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Can a metal surface repel electric charges?

We show that the interaction between a surface and a charge packet moving parallel to it can become repulsive above a critical relativistic energy. We find that this is true for a lossless dielectric surface and also for a Drude metallic surface--in apparent contrast with such common notions as image charge. This counterintuitive phenomenon occurs for packets larger in the transverse than in the longitudinal (parallel to the motion) direction. The repulsion does not occur for a point charge that is instead attracted at all energies. In addition to the above attractive or repulsive transverse force, there is a longitudinal decelerating force, which for a dielectric corresponds to the Čerenkov effect. Once again, the behavior of a line packet differs from that of a point charge: for a packet with infinite transverse size, the decelerating field decreases to zero as the relativistic factor γ → ∞, whereas, for a point charge, the asymptotic value is finite. These findings have a potential impact not only on fundamental electrodynamics but also on accelerator physics and electron spectroscopy.