Thread by @mcnees

The most energetic single particle ever detected, a cosmic ray dubbed the "Oh-My-God" particle, was observed by the Fly's Eye Cosmic Ray Detector #OTD in 1991. Its energy was about 3.2 x 10²⁰ eV ~ 51 J, equivalent to a baseball moving at almost 60 mph.
quantamagazine.org/ultrahigh-ener…

The "OMG Particle" should not to be confused with the “God Particle.” The latter is a terrible name that you should not use under any circumstances, while the former is a great name and all physicists are obligated to high-five whoever came up with it.

Also, the "OMG Particle" should not be confused with the "0mg particle," which is another name for a photon.

Reconstructing information about ultra-high energy cosmic rays like the OMG Particle is very tricky. Fly's Eye did it by studying emissions from the cascade of collisions after an energetic particle slammed into the upper atmosphere.

Their analysis gave an energy (3.2 ± 0.9) x 10²⁰ eV for the OMG Particle. So somewhere between 37 J at the low end and 66 J at the high end, with a central value of 51 J.

A standard baseball has a mass of 0.145 kg. How fast would it be moving if it had 51 J of kinetic energy?

½ (0.145 kg) v² = 51 J
v = 26.5 m/s ~ 60 mph

If a typical adult threw a baseball as hard as they possibly could, it'd have about as much kinetic energy as the OMG particle.

It isn't clear whether the Oh-My-God Particle was a single proton, or a heavier nucleus consisting of several protons and neutrons.

If the Oh-My-God Particle was a proton then it had about 50 million times as much energy as the most energetic proton accelerated in a human-built collider (the 6.5 TeV protons at the LHC). It would've been moving slower than c, the speed of light, by only about 4 parts in 10²⁴.

Let's think about that! The speed of light is about 3 x 10⁸ m/s. So 4 parts in 10²⁴ is a difference of about one femtometer per second. A femtometer is 10⁻¹⁵ m, which is about the size of a proton.

Light moves fast enough to circle the Earth at the equator about 7.5 times per second, and the OMG Particle – if it was a proton – would have lagged behind it by a distance comparable to the size of a proton! That is very, very fast for a massive particle.

It is not clear how such a particle, accelerated to tremendous energies by some astrophysical source, could even reach the Earth.

At those energies a proton would be subject to the "Greisen-Zatsepin-Kuzmin cutoff," causing it to lose energy and slow down dvia interactions with the ambient Cosmic Microwave Background. So it must have been produced nearby, cosmically speaking, to arrive with that much energy.

On the other hand, if it was a heavier nuclei, like Iron,then there'd be a little more wiggle room. Its energy would have to exceed 10²¹ eV before the energy-per-nucleon was high enough to trigger the effect pointed out by GZK.

A heavy nucleus with all its protons would also have more charge and therefore be more susceptible to deflection by our galaxy's magnetic field. So it could have been produced by some relatively nearby event and ended up on a trajectory that sent it here.

The OMG particle was about 50 million times more energetic than the protons accelerated by the LHC, but thanks to special relativity its collision with a stationary nucleon in our atmosphere would only have been around 100 times more energetic than the LHC's head-on collisions.

That's enough to tell us that the LHC isn't going to set off exotic catastrophes like vacuum decay, microscopic black holes, or the formation of strange matter. All the ambient ultra-energetic collisions in our atmosphere would have triggered such scenarios long ago.

Indeed, such ultra-high energy particles must be common throughout the universe. They probably participated in many very nearly head-on collisions over the past 13 billion years and the Universe is still here.

No accelerator here on Earth will ever collide particles at energies comparable to a head-on collision between two OMG Particles, so you can safely ignore most* doomsday scenarios the next time a new collider is switched on.

* For more details go read @AstroKatie's book!

(Also, microscopic black holes would likely flare out in a burst of Hawking radiation almost as soon as they formed. Their own radiation pressure would prevent anything from getting close enough to be swallowed. They are perfectly safe.)

I forgot: The name "Oh-My-God Particle" was coined by John Walker (@Fourmilab, who also founded @autodesk).
fourmilab.ch/documents/OhMy…