Electrodynamics and Relativity — David J. Griffiths
Physics · rapidity
Problem
The parallel between rotations and Lorentz transformations is even more striking if we introduce the rapidity:
θ≡tanh−1(v/c).(12.34)
(a) Express the Lorentz transformation matrix Λ (Eq. 12.24) in terms of θ, and compare it to the rotation matrix (Eq. 1.29).
(b) Express the Einstein velocity addition law in terms of rapidity.