Introduction to Electrodynamics · Problem 12.7

Electrodynamics and Relativity — David J. Griffiths

Physics · special-relativity-time-dilation

Problem

In a laboratory experiment, a muon is observed to travel 800 m before disintegrating. A graduate student looks up the lifetime of a muon (

2 \times 10^{-6}

s) and concludes that its speed was

v = \frac{800\text{ m}}{2 \times 10^{-6}\text{ s}} = 4 \times 10^8\text{ m/s}.

Faster than light! Identify the student's error, and find the actual speed of this muon.