On October 15, 1991, a cosmic ray detector in Utah observed something that had never been seen before or since.
It was a cosmic ray with more energy than anything ever observed and more energy than most scientists thought possible.
When one of the first researchers saw the data, they responded simply, “Oh, my God!”
Learn more about the OMG particle, what it was, and what it means on this episode of Everything Everywhere Daily.
The way science is supposed to work in theory is that someone comes up with a hypothesis and then develops an experiment to test it. Much of science works that way, but there are certain things you simply can’t conduct experiments on.
Astronomy is such a discipline. It is based on observation. They observe the universe as it is and then try to create hypotheses that explain all of their observations in a coherent manner.
Every so often, there is an observation that throws a wrench into everything. It requires a complete reappraisal of what we know and theories used to describe what we know.
This episode is about such an observation which took place in 1991.
Before I get into the details of what happened in 1991, there are several things I’ll need to explain to help make sense of everything.
The first thing is understanding what cosmic rays are. I’ve previously done an entire episode on cosmic rays, including the history of their discovery.
Cosmic rays are high-energy particles that are zooming around in space in pretty much every direction.
About 90% of all cosmic rays are hydrogen atomic nuclei stripped of their electrons, aka protons. The remaining 10% are atomic nuclei from helium or heavier atoms, which have been stripped of their electrons.
These particles are traveling at velocities near the speed of light, most of which originated well beyond our solar system.
When these particles hit the Earth’s atmosphere, they usually result in high-energy collisions with atoms and molecules. These collisions then create new particles and radiation, which then rain down on the Earth.
This is very similar to what is happening inside a particle accelerator like the Lage Hadron Collider. Particles are accelerated to high velocities and then smashed into other particles so the collision can be observed.
These collisions happen in the atmosphere all the time, often at energies far greater than what we can achieve in a particle accelerator. One square centimeter of the upper atmosphere is, on average, hit with a cosmic ray once every minute.
Cosmic rays can have a wide array of energies. The least energetic particles are the most common, and then they can increase in energy dramatically, with the most energetic particles being the most rare.
Extremely energetic particles can hit the atmosphere as rarely as once per century for every square kilometer.
The energy contained in these cosmic ray particles is what this episode is about.
The unit of energy used in particle physics is called an electronvolt. An electron volt is an extremely small unit of measurement. The definition of an electronvolt is the amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in a vacuum.
So, electron volts are a measure of energy similar to Joules, the only difference being that one Joule is about 1019 times larger than one electronvolt.
Electronvolts are so small that, in practice, particles are usually measured in mega, giga, or tera electron volts. Most common cosmic rays will be in the gigaelectronvolt range. The most energetic particles accelerated by the Large Hadron Collider have achieved 13.6 teraelectronvolts.
Because the mass of a proton remains stable, the only way to increase the kinetic energy in a particle is to increase its velocity. By the same token, a heavier particle, like a helium nucleus, would take more energy to reach the same velocity.
As you might know, there is a limit to how fast something can go. Nothing can go faster than the speed of light.
The closer you approach the speed of light, the more energy it takes to get ever closer. This is something we barely notice at the speeds we live at, but as you start approaching the speed of like, this becomes an issue.
To go the speed of light itself would require an infinite amount of energy.
All of that should lay the foundation for the subject of this episode.
Particle colliders like the Large Hadron Collider are great for doing controlled observations of what happens when particles collide. However, there are higher energy collisions taking place all the time above our heads with cosmic rays.
So, observatories were created to attempt to observe cosmic rays, or more accurately, the interaction of cosmic rays with the upper atmosphere.
One such observatory was the Fly’s Eye Cosmic Ray Detector, which was operated by the University of Utah. At the time, it was an experimental observatory that was operated in the Dugway Proving Ground, which was a large 800,000-acre tract of land in Utah that the US Army used for weapons testing.
The observatory had 100 detectors, each equipped with a 1.5-metre-wide mirror to look for the flash of particles colliding in the atmosphere.
When a cosmic ray hits the atmosphere, it creates a cascade of particles and radiation. The energy of the cosmic ray can be determined by the size of the cascade.
On October 15, 1991, an observation was made that astounded the researchers. They observed a cosmic ray that had an energy equivalent of 320 exaelectronvolts.
That number is so incredibly large that most of you probably don’t even know what the prefix exa even means.
Mega of course means million, giga means billions, tera means trillions, penta means quadrillions, and exa means quintillions.
