Ultra-high energy cosmic rays (UHECRs) are extremely energetic subatomic particles (mostly protons, but also some heavier atomic nuclei) with energies greater than 10¹⁵ eV. The record holder so far is a UHECR with an energy of 3×10²⁰ eV – equivalent to a baseball thrown at 160 km/hr!

Currently it is only possible to observe UHECRs through the cosmic ray showers produced as they interact with the Earth’s atmosphere. This indirect method of observation is required due to the extremely low numbers of incident cosmic rays at these energies. The most advanced ground-based experiments to detect cosmic ray showers extend over several kilometres and consist of both Cherenkov detectors monitoring several large tanks of water for light produced by high-energy particles, and fluorescence detectors used to track the glow of the particle as it descends through the atmosphere.

The source of UHECRs remains a mystery, as does the mechanism to accelerate particles to these energies. However, they have enough energy to escape the typical magnetic field of a spiral galaxy, and most astronomers believe that UHECRs are of extragalactic origin. Possible sources include active galactic nuclei, dormant quasars with associated supermassive black holes and galaxy mergers.

Even if UHECRs are created in extreme extragalactic environments, it is still not clear how we are able to detect them at such high energies. Above 5×10¹⁹ eV, cosmic rays should interact with the radiation of the cosmic microwave background within a distance of 150 million light years, a process that should reduce the cosmic ray’s energy below this threshold. This theoretical upper limit to the energy of a cosmic ray is called the Greisen-Zatsepin-Kuzmin limit (GZK limit), and the fact that we observe cosmic rays at energies larger than this appears to contradict the predictions of special relativity.

Although several exotic theories have been advanced to resolve this issue, other less-radical solutions have also been proposed:

• The detections were due to instrumental error or misinterpreted data,
• UHECRs have a local origin,
• UHECRs are weakly interacting particles.

It is hoped that the new generation of cosmic ray experiments will unequivocally verify or refute earlier measurements of energies greater than the GZK limit, and determine whether such extreme cosmic rays do indeed exist.