The Making of the Atomic Bomb

to estimate the size of the explosion.” The sticks were posted on the rim of the reservoir. “They were arranged so that their [height] corresponded to 1000 ft. at zero point.” Philip Morrison relayed the countdown to the Base Camp observers by loudspeaker.

The two-minute-warning rocket fizzled. A long wail of the Base Camp siren signaled the time. The one- minute warning rocket fired at 0529. Morrison also meant to look the beast in the eye and lay down on the slope of the reservoir facing Zero. He wore sunglasses and held a stopwatch in one hand and a piece of welder's glass in the other. The welder's glass was stockroom issue: Lincoln Super-visibility Lens, Shade #10.

At S-10000 someone heard Oppenheimer say, “Lord, these affairs are hard on the heart.” McKibben had been marking off the minutes and Allison broadcasting them. At 45 seconds McKibben turned on a more precise automatic timer. “The control post was rather crowded,” Kistiakowsky notes, “and, having now nothing to do, I left as soon as the automatic timer was thrown in… and went to stand on the earth mound covering the concrete dugout. (My own guess was that the yield would be about 1 kt [i.e., 1,000 tons, 1 kiloton], and so five miles seemed very safe.)”

Teller prepared himself further at Compania Hill: “I put on a pair of dark glasses. I pulled on a pair of heavy gloves. With both hands I pressed the welder's glass to my face, making sure no stray light could penetrate around it. I then looked straight at the aim point.”

Donald Hornig at S-10000 monitored a switch that could cut the connection between his X-unit in the tower and the bomb, the last point of interruption if anything went wrong. At thirty seconds before T = 0 four red lights flashed on the console in front of him and a voltmeter needle flipped from left to right under its round glass cover to register the full charging of the X-unit. Farrell noticed that “Dr. Oppenheimer, on whom had rested a very heavy burden, grew tenser as the last seconds ticked off. He scarcely breathed. He held on to a post to steady himself. For the last few seconds, he stared directly ahead.”

At ten seconds a gong sounded in the control bunker. The men lying in their shallow trenches at Base Camp might have been laid out for death. Conant told Groves he never imagined seconds could be so long. Morrison studied his stopwatch. “I watched the second-hand until T = — 5 seconds,” he wrote the day of the shot, “when I lowered my head onto the sand bank in such a way that a slight rise in the ground completely shielded me from Zero. I placed the welding glass over the right lens of my sun glasses, the left lens of which was covered by an opaque cardboard shield. I counted seconds and at zero began to raise my head just over the protecting rise.” Ernest Lawrence on Compania Hill had planned to watch the shot through the windshield of a car, allowing the glass to filter out damaging ultraviolet, “but at the last minute decided to get out… (evidence indeed I was excited!).” Robert Serber, his bottles of whiskey to succor him, stared twenty miles toward distant Zero with unprotected eyes. The last decisive inaction was Hornig's:

Now the sequence of events was all controlled by the automatic timer except that I had the knife switch which could stop the test at any moment up until the actual firing… I don't think I have ever been keyed up as I was during those final seconds… I kept telling myself “the least flicker of that needle and you have to act.” It kept on coming down to zero. I kept saying, “Your reaction time is about half a second and you can't relax for even a fraction of a second.”… My eyes were glued on the dial and my hand was on the switch. I could hear the timer counting… three… two… one. The needle fell to zero…

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 pluto-nium 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 0.65 ms [i.e., thousandths of a second]. 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 32 ms [when the fireball had expanded to 945 feet in diameter] 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 0.10 s..

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: