  Within the framework of general relativity, we investigate the tidal
acceleration of astrophysical jets relative to the central collapsed
configuration ("Kerr source"). We neglect electromagnetic forces throughout.
The rest frame of the Kerr source is locally defined via the set of
hypothetical static observers in the spacetime exterior to the source. Relative
to such a fiducial observer fixed on the rotation axis of the Kerr source, jet
particles are tidally accelerated to almost the speed of light if their outflow
speed is above a certain threshold, given roughly by one half of the Newtonian
escape velocity at the location of the reference observer; otherwise, the
particles reach a certain height, reverse direction and fall back toward the
gravitational source.
