The thing about the unthinkable is that somebody's bound to think it sooner or later. Not so long ago it was a man named J. Robert Oppenheimer who, with a little help from his friends, produced the first atomic bomb.

"Atomic Bomb" may sound like a quaint term to generations who've come of age living under the shadow of push-button mass destruction. But tonight at 8 on Channel 26, PBS presents a fascinating, quietly tremendous documentary about the birth of the bomb, "The Day After Trinity" -- produced and directed by 36-year-old filmmaker Jon Else -- that brings the bomb into focus in all its horrible, lethal beauty.

Earlier this year the film was nominated for an Academy Award as best documentary feature; although it has played in some theaters in major cities, the PBS airing will be most people's first chance to see it. Straightforward, sensitive and meticulous, it tells the story of the father of the bomb, the climate in which he lived and worked, and the aftermath of the decision to use it on two Japanese cities, Hiroshima and Nagasaki, to end World War II.

It isn't particularly alarmist or cautionary in nature, but somehow, in the faces of Oppenheimer's colleagues who talk about the era and the man, the momentousness of it is made stunningly graphic, and it evokes a time and a mentality with very little newsreel footage but a great deal of troubled reminiscence from those who were there.

"Talking heads" are supposed to be deadly dull on television, but that does depend on what the heads have to say. Else found fascinating figures to talk to, including Frank Oppenheimer, the physicist's surviving brother, who also worked on the bomb. Most of those interviewed are tweed-jacketed, twiddly and even lovable academic types; the film does not moralize about them or their work.

J. Robert Oppenheimer, a controversial figure in his later years for his opposition to the hydrogen bomb, is seen mainly in still photographs which haunt the 90-minute film; Oppenheimer's wide eyes look like the eyes of the Star Child at the end of "2001." In newsreel film, he quotes from the Bhagavad Gita, the Hindu holy book: "I am become Death, the destroyer of worlds."

At a press conference filmed during the LBJ years, he is asked about halting the spread of nuclear weapons. "It's 20 years too late," he says. "It should have been done the day after Trinity." Trinity Site is the name Oppenheimer gave to the spot in the New Mexico desert where the first bomb was detonated on July 16, 1945.

Rare films of preparations for that test are eerie and chilling even now, but more valuable still are the remembrances of those who, in one capacity or another, where there. "Once that happened, I was a different person from then on," recalls physicist Robert Wilson of the first nuclear explosion.

A woman who lived near Los Alamos, N.M., where 6,000 people involved in the Manhattan Project worked on the bomb, remembers her husband saying to her the night of the test, "I want you to come look -- the sun's coming up to come look -- the sun's coming up in the wrong direction." Another woman recalls that even her blind sister noticed the brightness of the light from the blast.

"It was the most expensive scientific project in the history of the world," says narrator Paul Frees, and Los Alamos represented the greatest assembly of scientific minds ever. But the air was not filled with dread and importance; people were doing a job, and an "anti-fascist" fervor, fear that Hitler's evil would sweep the world, compelled them forward.

Some who were there recall project as resembling "a camp-out." One calls it "fun . . . a lark," and there are memories of physicist Edward Teller (later an Oppenheimer foe and a developer of the hydrogen bomb who refused to be interviewed for the film) playing Beethoven late into the night, and of parties at which, because of a liquor shortage, drinks were mixed with laboratory alcohol -- "2001 proof," one scientist estimates.

"For many of them," says Frees, "it was the most marvelous time of their lives."

Does the film over-lionize the still-mysterious Oppenheimer? Not really. It delves into his leftist politics, the fact that his brother and sister-in-law were members of the Communist Party at one time, and deals with Oppenheimer's cold, curious treatment of an old mentor, Haakon Chevalier (interviewed in the film) during the nutty paranoia of the Cold War '50s.

From San Francisco, filmmaker Else says he did not set out to make a study of J. Robert Oppenheimer and the atomic bomb. He set out to make a study of the atomic bomb and the specter of Oppenheimer got in the way.

"I was a couple of weeks into the research for this thing before I even knew Oppenheimer was dead," says Else. "But the more I learned about him, the more he came to embody for me all the political, social and moral ambiguities of nuclear warfare -- of the whole 20th century."

Else confesses he has a political background that definitely tilts left, and that he is worried about the spread of nuclear weapons. But the diligent and moving film that he made is not a tract. It is a thoughtful, mesmerizing investigation into the most essential survival questions of technological man.

"I went into this film very doctrinaire and dogmatic," says Else, who previously directed "Stepping Out: The DeBolts Grow Up," for Home Box Office. "But in the end the experience of making the film asked more questions for me than it answered. I still think nuclear weapons are onerous and horrible, but I'm more puzzled than ever about what to do about it."

Else grew up in Sacramento, Calif., about 400 miles from the Nevada nuclear test site, and he remembers his father calling him into the back yard to "watch the sun come up" when a blast occured. It made a big impression on the Neveda desert and a big impression on him. Originally he and his researchers talked to 100 people associated with the Manhattan Project and they narrowed that group down to 15 to be interviewed in the film.

The first rough cut of the film was four hours long -- on a practical level, an unmanageable length for a TV documentary. Else cut the film down to 90 minutes because he wants it to be accessible to the largest possible audience.

"We have to convince people who would rather watch 'Charlie's Angels' to somehow stay tuned to it," he says. "There's a certain bending that has to go on when you make a film for television, and yet you have to be careful not to bend so far that you compromise everything you believe in."

One unhappy thing about the film is that it took so much struggling to raise the $150,000 to make it (Else says that hat-passing continued right up to the completion of the film), and that future independent documentaries like it are endangered by proposed Reagan budget cuts at the National Endowment for the Arts (NEA) and the National Endowment for the Humanities (NEH).

"This is the kind of film that would only be funded by NEA and NEH," says Else. "We can kiss those kinds of movies goodbye if the budget cuts go through. There is just no way Exxon or some other corporation is going to fund a film like this."

The film ended up in theaters prior to its airing because of a PBS scheduling goof."We broke our backs to finish the film by August because PBS said they wanted to put it on last fall," says Else. "Then they re-scheduled it for eight months down the road -- I don't pretend to understand how PBS works -- and we were faced with having a finished film on the shelf for eight months." A small distributor picked it up for theatrical showings in 25 theaters around the country, and the Oscar nomination followed. s

Did Else think he had a chance of winning? "I didn't realize how much I thought it would win until it didn't win," he says, but he is not discouraged and shouldn't be. He has made a film that speaks to the Great Issues but does it on intimate and riveting terms.

There have been other film and stage treatments of Oppenheimer and his era: a play called "In the Matter of J. Robert Oppenheimer" and, last fall on MBC, "Enola Gay," an inadequate account of the dropping of the bombs on Japan. None have been so affecting and thoughtful as "The Day After Trinity," the most relevant television imaginable and perhaps the best film ever made about living intimately with doom of our own design.