Forty years ago today, they waited before dawn in the desert. After months of feverish work on this extraordinary secret device, no one knew exactly what to expect. Would it, the "gadget" as it was called, work?

Then came the flash, the dazzling white reflection in the sky. It looked as if the ball of fire would never stop billowing, growing. Among those cool scientists, there was a frightful recognition of the power they had created. As the monstrous cloud rose in the distance, Robert J. Oppenheimer, director of the Manhattan Project, thought of a sacred Hindu passage:

I have become death, the shatterer of worlds.

Today, five of the major scientists who worked on the first atomic bomb, who were in Los Alamos, N.M., "the day the sun rose twice," have come to Washington. They are old now, but remain driven with a vibrant intensity that punctuates their every sentence. Propelled by urgency about the perilous future rather than reminiscences of their fateful past, they continue to do what they have done for 40 years -- urge the world powers to reduce the risk of nuclear war, to control the awesome power they had unleashed with their brainchild in the desert.

All express gratitude and some surprise that the world so far has survived.

"Through some fate of God we have existed for 40 years. I never thought for a minute it could go on for 40 years, building up stockpiles of weapons without a nuclear war," says Robert Bacher, who assembled the bomb's plutonium core in an old farmhouse near the Alamogordo, N.M., testing site. There was a moment's panic during the actual fitting of the bomb's central portion when an insertion wedged tightly and would go no further. "Dr. Bacher was undismayed," one account stated, and in three minutes the basic assembly was completed without further incident.

"One piece was very hot, another very cold," Bacher explains today, adding modestly that it was a simple matter to wait until the hot piece, which had expanded, cooled.

"Then I got in my government car and drove to the site with it the bomb in the back seat," recalls Bacher with a slight laugh. The evolution from that primitive image to the generation of sophisticated bombs that followed still astounds Bacher: "Just one of these submarines that carry multiple warhead missiles, if it were to deliver a full load of bombs, the amount would correspond to all of the bombs the United States dropped in World War I and World War II by a factor of about seven. One submarine!"

And Victor Weisskopf, who calculated the effects of that first bomb, adds, "had we known 40 years ago there would be 50,000 nuclear warheads deployed on both sides we probably would have had more guilt."

Now, intercontinental missiles can deliver warheads thousands of miles in minutes, with pinpoint accuracy. A new generation of scientists studies lasers and particle beams, the so-called Star Wars plans that would hand over the atomic reins to computers. The five Los Alamos scientists here are deeply troubled by Star Wars. "It's crazy. It won't protect the population and will make a higher rung on the spiral," says Weisskopf vehemently, circling his arm in midair. And Cyril Smith, "the old man of the project -- I was 43 then," shakes his head. "Star Wars is absolutely ridiculous."

It is this sense of clear and present danger that has brought these atomic physicists here today to testify on the Hill, to send letters to President Reagan, to offer their Trinity Day Appeal:

"We, who felt the pulse of heat at Trinity, call on you, our fellow citizens, for a response deep enough to match the insistent signal from the fireball.

"We ask that you join us in insisting on a policy for nuclear weapons that abandons two grand illusions of our times: that nuclear warfare can achieve rational military or political objectives, and that a defense of populations against nuclear attack is possible . . . "

Trinity was the code name for the Los Alamos test; its origin remains something of a puzzle to historians and even to the five scientists, who were all close friends of Oppenheimer. Philip Morrison, who at 29 helped assemble the Trinity device, thinks the word may come from the Spanish trinidad, used frequently for places in the Spanish southwest. One version is that Oppenheimer was reading John Donne's poems and one line, "Batter my heart, three person'd God," led him to call the site Trinity. Through the years, the lore of a death curse haunts the writings of that first explosion. A turquoise mine near Los Alamos, which had been laid under a curse and thus abandoned by the Indians, was called Trinidad. The area where the bomb was detonated had long before been named by the Spanish Jornada del Muerto -- "death tract."

Morrison, now a physics professor at the Massachusetts Institute of Technology, was among those who assembled before dawn at the base camp, 10 miles from the blast, fortified with sunburn cream and dark glasses.

"There was this remarkable experience of heat that came out of the cold desert night just before dawn," he remembers. And Weisskopf, a close friend and a colleague at MIT, adds, "we saw the color, from dazzling white to yellow and pink, surrounded by a blue halo due to the radioactive glow. The landscape was lit up 20 times stronger than midday sunlight. We felt a blast of intense heat on our faces. The Army people told us 'you have to lie down on your stomach and turn your head away.' All the Army people did, but of course we didn't even think of doing it! That would have meant we couldn't see."

Weisskopf and Morrison were two of the 600 or so "white badge" specialists among the thousands of Los Alamos workers who knew that this test was a requisite prelude to dropping the bomb in combat. Less than a month later, on Aug. 6, a city was destroyed instantly by a single bomb, and 130,000 perished. The intense heat felt by Morrison from 10 miles away would melt Hiroshima's roof tiles, incinerate people on the spot.

Before Hiroshima, there had been intense camaraderie. Secrecy was extraordinary in the months before the test. Wives and children were told nothing. A "what did you do today, dear?" question from wives drew a cryptic "worked on 'the project,' " recalls Smith with a smile. The pressure of the secret drew the scientists together in an "enormous spirit of friendship," Morrison says. "It was much more like a close-knit military expedition than the academic world." Such was the sense of historic accomplishment that otherwise reserved men would dance and hoot and clap each other on the back after this first man-made nuclear explosion in the desert.

