Cosmic rays are mostly protons from outer space that have kinetic energies as high as that of an apple falling a few meters in Earth's gravitational field. When a cosmic-ray proton strikes an air molecule—typically at an altitude of about 15 kilometers—the result is a shower of energetic particles and radiation. Because the muons produced move at close to the speed of light, their short lifetimes (2.1 microseconds) are extended by the time dilation effect of special relativity, which allows most of them to reach Earth's surface without decaying.

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