In 1912 Victor Hess was testing a hypothesis that this new fangled radiation came from rocks in the earth. He built a simple detector and made a series of ascents in a hydrogen balloon. The higher he went, the higher the radiation levels. The radiation was coming from above rather than below - he had discovered cosmic rays.
Cosmics proved frustrating to study. Very high energy particles - usually protons or light nuclei - hit the upper atmosphere where they'd strike a nitrogen or oxygen atom. That produced a shower of particles which would strike other atoms spreading the shower. It was difficult to say where the initial particle came from or even how much energy it had other than very high energy cosmic rays produced showers that spread out more than lower energy events.
By the 1960s physicists started building arrays of small water tanks and spreading them across large open areas. The tanks could detect small flashes of light made by fast moving particles moving faster than the speed of light in water. More tank signals at the same time meant a larger shower. By seeing how much area had been impacted by the shower it was possible to infer the original cosmic's energy.
By the 80s more sophisticated techniques began to lift the curtain a bit. Cosmics cover an enormous range of energy ranging from that of particle accelerators of the 1970s and up.. way up. Nature had built some amazingly power accelerators and we were building detectors for them. Learning where these accelerators are and how they work may teach us about the fundamental nature of the Universe. The focus shifted to extreme events over 1018 electron volts - that's over 100,000 times more energy than the highest energies produced in the Large Hadron Collider at CERN. These are among the fastest objects with mass in the Universe.
The really powerful ones are fairly rare. A square kilometer of the Earth will see about one 8 EeV (an EeV is 1018 electron volts - an exaelectron volt) per year. One array is about 3,000 square kilometers so it would see nearly ten of the 8 EeV events per day. It's effectively an enormous digital camera with car sized pixels spread over a huge area. Even higher energy cosmic rays occur less frequently - you expect a 100 EeV event per square kilometer per century.
It turns out the amount of kinetic energy an apple falling from a table is about one joule - that's about 6 EeV.1 These tiny incident cosmic rays have energies in ranges we're familiar with. Sarah can serve a beach volleyball at around 80 kilometers per hour giving it a kinetic energy of about 70 joules or 420 EeV. In 1991 an event known as the Oh My God particle was detected in Utah with an energy of about 320 EeV. That's only a bit less than Sarah's serve and about the same as a 60 mph baseball. It's almost certain higher energy cosmic rays exist, but we just haven't found any yet.
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1 An electron volt is a tiny amount of energy that's used mainly to study energies at the molecular and atomic level. A light photon useful for photosynthesis is on the order to 2 electron volts. A joule something more on the human scale.. 70 for Sarah's beach volleyball serve and 140 for a 100 mph. fastball.
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