The beginning of the nuclear age is not a single subject but a series of subjects that lead one to another in an unending chain reaction...That this is tacitly recognized is the most valuable aspect of The Day after Trinity: J. Robert Oppenheimer and the Atomic Bomb, Jon Else's documentary feature that opens today (January 20, 1981) at the Public Theater. The film serves as a kind of introduction to a period of history that is very easily ignored in favor of subjects of far less immediate concern. Mr. Else, and the movie, share with Oppenheimer an awful suspicion that when the first bomb was successfully detonated on the New Mexico desert in July 1945, it signaled the beginning of the end.