E=mc2 (17 page)

Heisenberg would be welcomed as a hero in Germany when he was finally released in 1946, while Oppenheimer, even before the war ended, knew his postwar life wouldn't be so simple. He had been a leftist in the late 1930s, and although a Berkeley physics professor might not suffer harm from that, once he was head of Los Alamos the FBI had dug up everything. Then he'd lied about some of the details, in his first interviews with military intelligence. Several important individuals wanted him out, but Groves was protecting him, so in revenge his enemies simply tormented him: much of his time as director his phone was tapped, his living quarters bugged, his past friends interrogated, and his trips shadowed. His wife had started drinking, a lot, and although he hadn't yet been attacked, he knew he was open to blackmail: the FBI had followed him on visits to San Francisco, where he'd spent nights with a girlfriend he'd been close to in the past.

More important, he knew what had happened on Lake Tinnsjo; he knew what was in store in the Pacific. It's common today to state that the atomic bombing of Japan was obviously justified, on the grounds that the alternative would have been an invasion that had to be much worse. But at the time it was not so clear. The bulk of Japan's army was no threat to American forces: it was sequestered up in China, with American submarines keeping it from crossing to the home islands, and the great weight of Russia's army looming above, able to destroy it once a sufficient buildup had occurred. Japan's industry had largely been burned out. Early in 1945, U.S. strategic bombers had been assigned the task of destroying thirty to sixty large and small cities. By August, they had burned out fifty-eight of them.

Douglas MacArthur, who had run much of the Pacific campaign, didn't expect an invasion would be needed; Admiral Leahy, chairman of the Joint Chiefs of Staff, was later adamant that there had been no need for an atomic bomb; Curtis LeMay, the head of the strategic bombing force, agreed. Even Eisenhower, who'd had no qualms about killing thousands of opponents when it was necessary to safeguard his troops, was strongly hostile to it, as he explained at the time to Henry Stimson, the elderly secretary of war: "I told him I was against it on two counts. First, the Japanese were ready to surrender and it wasn't necessary to hit them with that awful thing. Second, I hated to see our country be the first
to
use such a weapon. Well . . . the old gentleman got furious. . . ."

The feeling it might not be needed was so strong that there was talk about having demonstrations first, or at least adjusting the phrasing in the surrender demands to make clear that the emperor could remain in place. Oppenheimer had been at many of those meetings: listening intently, arguing—often in a slightly hedged way—for use if it was needed, but supporting the clause about safeguarding the emperor.

These arguments didn't take. Truman's most forceful adviser was Jimmy Byrnes, a man of Lyman J. Briggs's generation, but far less mild in temperament. The ethos Byrnes had been brought up with was that when you fought, you fought with everything you had. He'd been raised in South Carolina in the 1880s, with no father and not a great deal of schooling. Visitors to his state during earlier times reported their amazement that it was rare on a jury to find twelve men who had all their eyes and ears: South Carolina still had the ethos of a frontier society, and gouging, biting, and knife slashes were the way fights were settled. It was Byrnes who ensured that the clause protecting the emperor—which might mollify Japanese opponents of a settlement—was taken out. There would be no nonsense either about just waiting for the submarine blockade to tighten, or getting advancing Russians to do the dirty work.

Notes from the Presidential "Interim Committee," June 1, 1945:

Mr. Byrnes recommended,
and the Committee
agreed,
that. . . the bomb should be used against japan as soon as possible; that it be used on a war plant surrounded by workers' homes; and that it be used without prior warning."

Part of Oppenheimer accepted that; part of him— especially when away from Washington—was unsure. But did it matter? He'd helped bring out these powers but now was the least part of it. Oppenheimer's superior, Leslie Groves, was General Groves. Los Alamos was a project of the United States Army. The army built weapons to use them.

The atomic bomb was going to be loaded onto that airplane.

8:16 A.M.—Over Japan
I3

Whistling, spinning, the bomb ("an elongated trash can with fins") had taken forty-three seconds to fall from the B-29 that released it. There were small holes around its midpoint where wires had been tugged out of it as it dropped away: this had started the clock switches of its first arming system. More small holes had been drilled farther back on its dark steel casing, in New Mexico, and those took in samples of air as the free fall continued. When it had tumbled to 7,000 feet above the ground, a barometric switch was turned, priming the second arming system.

From the ground the B-29 was just visible as a silvery outline, but the bomb—a bare ten feet long, two and a half feet wide—would have been too small a speck to see. Weak radio signals were being pumped down from the bomb to the Shina Hospital directly below. Some of those radio signals were absorbed in the hospital's walls, but most were bounced back skyward. Sticking out of the bomb's back, near the spinning fins, were a number of whiplike thin radio antennae. Those collected the returning radio signals, and used the time lag each took to return as a way of measuring the height remaining to the ground.

At 1,900 feet the last rebounded radio signal arrived. John von Neumann and others had calculated that a bomb exploding much higher would dissipate much of its heat in the open air; exploding much lower,
it
would dig a huge crater in the ground. At just under 2,000 feet the height would be ideal.

An electric impulse lit cordite sacs, producing a conventional artillery blast. A small part of the total purified uranium was now pushed forward down a gun barrel that was actually inside the bomb. In the early planning this gun had been a very heavy device, being simply a copy of large U.S. Navy weapons. Only after several months had one of Oppenheimer's men realized that navy guns were so heavy because they had to survive the recoil of shot after shot. Here, of course, it wouldn't matter: this gun was only going to be fired once. Instead of weighing 5,000 pounds it was machined to weigh barely a fifth of that.

