The Earth’s atmosphere is bombarded constantly by radiation from outer space, generating elementary particles like muons, which exist for just a few millionths of a second. Even traveling at the speed of light they would only be able to cover a distance of a few hundred meters. Yet according to Einstein’s theory of relativity, clocks in motion tick more slowly; thus time passes more slowly for a muon. Otherwise there would be no way for cosmic radiation to reach the surface of the Earth.