When high-energy cosmic rays hit Earth’s upper atmosphere, new particles including muons are created. These particles are often unstable and decay in a known short amount time. The decay of muons travelling through our atmosphere was used in 1962 to confirm the relativistic concepts of time dilation and length contraction.
“Detectors are set up high on a mountain and at the base of the mountain. The distance between detectors is known, and by detecting the number of muons at both positions, relevant data are obtained. We just need to measure the number of muons per unit time at both positions. The muons act like precision clocks, and they have a known half-life.”
– Kognity, Inc
You can use the applet below to try the experiment yourself. Note that you’ll need Adobe Flash.