  The above title is the same, but with "semisimple" instead of "simple," as
that of a notice by N. Kowalsky. There, she announced many theorems on the
subject of actions of simple Lie groups preserving a Lorentz structure.
Unfortunately, she published proofs for essentially only half of the announced
results before her premature death. Here, using a different, geometric
approach, we generalize her results to the semisimple case, and give proofs of
all her announced results.
