Trinity

J. Robert Oppenheimer, one of my scientific heroes

Enrico Fermi, my other great scientific hero

Trinity test, T+0.016ms. 18 kilotons. Dropped a gem on 'em.

Time: 0529:45. The firing circuit closed; the X-unit discharged; the detonators at thirty-two detonation points simultaneously fired; they ignited the outer lens shells of Composition B; the detonation waves separately bulged, encountered inclusions of Baratol, slowed, curved, turned inside out, merged to a common inward-driving sphere; the spherical detonation wave crossed into the second shell of solid fast Composition B and accelerated; hit the wall of dense uranium tamper and became a shock wave and squeezed, liquefying, moving through; hit the nickel plating of the plutonium core and squeezed, the small sphere shrinking, collapsing into itself, becoming an eyeball; the shock wave reaching the tiny initiator at the center and swirling through its designed irregularities to mix its beryllium and polonium; polonium alphas kicking neutrons free from scant atoms of beryllium: one, two, seven, nine, hardly more neutrons drilling into the surrounding plutonium to start the chain reaction. Then fission multiplying its prodigious energy release through eighty generations in millionths of a second, tens of millions of degrees, millions of pounds of pressure. Before the radiation leaked away, conditions within the eyeball briefly resembled the state of the universe moments after its first primordial explosion.

Then expansion, radiation leaking away. The radiant energy loosed by the chain reaction is hot enough to take the form of soft X rays; these leave the physical bomb and its physical casing first, at the speed of light, far in front of any mere explosion. Cool air is opaque to X rays and absorbs them, heating; "the very hot air," Hans Bethe writes, "is therefore surrounded by a cooler envelope, and only this envelope" - hot enough at that - "is visible to observers at a distance." The central sphere of air, heated by the X rays it absorbs, reemits lower-energy X rays which are absorbed in turn at its boundaries and reemitted beyond. By this process of downhill leapfrogging, which is known as radiation transport, the hot sphere begins to cool itself. When it has cooled to half a million degrees - in about one ten-thousandth of a second - a shock wave forms that moves out faster than radiation transport can keep up. "The shock therefore separates from the very hot, nearly isothermal [i.e., uniformly heated] sphere at the center," Bethe explains. Simple hydrodynamics describes the shock front: like a wave in water, like a sonic boom in air. It moves on, leaving behind the isothermal sphere confined within its shell of opacity, isolated from the outside world, growing only slowly by radiation transport on this millisecond scale of events.

What the world sees is the shock front and it cools into visibility, the first flash, milliseconds long, of a nuclear weapon's double flash of light, the flashes too closely spaced to distinguish with the eye. Further cooling renders the front transparent; the world if it still has eyes to see looks through the shock wave into the hotter interior of the fireball and "because higher temperatures are now revealed," Bethe continues, "the total radiation increases toward a second maximum": the second, longer flash. The isothermal sphere at the center of the expanding fireball continues opaque and invisible, but it also continues to give up its energy to the air beyond its boundaries by radiation transport. That is, as the shock wave cools, the air behind it heats. A cooling wave moves in reverse of the shock wave, eating into the isothermal sphere. Instead of one simple thing the fireball is thus several things at once: an isothermal sphere invisible to the world; a cooling wave moving inward toward that sphere, eating away its radiation; a shock front propagating into undisturbed air, air that has not yet heard the news. Between each of these parts lay further intervening regions of buffering air.

Eventually the cooling wave eats the isothermal sphere completely away and the entire fireball becomes transparent to its own radiation. Now it cools more slowly. Below about 9000°F it can cool no more. Then, concludes Bethe, "any further cooling can only be achieved by the rise of the fireball due to its buoyancy, and the turbulent mixing associated with this rise. This is a slow process, taking tens of seconds."

The high-speed cameras at W-10000 recorded the later stages of the fireball's development, Bainbridge reports, tracking its huge swelling from the eyeball it had been:

The expansion of the ball of fire before striking the ground was almost symmetric… except for the extra brightness and retardation of a part of the sphere near the bottom, a number of blisters, and several spikes that shot radially ahead of the ball below the equator. Contact with the ground was made at 0.65ms. Thereafter the ball became rapidly smoother… Shortly after the spikes struck the ground (about 2 ms) there appeared on the ground ahead of the shock wave a wide skirt of lumpy matter… At about 32ms there appeared immediately behind the shock wave a dark front of absorbing matter, which traveled slowly out until it became invisible at 0.85 s [the expanding front about 2,500 feet across]. The shock wave itself became invisible earlier at about 0.10s The ball of fire grew even more slowly to a [diameter] of about [2,000 feet], until the dust cloud growing out of the skirt almost enveloped it. The top of the ball started to rise again at 2 s. At 3.5 s a minimum horizontal diameter, or neck, appeared one-third of the way up the skirt, and the portion of the skirt above the neck formed a vortex ring. The neck narrowed, and the ring and fast-growing pile of matter above it rose as a new cloud of smoke, carrying a convection stem of dust behind it… The stem appeared twisted like a left-handed screw.

