  Imagine a swarm of free particles near a point P outside a gravitating mass M
and a free reference particle at P that is on a radial escape trajectory away
from M. Relative to this reference particle and in a Fermi normal coordinate
system constructed along its worldline, the particles in the swarm that move
along the radial direction and are ultrarelativistic (that is, they have speeds
above c/sqrt(2)) decelerate toward this terminal speed. On the other hand, the
swarm particles that are ultrarelativistic and move in directions normal to the
radial (jet) directions accelerate to almost the speed of light by the
gravitational tidal force of the mass M. The implications of these effects as
well as the influence of the higher-order terms on the tidal acceleration
mechanism are investigated. The observational evidence in support of these
general relativistic effects is briefly discussed.
