1. R. P. Kerr:Phys. Rev. Lett.,11, 237 (1963).
2. G. C. Debney, R. P. Kerr andA. Schild:Journ. Math. Phys.,10, 1842 (1969).
3. J. L. Synge:Relativity: The General Theory, Chap. 2 (Amsterdam, 1966); we have usedη ij instead ofg ij , to indicate the parallel propagator in order to emphasize that we are working in Minkowski space-time.
4. Equation (1.9) represents the special case of Born rigidity (cf. (3),J. L. Synge:Relativity: The General Theory, Chap. 2 (Amsterdam, 1966); p. 173) when the world-lines of the congruence λ i are geodesics.
5. F. A. Pirani: inLectures on General Relativity, Brandeis 1964 Summer Institute on Theoretical Physics, Vol.1 (Englewood Cliffs, N. J., 1964).