An experiment has been carried out employing time-of-flight techniques to compare the velocity of propagation of short pulses of eV (visible) and GeV electromagnetic radiation. The results give a relative velocity difference $\frac{\left[c\right(\mathrm{GeV})-c(\mathrm{eV}\left)\right]}{c\left(\mathrm{eV}\right)}=(1.8\mathrm{}\pm 6)\times {10}^{-6\mathrm{}}$. The velocity of 11-GeV energy electrons $v_e$ is measured and found to be approximately equal to the velocity of visible light: $\frac{(c-v_e)}{c}=(-1.3\mathrm{}\pm 2.7)\times {10}^{-6\mathrm{}}$.

• Received 26 February 1973

DOI:https://doi.org/10.1103/PhysRevLett.30.763