Relative Velocity Measurements of Electrons and Gamma Rays at 15 GeV

Measurements were made to detect differences in the velocity of propagation of $\ensuremath{\gamma}$ 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 \ensuremath{\sim} 1 km. A relative velocity difference larger than $1\ensuremath{-}{\ensuremath{\beta}}_{e}$ (\ensuremath{\sim} 5\ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$) would imply a breakdown of special relativity. No significant difference in the velocities of light and electrons was observed to within \ensuremath{\sim} 2 parts in ${10}^{7}$.