Our planet is under a constant bombardment of radiation—from space.
Well, maybe it’s not as scary as that makes it seem. “Radiation” is a catchall term astronomers use for forms of light—including visible light, the kind we see—and also for subatomic particles sleeting through space. We don’t normally think of such particles as “rays”—cosmic rays, to be precise—but we still use that nomenclature because of lingo inertia.
Some cosmic rays come from the sun, some from elsewhere in our Milky Way, and others, called extragalactic cosmic rays, trace their origins across vast distances to other galaxies.
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That’s a remarkable thought, actually: Earth gets hit routinely by particles from other galaxies. That’s a long hike—a journey of tens of millions of light-years, sometimes more, ending when one of these wayward rays is absorbed harmlessly by our atmosphere, high above our heads.
These particles come in with a broad range of velocities, which in turn gives them a broad range of kinetic energy, the energy of motion. In our macroscopic universe, we use a unit such as joules to measure energy, which is still rather small. (It takes about four joules to raise a cubic centimeter of water 1 degree Celsius.) Particle physicists, however, use a far smaller unit called an electron volt (or eV). It takes 26 million trillion of them to heat that same amount of water! That’s a more appropriate unit for particles, most of the time. But cosmic rays are moving so rapidly—near the speed of light—that they can have a very high kinetic energy, easily reaching the mega electron volt (MeV) and giga electron volt (GeV) level.
You still wouldn’t feel it if one of these struck you. But shockingly, some cosmic rays have far, far higher energies than this.
In 1991 the Fly’s Eye detector, which monitored the sky for the glow caused by energetic particles slamming into our atmosphere, detected a flash so huge it defied belief: the cosmic ray that sparked it had an energy of 320 quintillion eV, or 320 billion GeV. That’s millions of times the kinetic energy of protons we can spin up in our most powerful particle accelerators. It’s so energetic, in fact, that it actually has a decent macroscopic equivalent: this cosmic ray carried 51 joules of kinetic energy, which is about the same as a slow curveball—but this energy came from a single subatomic particle.
It’s been nicknamed the “Oh-My-God” particle, and it makes the hair on the back of my neck stand up.
Why? Because protons are almost incomprehensibly small—as an analogy, the size of a proton compared to the size of an orange is roughly the same as the size of an orange compared to the diameter of Neptune’s orbit around the sun.
The OMG particle is a big mystery. For one thing, to have that much energy, it must have been traveling incredibly fast relative to Earth. Assuming it was a proton, it was moving at a speed of 99.9999999999999999999995 percent the speed of light. If a photon and the OMG particle had been in a race since the universe first formed, the particle now would only be about 600 meters behind.
So what could kick a particle like this up to such ridiculously high speeds? The answer may shock you.
That’s not clickbait: shock waves, specifically in catastrophically high-energy structures such as the focused beams of matter and energy pouring forth from a supermassive black hole. Ionized gas moving rapidly outward from such events carries along extremely strong magnetic fields. Charged subatomic particles (such as protons, which carry a positive electrical charge) are accelerated when moving through such fields, sometimes to high speed. But if the gas collides with other gas clouds, the subatomic particles can ping-pong between them, gaining energy every time they bounce. (This is called first-order Fermi acceleration, a term I love for its Star Trek–like cadence.) They can become so energetic they’re flung out like a rock from a trebuchet.
Even so, getting particles up to mere femtometers-per-second-slower than light itself is extraordinary, and it’s not clear what specific processes are involved. There are no known sources capable of this in the Milky Way, so the OMG particle very likely came from another galaxy. The second-highest-energy cosmic ray ever seen, nicknamed Amaterasu after the Shinto sun goddess, had an energy of 244 quintillion eV, and it seems to have come from a patch of sky that overlaps with the galaxy PKS 1717+177, known to have extremely powerful jets blazing forth from its central black hole. Many others have been associated with other active galaxies as well.
And there’s more mystery afoot. The speed of the OMG particle actually violates a cosmic rule of thumb used by particle astrophysicists. The universe is filled with radiation leftover from the big bang called the cosmic microwave background. This is pretty low-energy stuff, assuming you’re not moving rapidly relative to it.
But a particle moving near the speed of light will see that radiation coming from ahead of it hugely amplified in energy because of the Doppler shift, and at these speeds, that effect operates on a ridiculously extreme level. A proton hit by such high-energy photons should lose energy, slowing it down, so at very high speed it actually gets decelerated rapidly. There’s an even more stringent cutoff; if the photons it sees are energetic enough, the proton will be converted into two other subatomic particles, a neutron and a pion. Both of these decay rapidly into even more particles, so in the end, ultrahigh-energy protons (with more than 50 quintillion eV) from distant galaxies should never reach us.
So how did the OMG particle get here?
The answer may simply be that it wasn’t a proton. Cosmic rays are a mix of different subatomic particles, including helium nuclei (two protons and two neutrons bound together) or even heavier elements. An iron nucleus, a common cosmic-ray culprit, wouldn’t be affected the same way a proton is and could make that long journey to Earth.
The OMG particle is the highest energy cosmic ray ever detected, but many others have been seen with somewhat lower but still startling energies. Clearly the universe has no issues making them, even if they’re rare.
Besides the gee-whiz aspect of them, they’re also telling us something important about the cosmos. There are engines out there of extreme power, capable of producing far more energetic particles than we could hope to on Earth. Energies like this were common, even ubiquitous, in the very early universe, so finding particles like this is like having a window into the fraction of a second after the big bang.
The universe is teaching us about itself, and all we have to do is pay attention to the little things.