1. R.C. Tolman, Relativity, Thermodynamics and Cosmology, Oxford University Press, Oxford, 1934, p. 272.
2. General-relativistic treatment of the gravitational coupling between laser beams
3. G.R. Fowles, Introduction to Modern Optics, Dover Publications, New York, 1989, p. 290.
4. I. Ciufolini, J.A. Wheeler, Gravitation and Inertia, Princeton Univ. Press, Princeton, 1995, Chap. 6.
5. H.C. Ohanian, R. Ruffini, Gravitation and Spacetime, W.W. Norton, New York, 2nd ed., Chap. 3.