After Hiroshima, the doubts and fears, the effort to halt the nuclear buildup, began. Morrison, who went to the island of Tinian with the Nagasaki bomb, flew over Hiroshima a few days after the bomb was dropped. "We circled finally low over Hiroshima and stared in disbelief. There below was the flat level ground of what had been a city, scorched red . . . One bomber, and one bomb, had, in the time it takes a rifle bullet to cross the city, turned a city of 300,000 into a burning pyre. That was," Morrison reported, "the new thing."

This "new thing," this incredible ease of human destruction, produced endless debate of conscience and a crusade among atomic scientists. The nuclear scientists here were among the many who spoke out vigorously against the hydrogen bomb. They understand, sometimes obsessively so, that they had given birth to a force of shattering destructiveness. While feeling "awe" and "pride," Weisskopf also "immediately felt that morning in the desert deep concern about what will come out of this for mankind." Many share Weisskopf's view that the second drop on Nagasaki, three days later, was a "crime." Still, like Weisskopf, they cling to the view that Hiroshima was necessary to "end a murderous war."

Nobel laureate Hans Bethe, the "dean" of American physicists and director of the Trinity detonation project, was among those troubled by the responsibility he bore in its creation. Before Hiroshima, recalls Bethe, "I knew there was tremendous power. I knew this when we first started the Manhattan Project in 1942, but it is different knowing it theoretically and seeing the actual destruction."

Bethe and Weisskopf, Jews who both fled Nazi Germany, joined the project for similar reasons -- fear that Hitler would develop the bomb first. "Fission was discovered in Germany," explains Weisskopf. Adds Bethe, "I have no regrets on that whatever. Furthermore, I believe it was necessary to drop it on Hiroshima. It was the only way to get the attention to the emperor of Japan."

At the time the bomb was dropped on Hiroshima and Nagasaki, the United States was certain that Japan would fight to the end and that a conventional war would cost millions of lives. "It was awful, but it was the only way. A demonstration on an uninhabited site would not have done it. The emperor would not have been there. It was awful," Bethe repeats, "but far more awful is that this weapon is now in the world and in enormous quantity."

Bethe, at 79 an emeritus professor at Cornell University, says of arms reduction, "At half of what we have now it is still far too much, but you have to make a start. We would start to go down at last -- rather than always up."

This quest has consumed much of Bethe's life since Hiroshima. "I try to do as much as I can in favor of sanity."

Forty years later, there is also ambivalence about Hiroshima.

Morrison says, "I suspect if Roosevelt had lived it might have been done better. The most important thing was perhaps a warning and description; we were going to do thus and so and that it was this new thing. It would not have made a difference to the Japanese," adds Morrison, pointing out that conventional bombs had already destroyed many of Japan's cities and the Japanese continued to fight. "But it would have made a difference to us -- and to the future. To our conscience and the whole way it was introduced to the world." Just as the bomb was referred to as the "gadget" in Los Alamos, atomic scientists seem to refrain from using the word "bomb" today; Morrison is not the only one to refer to the bomb as "it."

"We meant so well," says Weisskopf, a tall, vigorous man of 76, recalling what was their climactic moment, lived half a lifetime ago. "We thought such powerful weapons would make wars between great powers unthinkable. We tried to create an agency to avoid an arms race by keeping nuclear weapons under international control but we were naive. It was a complete failure. There may have been some guilt that drove us on, but more importantly it was because we were so close to it. We were scared by the effects."

Weisskopf shakes his head. "The race is getting worse and worse. A silly race and our security is decreasing. The more the Russians feel insecure, then they will start doing something crazy. Russia is a terrible country but we can't change it. Hitler's was the last despicable regime we could get rid of by force. Because of the bomb we cannot change anymore by such force. We must sit down with them."

And Cyril Smith, now 83, looks with brilliant blue eyes at world leaders, at actions he does not comprehend, knowing that both superpowers have the force to blow up the planet many times over.

"What is hardest to understand is that the very people in high levels of government continue to chant the same old refrain about the other side. Each side repeating the same old lies, having the same old suspicions of one another. The absence of real knowledge of what each one is doing produces this foolish imagination. Secrecy cloaks error -- and encourages the growth of fearful imaginary monsters."

Philip Morrison says of their effort, "I have no idea if coming to Washington today will help. I've just been doing the same thing day in and day out for 40 years. One thing," Morrison continues after a slight pause. "I don't think not doing it is of any help."

The urgency is in the voice of Morrison's colleague, Robert Bacher, now 79, an emeritus professor of physics at the California Institute of Technology. "When the Russians got the bomb, I felt it would not be long before there was Grade A trouble. It took much longer, but the situation has been very, very difficult for nearly 20 years now."

As for dropping the bomb 40 years ago, Bacher says, "I knew perfectly well it would be the president, not us at Los Alamos, who would make that decision. I feel it would have been better if it somehow could have been used by persuasion rather than by use. Good Lord, I certainly feel that! I don't know anyone who dealt with it who didn't feel that way."

But he, like other atomic scientists, looks back to that urgent time when war seemed endless; looks back at the belief that the bomb would halt even more death.

"Time," recalls Bacher, "had to move too fast."