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.

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.

[5] 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.

"[5] In the late 1940s and 1950s, Fox was part of the Carnegie Tech team that created the then state-of-the-art 450 MeV synchrocyclotron at the Nuclear Research Center in nearby Saxonburg.

In 1962/63, Fox spent a sabbatical year in France at the Laboratoire Joliot-Curie de Physique Nucléaire d'Orsay, in part because of the new synchrocyclotron installed there.