Validity of Quantum Electrodynamics at Extremely Small Distances

We have measured the yield of electron-positron pairs from the reaction $\gamma +\mathrm{C}\to e^{+}+e^{-}+\mathrm{C}$ as a test of the validity of quantum electrodynamics at very small distances. Our results show that first-order quantum electrodynamics correctly predicts the $e^{+}{e}^{-}$ pair yield up to an invariant pair mass of 900 MeV/${\mathit{c}}^2$.