|One of the two Fly's Eye Cosmic Ray Detectors.|
The Oh-My-God particle was an ultra-high-energy cosmic ray—most likely a proton—detected on October, 1991 in the skies over western Utah. Its observation by the University of Utah's Fly's Eye Cosmic Ray Detector was a shock to astrophysicists, who estimated its energy to be approximately 50 J. In other words, a subatomic particle with kinetic energy equal to that of a baseball traveling at about 90 kilometers per hour.
The particle was traveling at almost the speed of light. Assuming it was a proton, its speed was only about 1.5 quadrillionth of a meter per second less than the speed of light. In other words, if it were in a race with a beam of light, the Oh-My-God particle would fall behind only one centimeter every 220,000 years.
The energy of this particle is some 40 million times that of the highest energy protons that have been produced by the Large Hadron Collider. However, only a small part of this energy would be available for an interaction with another proton or neutron. Most of the energy would remain as kinetic energy. The effective energy available for such an interaction is still 50 times greater than the collision energy of the Large Hadron Collider.
Applying special relativity to such a fast particle yields some incredible results. Time passes more slowly as velocity increases, and for anyone hypothetically travelling on the back of this particle time would nearly stop. For example, a trip to the Andromeda Galaxy, which is more than two million light years away, would have a perceived travel time of only three and a half minutes. Special relativity also tells us that there is a length contraction in the direction of motion. If the Earth were somehow able to match the speed of the Oh-My-God particle, it would pancake down to a thickness of less than four hundredths of a millimeter!
The University of Utah experiment relied on two telescopes searching the sky for the characteristic flashes of ultraviolet light that are produced when a cosmic ray collides with a molecule in Earth’s atmosphere and creates a shower of secondary particles. The two telescopes were covered in photomultiplier tubes and looked like the compound eyes of a fly. By capturing almost all the light in the shower, they were able to make a good measurement of the particle’s energy.
These ultra-high-energy cosmic rays are very rare. Since the first observation, only about fifteen similar events have been recorded to confirm the phenomenon. What cosmic process transforms an ordinary particle into an Oh-My-God particle? A supernova or supermassive black hole might explain it, but when astronomers followed the impact track back to its source they found nothing unusual in that direction.