But we should count that as a blessing. Apollo astronauts traveling outside Earth's magnetosphere reported strange flashes of light, which may be from Cherenkov radiation from cosmic rays in their eye's vitreous humor, or from the particles hitting their optic nerves. Even ISS astronauts are subject to a significant radiation risk. Along with solar outbursts, cosmic rays are one of the most serious obstacles to manned interplanetary travel. 

As we figure out the origins of these particles, cosmic ray astronomy is becoming an increasingly powerful tool for investigating our amazing universe. But even now these particles are extremely useful. The highest energy cosmic rays like the Oh-My-God particle generate collisions far more energetic than our largest particle accelerator - the large hadron collider. Studying cosmic rays may crack open the mysteries of both the largest and smallest scales of space time.