Muonet

Meet Muonet, a particle born high in the sky who carries a clock that runs slow.

Cosmic rays smash into air molecules about 15 km up and make muons, heavy cousins of the electron. A muon at rest decays in about 2.2 microseconds on average. Even at nearly the speed of light, that is only enough time to travel about 660 m.

Yet huge numbers of muons reach the ground. Special relativity explains it: a moving clock runs slow by the factor γ, so Δt = γΔt₀. At 0.995c, γ ≈ 10. From the ground, the muon lives about 22 μs and can cover about 6.6 km.

The faster the muon moves, the bigger γ gets and the farther it travels before decaying. From the muon's own point of view, its clock is normal and the atmosphere is squashed shorter instead.

Muon detectors on mountains and at sea level were used in the 1940s and 1960s to measure this effect directly.