Ultra-high-energy cosmic ray



Unsolved problems in physics: Why is it that some cosmic rays appear to possess energies that are theoretically too high?

In high-energy physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray (subatomic particle) which appears to have extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays. These particles are significant because they have energy comparable to (and sometimes exceeding) the Greisen-Zatsepin-Kuzmin limit.