The first uranium segment traveled about four feet within the thinned gun barrel, and then it impacted the remaining bulk of the uranium. Nowhere on Earth had a ball of several dozen pounds of such purified uranium ever been accumulated. There were a number of stray neutrons loose inside it, and although the uranium atoms were densely protected by their outer flurries of electrons, the escaped neutrons, having no electrical charge, weren't affected by the electrons. They flew through the outer electron barrier—as we saw, like a probe skimming past the planets down toward our sun—and while many of them flew straight through out the other side, a few were on a collision course for the speck of a nucleus far down at the center.

That nucleus normally blocked outside particles from entering, for it was seething with positively charged protons. But since neutrons have no electric charge they're invisible to the protons as well. The arriving neutrons pushed in to the nucleus, overbalancing it; making it jostle and wobble.

The uranium atoms mined on Earth were each over 4.5 billion years old. Only a very powerful force, before the Earth was formed, had been able to squeeze their electrically crackling protons together. Once that uranium had been formed, the strong nuclear force had acted, gluelike, to hold these protons in place over all that long span: while the Earth cooled, and continents formed; as America separated from Europe, and the North Atlantic Ocean slowly filled; as volcanic bursts widened on the other side of the globe, forming what would become Japan. A single extra neutron unbalanced that stability now.

Once the wobbling in the nucleus was enough to break the strong force glue, then the ordinary electricity of the protons was available to force them apart. A single nucleus doesn't weigh much, and the fragmentary section of one weighs even less. Its speeding impact into the other parts of the uranium didn't heat it up much. But the density of uranium was enough that a chain reaction started, and soon there weren't just two speeding fragments of uranium nuclei, there were four, then eight, then sixteen, and
so
on. Mass was "disappearing" within the atoms, and coming out as the energy of speeding nuclei fragments. E=mc
²
was now under way.

The entire sequence of multiplying releases was finished in barely a few millionths of a second. The bomb was still suspended in the humid morning air with a faint layering of condensation on its outer surface, for it had been up in the cold air of 31,000 feet just forty-three seconds earlier, and now, 1,900 feet over the hospital, it was a balmy 80°F. The bomb fell downward just an additional fraction of an inch in the time of most of the reaction; from the outside there would only be the first odd bucklings of its steel surface to suggest what was going on inside.

The chain reaction went through eighty "generations" of doubling before it ended. By the last few of those, the segments of broken uranium nuclei were so abundant, and moving so fast, that they started heating up the metal around them. The last few doublings were the crucial ones. Imagine you have a pond in your garden, with a lily plant floating on it that doubles in size every day. In eighty days the lily entirely covers the pond. On which day is half the pond still uncovered, open to the sun and outside air? It's the seventy-ninth day.

From this point on, all the action of the E=mc
²
reaction was over. No more mass was "disappearing"; no more fresh energy appeared. The energy in the movement of those nuclei was simply being transformed to heat energy—just as rubbing your hands together will make your palms warm up. But the uranium fragments were rubbing against resting metal at immense speed, due to the multiplication by c
²
. They soon were traveling at a substantial fraction of the speed of light.

The rubbing and battering made the metals inside the bomb begin to warm. They had started at near body temperature—98.6°F or 37°C—and then they reached water's boiling temperature—212°F or 100°C—and then that of lead—560°C. But the generations of chain reaction doubling had gone on, as yet more uranium atoms had been splitting, so it reached 5,000°C (the surface of the sun) and then several million degrees (the temperature of the center of the sun) and then it kept on rising. For a brief period, in the center of the suspended bomb, conditions similar to those in the early moments of the creation of the universe were produced.

The heat moves out. It goes through the steel tamping around the uranium, and just as easily through what had been the several-thousand-pound massive casing of the bomb, but then it pauses. Entities as hot as that explosion have energy that must be released. It starts pushing X rays out of itself, a very large number of them, some of them angling up, and some to the side, and the rest in a wide stretching arc toward the ground.

The explosion is hovering; the fragments are trying to cool themselves off. They remain that way, pouring out a large part of their energy. Then, after 1/10,000th of a second, when the X ray spraying is over, the heat ball resumes its outward spread.

Only now does the central eruption become visible. Ordinary light photons could not push through the X ray sprays; only the glows on the outside of the sprays would have been seen. When the full flash appears, it's as if a rip in the sky has opened. An object resembling one of the giant suns from a distant part of our galaxy now appears. It fills several hundred times more of the sky than Earth's ordinary sun.

The unearthly object burns at full power for about one-half of a second, then begins to fade away, taking two or three seconds to empty itself out. This "emptying" is accomplished, in large part, by spraying heat energy outward. Fires begin, seemingly instantaneously; skin explodes off, hanging in great sheets from the bodies of everyone below. The first of the tens of thousands of deaths in Hiroshima begin.

At least a third of the energy from the chain reactions comes out in this flash. The rest now follows soon behind. The strange object's heat pushes on ordinary air, accelerating it to speeds that have never occurred here before, unless at some time in the distant past a large meteor or comet arrived. It travels several times faster than any hurricane could achieve—so fast, in fact, that it's silent, for it outruns any sound its immense force might make. After it there's a second air pulse, a little slower; after that the atmosphere sloshes backward, to fill up the gap pushed out. This briefly lowers the air density to virtually zero. Far enough from the blast, life-forms that have survived will now begin to explode outward, having been exposed—briefly—to the vacuum of outer space.

An atomic bomb exploding in the very first milliseconds (top) and the ground being churned up as it expands, prior to the bomb producing a mushroom cloud (center; bottom).

TOP: PHOTOGRAPH BY DR. HAROLD E. EDGERTON.

COPYRIGHT © THE HAROLD E. EDGERTON 1992 TRUST. PALM PRESS, INC.

CENTER AND BOTTOM: LOS ALAMOS NATIONAL LABORATORY. PHOTO RESEARCHERS, INC.

And when that great mushroom cloud appeared, E=mc
²
's first work on planet Earth was done.