Muon decays in the Earth's atmosphere, differential aging and the   paradox of the twins
  Observation of the decay of muons produced in the Earth's atmosphere by cosmic ray interactions provides a graphic illustration of the counter-intuitive space-time predictions of special relativity theory. Muons at rest in the atmosphere, decaying simultaneously, are subject to a universal time-dilatation effect when viewed from a moving frame and so are also observed to decay simultaneously in all such frames. The analysis of this example reveals the underlying physics of the differential aging effect in Langevin's travelling-twin thought experiment. 
J.H.Field