Oh-My-God particle

The Oh-My-God particle was an ultra-high-energy cosmic ray detected on 15 October 1991 by the Fly's Eye camera in Dugway Proving Ground, Utah, United States.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=q/> Template:Asof, it is the highest-energy cosmic ray ever observed.<ref>Template:Cite journal</ref> Its energy was estimated as Template:Val (320 exa-eV). The particle's energy was unexpected and called into question prevailing theories about the origin and propagation of cosmic rays.

Speed

It is not known what kind of particle it was, but most cosmic rays are protons. If <math>m_\mathrm{p}</math> is the rest mass of the particle and <math>E_\mathrm{K}</math> is its kinetic energy (energy above the rest mass energy), then its speed was very close to <math display=inline>\sqrt{1-[m_\mathrm{p}c^2/(E_\mathrm{K}+m_\mathrm{p}c^2)]^2}</math> times the speed of light. Since <math display=inline>E_\mathrm{K} \gg m_\mathrm{p}c^2</math>, this ratio can be simplified to <math display=inline>1-\frac{1}{2}[m_\mathrm{p}c^2/E_\mathrm{K}]^2</math>. Assuming it was a proton, for which <math>m_\mathrm{p}c^2</math> is 938 MeV, this means it was traveling at Template:Val times the speed of light, its Lorentz factor was Template:Val and its rapidity was Template:Val. This is 1.3 femtometers per second less than the speed of light, so if a photon were traveling alongside the proton, it would take over 245,000 years for the photon to gain a 1 cm lead, as seen from the Earth's reference frame. Due to special relativity, the relativistic time dilation experienced by a proton traveling at this speed would be extreme. If the proton originated from a distance of 1.5 billion light years, it would take approximately 1.71 days in the reference frame of the proton to travel that distance.

Collision energy

The energy of the particle was some 40 million times that of the highest-energy protons that have been produced in any terrestrial particle accelerator. However, only a small fraction of this energy was available for its interaction with a nucleus in the Earth's atmosphere, with most of the energy remaining in the form of kinetic energy of the center of mass of the products of the interaction. If <math>m_\mathrm{t}</math> is the mass of the "target" nucleus, the energy available for such a collision is<ref>Template:Cite book</ref>

<math display=block>\sqrt{ 2E_\mathrm{K}m_\mathrm{t}c^2+(m_\mathrm{p}+m_\mathrm{t})^2c^4 }-(m_\mathrm{p}+m_\mathrm{t})c^2</math>

which for large <math>E_\mathrm{K}</math> is approximately

<math display=block>\sqrt{ 2E_\mathrm{K}m_\mathrm{t}c^2}.</math>

For the Oh-My-God particle hitting a nitrogen nucleus, this gives 2900 TeV, which is roughly 200 times higher than the highest collision energy of the Large Hadron Collider, in which two high-energy particles going opposite directions collide.<ref>Template:Cite news</ref><ref>Template:Cite news</ref> In the center-of-mass frame of reference (which was moving, in our frame of reference, at almost the speed of light), the products of the collision would therefore have had around 2900 TeV of energy. This would have transformed the nucleus into many particles moving apart at almost light speed in the center-of-mass frame of reference. As with other cosmic rays, the collision generated a cascade of relativistic particles as the particles interacted with other nuclei.

Comparisons

Template:Original research section The Oh-My-God particleTemplate:'s energy was estimated as Template:Val, or Template:Val. Although this amount is phenomenally large for a single elementary particle – far outstripping the highest energy that human technology can generate in a particle – it is still far below the level of the Planck scale, where exotic physics is expected. Though a subatomic particle, its energy was comparable to the gravitational potential energy of a 1 kilogram object that could fall 5 meters off a two-story building.

The Oh-My-God particle had Template:10^ (100 quintillion) times the photon energy of visible light, equivalent to a Template:Convert baseball travelling at about Template:Convert. Template:Sky Its energy was 20 million times greater than the highest photon energy measured in electromagnetic radiation emitted by an extragalactic object, the blazar Markarian 501.<ref>Template:Cite journal</ref>Template:Update-inline

It had a relativistic mass equivalent to around Template:Val daltons, or Template:Val nitrogen nuclei.

High energy, but far below the Planck scale

While the particle's energy was higher than anything achieved in terrestrial accelerators, it was still about 40 million times lower than the Planck energy (Template:Val). Particles of that energy would be required in order to expose effects on the Planck scale. A proton with that much energy would travel Template:Val times closer to the speed of light than the Oh-My-God particle did. As viewed from Earth and observed in Earth's reference frame, it would take about Template:Val (Template:Val times the current age of the universe) for a photon to overtake a Planck energy proton with a 1 cm lead.Template:Citation needed

Later similar events

Since the first observation, hundreds of similar events (energy Template:Val or greater) have been recorded, confirming the phenomenon.<ref>Template:Cite journal</ref><ref name="Abbasi">Template:Cite journal</ref> These ultra-high-energy cosmic ray particles are very rare; the energy of most cosmic ray particles is between Template:10^ eV and Template:10^ eV.

More recent studies using the Telescope Array Project have suggested a source of the particles within a 20 degree radius "warm spot" in the direction of the constellation Ursa Major.<ref name="q">Template:Cite journal</ref><ref name="Abbasi" /><ref>Template:Cite journal</ref>

The Amaterasu particle, named after the sun goddess in Japanese mythology, was detected in 2021 and later identified in 2023, using the Telescope Array observatory in Utah, United States. It had an energy exceeding 240 exa-electron volts (Template:Val eV).<ref>Template:Cite journal</ref> This particle appears to have emerged from the Local Void, an empty area of space bordering the Milky Way galaxy.<ref>Template:Cite journal</ref> It contained an amount of energy comparable to dropping a brick from the height of the waist. No promising astronomical object matching the direction from which the cosmic ray arrived has been identified.<ref>Template:Cite journal</ref>

See also

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• Amaterasu particle (2021) – 240 Eev