Relative Velocity Measurements of Electrons and Gamma Rays at 15 GeV

Measurements were made to detect differences in the velocity of propagation of $\gamma$ rays and electrons in the energy range 15-20 GeV, by using a time-of-flight technique with 1-psec sensitivity and a flight path of ∼ 1 km. A relative velocity difference larger than $1-{\beta }_e$ (∼ 5× ${10}^{-10\mathrm{}}$) would imply a breakdown of special relativity. No significant difference in the velocities of light and electrons was observed to within ∼ 2 parts in ${10}^7$.