only a "small gap" in the experimental record. Fox suggested that better experiments were possible, in order to close that "small gap". Since photons with higher energies will, on average, travel much farther in any material before being extinguished and re-emitted, experiments using gamma rays instead of lower-energy visible light or x-rays would be dramatically less sensitive to the extinction problem. In 1963, along with T. A. Filippas (also of Carnegie Tech), Fox examined 68 MeV gamma rays emitted in the forward and backward directions by neutral pions moving at 0.2c, i.e., two-tenths the speed of light. The classical velocities of those photons should have been 1.2c and 0.8c, respectively, but this was not the case. The experiment, perhaps the earliest to be considered free of extinction effects, was successful: "We conclude that our results provide strong evidence that the velocity of radiation from a moving source is not the classical vector sum of c and the velocity of the source. Within our accuracy, the resultant sum is c as required by special relativity." Reexamination of Ritz's emission theory The emission theory of Walter Ritz was considered early on to be an alternative to Einstein's special relativity. A key difference between the two is that under emission theory, the speed of light is expected to vary along with the speed of its source. Emission theory had long been disfavored by the 1960s, but Fox realized that the effects of extinction on the measurement of the speed of light nullified much of the accepted evidence against it. So in 1964, while still a firm believer in special relativity, Fox decided it was necessary to critically reexamine all of the evidence against Ritz's theory. In doing so, he showed that most of the previous rejections of Ritz's ideas based on theoretical arguments were invalid, including all of those enumerated by Wolfgang Pauli in his 1921 monograph on relativity. In addition, most previous experimental results disfavoring emission theory could be discarded as well, once extinction effects were considered, since the light (or other radiation) whose speed was measured was all actually re-emitted somewhere other than the original moving source. But Fox determined that his own work with Filippas, as well as very current work done at the CERN laboratory using 6 GeV gamma rays from neutral pions moving at very close to c, did, conclusively, support special relativity. His summary: "Our general conclusion