" All of the lightest stable quarks and leptons have heavier replicas. No one knows why they are there, or what they are good for. When physicists first realized that the muon, a particle first seen in cosmic rays, was nothing other than a heavier version of the electron (200 times heavier), the physicist I.I. Rabi asked, “Who ordered that? "

― Lisa Randall , Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions