1. J. A. Wheeler and R. P. Feynman,Rev. Mod. Phys. 17, 157 (1945).
2. J. A. Wheeler and R. P. Feynman,Rev. Mod. Phys. 21, 425 (1949).
3. P. C. W. Davies,The Physics of Time Asymmetry (University of California Press, Berkeley, 1974).
4. D. Layzer,Vistas of Astronomy, Vol. XIII (Pergamon Press, New York, 1972).
5. S. W. Hawking,Phys. Rev. D32, 2489 (1985).