REACTION FORCES ON A RELATIVISTIC POINT-CHARGE MOVING ABOVE A DIELECTRIC OR A METALLIC HALF-SPACE

We investigate the forces that act on a particle as it moves at a heig ht h above a half-space of dielectric or lossy material. The expressio ns for the longitudinal and transverse forces are calculated numerical ly and analytic expressions are presented for limit cases. In a dielec tric it is shown that the longitudinal force is zero at velocities bel ow the Cherenkov velocity and it reaches a constant value at high ener gies. The transverse force below the Cherenkov velocity is the result of a superposition of evanescent waves only; in this regime it increas es with the momentum. Above the Cherenkov velocity propagating waves a dd their contribution to the transverse force. For relatively low ener gies their contribution tends to increase the total force. At high ene rgies the total transverse force decays as 1/gamma. In the case of a m etallic medium (sigma), the longitudinal force is proportional to root (gamma beta)(3)/sigma eta(0)h when this quantity is smaller than unity and it reaches a constant value when this parameter is much larger th an unity. The transverse attraction force decreases monotonically with the relativistic factor gamma.