Cosmic rays are energetic particles that can be detected when they collide with the upper atmosphere, allowing us to infer their energy and direction. Ultrahigh-energy cosmic rays (UHECRs) are those that exceed the Greisen-Zatsepin-Kuzmin (GZK) limit, and their existence is a mystery that may hint at new physical theories. The Oh-My-God particle, observed in 1991, had an energy of about 3 x 1020 electron volts and has been followed by at least fifteen similar events. These particles are unlikely to be affected by magnetic fields and travel in a direct line from their point of origin.
Cosmic rays are subatomic particles that fly through space at great speed in all directions. About 90% are protons, 9% are helium nuclei and 1% are electrons. Contrary to their denomination, cosmic rays are energetic particles rather than rays themselves. On Earth, we can detect cosmic rays when they collide with the upper atmosphere and release a shower of charged particles. By observing the intensity of the particle shower, we can infer the energy and direction of the cosmic ray.
A very high energy cosmic ray (UHECR) is a type of cosmic ray with an abnormally high energy. There is no official cutoff limit that defines an ultrahigh-energy cosmic ray, but the term generally refers to cosmic rays that approach or exceed the Greisen-Zatsepin-Kuzmin (GZK) limit on cosmic ray energy. The GZK limit corresponds to a threshold energy of 6 x 1019 electron volts, about ten million times more energetic than particles in the most powerful particle accelerators. Above this limit, cosmic rays are thought to interact with the CMB to produce exotic particles called pions, thereby lowering their energy to the GZK limit.
But cosmic rays exceeding the GZK limit have been observed. This is a physics mystery and may be a hint of physical theories succeeding Einstein’s, according to respected physicists such as Lee Smolin. One such cosmic ray, observed at Dugway Proving Grounds in Utah and dubbed the Oh-My-God particle, had an energy of about 3 x 1020 electron volts. This is roughly equivalent to that of a well-thrown baseball, but we’re talking here about a particle smaller than a typical atomic nucleus. It is estimated that this particle traveled at about 1 – 5 x 10-24 the speed of light. This means that if the cosmic ray were racing against a photon (particle of light), after a full year of travel, the cosmic ray would be only 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 ultrahigh-energy cosmic rays are a real phenomenon. Because such particles are so energetic, they are unlikely to 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 the sky where the cosmic ray originated. Why? We do not know.
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