Ultrarelativistic Motion: Inertial and Tidal Effects in Fermi   Coordinates
  Fermi coordinates are the natural generalization of inertial Cartesian coordinates to accelerated systems and gravitational fields. We study the motion of ultrarelativistic particles and light rays in Fermi coordinates and investigate inertial and tidal effects beyond the critical speed c/sqrt(2). In particular, we discuss the black-hole tidal acceleration mechanism for ultrarelativistic particles in connection with a possible origin for high-energy cosmic rays. 
C. Chicone and B. Mashhoon