A muon detector on Mount Washington observes greater numbers of muons than an identical detector located at a lower elevation, for example in Boston. The number detected in Boston, however, is much greater than the number predicted when considering the elevation separation between Boston and Mount Washington, the muons' speed, and the time of the muons' half-life. Why is this so?


The muons are traveling faster than the speed of light
The addition of velocities increases the muons' half-life


The muons' speeds are large enough for relativistic effects to occur
The detectors must be faulty—the number predicted is classically correct



Einstein's special theory of relativity implies that moving objects experience time dilation. The half-life of "stationary" muons is a known value, and the explanation for greater numbers than classically predicted lower in the atmosphere comes from using the distance covered by the muons and the stationary half-life. The correction comes from the time dilation the muons experience.

Physics & Space Science

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