But men saw what theoretical physics cannot notice and what cameras cannot record, saw pity and terror. Rabi at Base Camp felt menaced:

We were lying there, very tense, in the early dawn, and there were just a few streaks of gold in the east; you could see your neighbor very dimly. Those ten seconds were the longest ten seconds that I ever experienced. Suddenly, there was an enormous flash of light, the brightest light I have ever seen or that I think anyone has ever seen. It blasted; it pounced; it bored its way right through you. It was a vision which was seen with more than the eye. It was seen to last forever. You would wish it would stop; altogether it lasted about two seconds. Finally it was over, diminishing, and we looked toward the place where the bomb had been; there was an enormous ball of fire which grew and grew and it rolled as it grew; it went up into the air, in yellow flashes and into scarlet and green. It looked menacing. It seemed to come toward one. A new thing had just been born; a new control; a new understanding of man, which man had acquired over nature.

To Teller at Compafiia Hill the burst "was like opening the heavy curtains of a darkened room to a flood of sunlight." Had astronomers been watching they could have seen it reflected from the moon, literal moonshine.

Joseph McKibben made a comparison at S-10000: "We had a lot of flood lights on for taking movies of the control panel. When the bomb went off, the lights were drowned out by the big light coming in through the open door in the back."

It caught Ernest Lawrence at Compania Hill in the act of stepping from his car: "Just as I put my foot on the ground I was enveloped with a warm brilliant yellow white light - from darkness to brilliant sunshine in an instant and as I remember I momentarily was stunned by the surprise."

To Hans Bethe at Compania Hill "it looked like a giant magnesium flare which kept on for what seemed a whole minute but was actually one or two seconds."

Serber at Compania Hill risked blindness but glimpsed an earlier stage of the fireball:

At the instant of the explosion I was looking directly at it, with no eye protection of any kind. I saw first a yellow glow, which grew almost instantly to an overwhelming white flash, so intense that I was completely blinded… By twenty or thirty seconds after the explosion I was regaining normal vision… The grandeur and magnitude of the phenomenon were completely breathtaking.

Segre at Base Camp imagined apocalypse:

The most striking impression was that of an overwhelmingly bright light… I was flabbergasted by the new spectacle. We saw the whole sky flash with unbelievable brightness in spite of the very dark glasses we wore… I believe that for a moment I thought the explosion might set fire to the atmosphere and thus finish the earth, even though I knew that this was not possible.

Not light but heat disturbed Morrison at Base Camp:

From ten miles away, we saw the unbelievably brilliant flash. That was not the most impressive thing. We knew it was going to be blinding. We wore welder's glasses. The thing that got me was not the flash but the blinding heat of a bright day on your face in the cold desert morning. It was like opening a hot oven with the sun coming out like a sunrise.

It unfolded in silence, a ballistics expert watching from Compania Hill realized with awe:

The flash of light was so bright at first as to seem to have no definite shape, but after perhaps half a second it looked bright yellow and hemispherical with the flat side down, like a half-risen sun but about twice as large. Almost immediately a turgid rising of this luminous mass began, great swirls of flame seeming to ascend within a rather rectangular outline which expanded rapidly in height… Suddenly out of the center of it there seemed to rise a narrower column to a considerably greater height. Then as a climax, which was exceedingly impressive in spite of the fact that the blinding brightness had subsided, the top of the slenderer column seemed to mushroom out into a thick parasol of a rather bright but spectral blue… All this seemed very fast… and was followed by a feeling of letdown that it was all over so soon. Then came the awe-inspiring realization that it was twenty miles away, that what had flared up and died so brilliantly and quickly was really a couple of miles high. The feeling of the remoteness of this thing which had seemed so near was emphasized by the long silence while we watched the grey smoke grow into a taller and taller twisting column, a silence broken after a minute or so that seemed much longer by a quite impressive bang, about like the crack of a five-inch anti-aircraft gun at a hundred yards.

"Most experiences in life can be comprehended by prior experiences," Norris Bradbury comments, "but the atom bomb did not fit into any preconceptions possessed by anybody."

