1. M. Gell-Mann, Lectures delivered at the Summer School of Theoretical Particle Physics, University of Hawaii, Honolulu, Hawaii, 1969.
2. J. Stratdhee andY. Takahashi, Nucl. Phys.,8, 113, 1958. See alsoY. Takahashi, Phys. Rev.,D3, 622, 1971.
3. See, for example,H. Y. Chiu andW. F. Hoffmann, “Gravity and Relativity”, W. A. Benjamin, Inc., New York, 1964, p. 143.
4. Y. Fujii, Phys. Rev.,D2, 896, 1970.
5. R. C. Arnold, Phys. Rev.,D3, 2250, 1970.