“The Earth is being constantly bombarded from space by cosmic rays of an unknown origin!”
This may seem like the cry of a panicking news reporter in a lurid sci-fi tale, but it’s actually a scientific fact — albeit a slightly hyperbolic one.
Mysterious cosmic rays traveling at speeds approaching that of light constantly pelt Earth’s upper atmosphere from the depths of space, creating high-energy collisions that dwarf those produced in even the most powerful particle colliders. The atmospheric crashes rain down gigantic showers of secondary particles to the surface of our planet.
But despite being discovered more than a century ago, physicists still don’t know where cosmic rays come from.
“The short answer to why we can’t trace cosmic rays back to their source: magnetic fields,” Julia Tjus, professor at the School of Physics and Astronomy at Ruhr University in Bochum, Germany, tells Astronomy. She explains that charged cosmic-ray particles are redirected by the magnetic fields they pass through on their long journey through space. “As magnetic fields in space have local, small, randomly oriented structures, a prediction of the exact path of a cosmic-ray particle is impossible,” she says.
“It’s as if it wandered through the universe like a ball in a pinball machine.”
This doesn’t mean that we are completely in the dark about these tiny space bullets, though.
What are cosmic rays?
One thing we certainly do know about cosmic rays is that they are comprised of extremely energetic charged particles — like protons, alpha particles, and atomic nuclei like helium and iron, with miniscule proportions of antiparticles thrown into the mix.
It’s hard to imagine just how shocking the discovery of cosmic rays must have been to physicists in the early 1900s. The energies of these particles were monumental in comparison to those of every other particle they had observed until that point. For example, the average energy of a solar photon is approximately 1.4 electron volts (eV). For reference, a flying mosquito has an energy of about 1 trillion eV, or 1x1012 eV, but a mosquito is also much, much larger than a single particle. Meanwhile, an alpha particle emitted during the decay of Uranium-238 possess 4.27x10⁶ eV of energy.
Compare that to a cosmic ray proton, which has an energy of some 1x10²⁰ eV.
“Imagine a proton that is accelerated so that it has an energy of 100 Joule,” Says Tjus, whose recent paper explains how new developments in astronomy could help trace the origins of cosmic rays. “This energy corresponds to that of a tennis ball smashed by someone like Raphael Nadal with a velocity of around 200 kilometers [124 miles] per hour. Only, the tennis ball 10²⁹ times heavier than the proton.
“That means a proton can only reach [that] extreme, macroscopic energy by travelling at almost the speed of light. The universe must be able to accelerate particles to these energies, but we still do not know how it does it.”