13. Ultra-High-Energy Cosmic Rays
Occasionally, Earth is struck by ultra-high-energy cosmic rays—particles traveling at energies millions of times greater than anything produced in particle accelerators. Their immense power and rarity puzzle astrophysicists, as their origins remain uncertain. Potential sources include violent cosmic events like supernovae or distant active galactic nuclei, but no single explanation fits all observations. The journey of these particles across the universe continues to challenge and expand our understanding of fundamental physics. Learn more at Fermilab.