The cosmic ray had an energy of 320 qunitillion electron volts or the equivalent of 51 joules of energy. The energy equivalent in that single particle was the equivalent of 56 mile per hour or 90 kilometer per hour baseball, or the equivalent of dropping a bowling ball from shoulder height.
….all packed into a single particle that was believed to have been a proton. It was about 40 million times greater than the most powerful particle that could be accelerated in the Large Hadron Collider.
When they saw the data, one of the researchers exclaimed, “Oh my God!” and the name stuck. It became known as the “Oh my God” or OMG particle. Actually, it was supposedly originally dubbed the WTF particle, but when they went public, they changed the name.
There are several reasons why this observation was so shocking.
The first of which was the speed it had to have been traveling to have this much energy.
To say it was traveling near the speed of light would be an understatement. Its speed would have been 99.999 999 999 999 999 999 999 51% the speed of light.
If this particle and a photon of light had both been emitted at the same time in a race, it would take 220,000 years for the photon to gain a 1-centimeter lead.
That, of course, would be 220,000 years for a stationary observer. If you are familiar with special relativity, you know that time for an observer going close to the speed of light goes much slower.
From the perspective of the particle, it could have traveled the distance from Earth to the nearest star to the Sun, Proxima Centauri, in just 0.43 milliseconds. The relative time it would take to travel the 100,000 light years to cross the Milky Way Galaxy would be just 10 seconds.
The absurd speed of this particle and the incredible energy it held was just part of what made the observation astonishing.
The other reason was that such a particle wasn’t supposed to be possible.
There is a theoretical limit for the amount of kinetic energy that a proton could have. Known as the Greisen–Zatsepin–Kuzmin or GZK limit, it postulates that the limit for a proton was 50 exaelectronvolts.
The OMG particle exceeded this by sixfold.
When incredible observations like this are made that overturn theoretical estimates, the first thing you look for is a problem with the observation.
The observed energy in the OMG particle was so great that the researchers at the University of Utah thought there was a problem with the equipment. Checking the validity of the data was one of the reasons it took them a year before announcing their results.
When they did, other researchers were naturally suspicious of their findings.
In the years since the OMG particle was observed, there have been more observations made of what are now known as ultra-high-energy cosmic rays.
Ultra-high-energy cosmic rays are defined at anything over 1 exaelectronvolt. Very large in terms of cosmic rays, but still much smaller than the OMG particle.
With observations of more and more ultra-high-energy cosmic rays, the OMG particle seemed less and less implausible. It was a rare event, to be sure, but not necessarily impossible. In 2022, a team working on the multinational Telescope Array project announced they observed a whopper 244 exaelectronvolt cosmic ray, which was also above the GZK limit.
Even if the OMG particle was possible, the question still remained: what was it, and where did it come from?
As to what it was, the GZK limit only applies to protons. 90% of cosmic rays are protons, so that is what it was assumed that the OMG particle was. All of the velocity estimates I gave were under the assumption that it was a proton.
However, there is some speculation that these ultra-high energy particles are not, in fact, protons but the nuclei of heavier elements. If the OMG particle had a mass larger than a proton, then the velocity could have been less. It still would have been incredibly close to the speed of light, but maybe not with quite so many 9s in the percentage.
Regardless of what the particle was, the other big question is where did it come from?
The particle had to have originated at some incredible source of magnetism.
One theory holds that it might have some from a neutron star. These rapidly rotating, incredibly dense stars have some of the strongest magnetic fields in the universe. The magnetic field of a neutron star would make it possible to accelerate a particle to become an Ultra-high-energy cosmic ray.
Another contender would be what is called Active galactic nuclei. These are massive black holes in the middle of galaxies that have rapidly swirling masses of matter that can emit incredible amounts of electromagnetic radiation.
Given the direction of the cosmic ray at the date of the OMG particle observation, one of the suspects was the active galactic nucleus of a galaxy known as Centaurus A, which is about 16 million light-years away.
However, this theory has fallen out of favor given how easily charged particles can be influenced by other magnetic fields. It is now thought to be highly unlikely that the particle traveled in a straight line from Centaurus A.
The truth is, we don’t really know what the OMG particle was or where it came from. The entire act of observing cosmic rays is seeing what they do after they are destroyed. It places a limit on what we can know about them.
While there might be a limit on what we can know, we are still learning more. We know that while ultra-high-energy cosmic rays are rare, the OMG particle probably wasn’t a singular event. Even if over 30 years later, it remains the most powerful cosmic ray ever witnessed.
The Executive Producer of Everything Everywhere Daily is Charles Daniel.
The associate producers are Peter Bennett and Cameron Kieffer.
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