What is an Ultra-High Energy Cosmic Ray?

Cosmic rays are subatomic particles flying through space at great speed in all directions. About 90% of them are protons, 9% are helium nuclei, and 1% are electrons. Contrary to their naming, cosmic rays are energetic particles rather than rays per se. On Earth, we can detect cosmic rays when they collide with the upper atmosphere and release a shower of charged particles. By observing how intense the particle shower is, we can infer the energy and direction of the cosmic ray.

An ultra-high energy cosmic ray (UHECR) is a type of cosmic ray with an anomalously high energy. There is no official cutoff limit defining an ultra-high energy cosmic ray, but the term generally refers to cosmic rays approaching or exceeding the Greisen-Zatsepin-Kuzmin (GZK) limit on cosmic ray energy. The GZK limit corresponds to a threshold energy of 6 x 10¹⁹ electronvolts, about ten million times more energetic than the particles in the most powerful particle accelerators. Above this limit, it is thought that cosmic rays interact with the cosmic microwave background to produce exotic particles called pions, thereby lowering their energy to the GZK limit.

But cosmic rays have been observed exceeding the GZK limit. This is a mystery of physics and might be a hint to physical theories that succeed those of Einstein, according to respected physicists such as Lee Smolin. One such cosmic ray, observed at Dugway Proving Grounds in Utah and nicknamed the Oh-My-God particle, had an energy of about 3 x 10²⁰ electronvolts. This is roughly equivalent to that of a well-thrown baseball – yet here we are talking about a particle smaller than a typical atomic nucleus. It is estimated that this particle was traveling at about 1 – 5 x 10^-24 the speed of light. This means that if the cosmic ray were racing a photon (light particle), after an entire year of travel, the cosmic ray would only be 46 nanometers behind the original photon.

Since the discovery of the Oh-My-God particle in 1991, at least fifteen similar events have been recorded, proving that ultra-high energy cosmic rays are a real phenomenon. Since such particles are so energetic, it is unlikely that they would be affected by the magnetic fields of galaxies, so they would likely travel in a direct line from their point of origin. But, nothing of interest was found in the direction of sky from where the cosmic ray originated. Why is that? We don’t know.