  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.
