Cosmic rays are very high energy particles, mainly protons, from outer space.
When they enter Earth's atmosphere they interact with the nuclei of the atmospheric gas molecules. One of the products of these reactions is a subatomic particle called a muon. In the laboratory, muons decay with a mean life of . This speed corresponds to a gamma factor of 7.1. Were it not for the time dilation factor, the muons would travel, on average, about before decaying and thus very few would reach Earth's surface.