Muon
Definition
A heavy, short-lived cousin of the electron — born from cosmic rays, living 2.2 microseconds, and proving time dilation.
Symbol: ·
Technical definition
A lepton of mass 105.7 MeV/c² (≈207 m_e) with a 2.2 μs mean lifetime. Atmospheric muons reach the ground only because relativity stretches their lifetime.
Example
Muons created 10 km up should decay after ~660 m of travel — yet they reach the ground, a walking proof of Einstein's time dilation detectable with a school counter.
Common confusion
Muons are not merely 'fast electrons' — they are a distinct particle with 207 times the mass. They behave similarly only because both are leptons.