As the fireball rose into the air, Joseph W. Kennedy reports, "the overcast of strato-cumulus clouds directly overhead [became] pink on the underside and well illuminated, as at a sunrise." Weisskopf noticed that "the path of the shock wave through the clouds was plainly visible as an expanding circle all over the sky where it was covered by clouds." "When the red glow faded out," writes Edwin McMillan, "a most remarkable effect made its appearance. The whole surface of the ball was covered with a purple luminescence, like that produced by the electrical excitation of the air, and caused undoubtedly by the radioactivity of the material in the ball."

Fermi had prepared an order-of-magnitude experiment to determine roughly the bomb's yield:

About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during and after the passage of the blast wave. Since, at the time, there was no wind, I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 21/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of T.N.T.

"From the distance of the source and from the displacement of the air due to the shock wave," Segre explains, "he could calculate the energy of the explosion. This Fermi had done in advance having prepared himself a table of numbers, so that he could tell immediately the energy liberated from this crude but simple measurement." "He was so profoundly and totally absorbed in his bits of paper," adds Laura Fermi, "that he was not aware of the tremendous noise." Frank Oppenheimer found his brother watching beside him outside the control bunker at S-10000:

And so there was this sense of this ominous cloud hanging over us. It was so brilliant purple, with all the radioactive glowing. And it just seemed to hang there forever. Of course it didn't. It must have been just a very short time until it went up. It was very terrifying.

And the thunder from the blast. It bounced on the rocks, and then it went - I don't know where else it bounced. But it never seemed to stop. Not like an ordinary echo with thunder. It just kept echoing back and forth in that Jornada del Muerto. It was a very scary time when it went off.

And I wish I would remember what my brother said, but I can't - but I think we just said, "It worked." I think that's what we said, both of us. "It worked."

Trinity director Bainbridge appropriately pronounced its benediction: "No one who saw it could forget it, a foul and awesome display."

At Base Camp Groves "personally thought of Blondin crossing Niagara Falls on his tightrope, only to me the tightrope had lasted for almost three years, and of my repeated, confident-appearing assurances that such a thing was possible and that we would do it." Sitting up in their trenches before the blast wave arrived, he and Conant and Bush ceremoniously shook hands.

The blast had knocked Kistiakowsky down at S-10000. He scrambled up to watch the fireball rise and darken and mushroom purple auras, then moved to claim his bet. "I slapped Oppenheimer on the back and said, ‘Oppie, you owe me ten dollars.'" The distracted Los Alamos director searched his wallet. "It's empty," he told Kistiakowsky, "you'll have to wait." Bainbridge went around congratulating the S-10000 leaders on the success of the implosion method. "I finished by saying to Robert, ‘Now we are all sons of bitches.'… [He] told my younger daughter later that it was the best thing anyone said after the test."

"Our first feeling was one of elation," Weisskopf remembers, "then we realized we were tired, and then we were worried." Rabi elaborates:

Naturally, we were very jubilant over the outcome of the experiment. While this tremendous ball of flame was there before us, and we watched it, and it rolled along, it became in time diffused with the clouds… Then it was washed out with the wind. We turned to one another and offered congratulations, for the first few minutes. Then, there was a chill, which was not the morning cold; it was a chill that came to one when one thought, as for instance when I thought of my wooden house in Cambridge, and my laboratory in New York, and of the millions of people living around there, and this power of nature which we had first understood it to be - well there it was.

Oppenheimer looked again into the Gita for a model sufficiently scaled:

We waited until the blast had passed, walked out of the shelter and then it was extremely solemn. We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita: Vishnu is trying to persuade the Prince that he should do his duty and to impress him he takes on his multi-armed form and says, "Now I am become Death, the destroyer of worlds." I suppose we all thought that, one way or another.

Other models also came to mind, Oppenheimer told an audience shortly after the war:

When it went off, in the New Mexico dawn, that first atomic bomb, we thought of Alfred Nobel, and his hope, his vain hope, that dynamite would put an end to wars. We thought of the legend of Prometheus, of that deep sense of guilt in man's new powers, that reflects his recognition of evil, and his long knowledge of it. We knew that it was a new world, but even more we knew that novelty itself was a very old thing in human life, that all our ways are rooted in it.

The successful director of the Los Alamos bomb laboratory left with Farrell in a jeep. Rabi watched him arrive at Base Camp and saw a change:

He was in the forward bunker. When he came back, there he was, you know, with his hat. You've seen pictures of Robert's hat. And he came to where we were in the headquarters, so to speak. And his walk was like "High Noon" - I think it's the best I could describe it - this kind of strut. He'd done it.

See Also

• Nuclear weapons
• Rutherford