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hellfire
Nov3-04, 03:42 AM
I am going through the exercises in Schutz "A first course in General Relativity". Chapter 2, nr. 17:

Prove that any timelike vector U for which U0 > 0 and U U = -1 is the four velocity of some world line.
So it seams that first one has to prove that it exists a Lorentz transformation to some reference frame in which all components of U vanish except U0. Seams not to be complicated, but it is not clear to me how proceed then.