
 Lorentz transformation  Wikipedia 
  The group transformation known as the Lorentz transformation, after physicist H. 
  The Lorentz transformation, as a set of equations governing two reference frames in spacetime, S and S', with S' traveling at a relative speed of u to S; an event has spacetime coordinates of (x,y,z,t) in S and (x',y',z',t') in S': 
  In cases where u does not point along the xaxis of S, it is generally easier to perform a rotation so that u does point along the xaxis of S than to bother with the general case of the Lorentz transformation. 
 nostalgia.wikipedia.org /wiki/Lorentz_invariance (233 words) 
