The design for the first atomic bomb was frighteningly simple: One lump of a special kind of uranium, the projectile, was fired at a very high speed into another lump of that same rare uranium, the target. When the two collided, they began a nuclear chain reaction, and it was only a tiny fraction of a second before the bomb exploded, forever splitting history between the time before the atomic bomb and the time after.

At 17 seconds past 8:15 a.m. on August 6, 1945, the Enola Gay released the bomb from a height of 31,600 feet above the target, a T-shaped bridge in the center of Hiroshima, Japan.

The morning was cloudless, as the weather plane sent to scout for the Enola Gay had reported in the hour before. If the weather had been poor, the plane would have set its course to one of the two alternate targets. As the bomb fell, a schoolboy closed his eyes and began to count as his friends hid along the way to school.

“I think we had all concluded that it was a dud,” Theodore Van Kirk, navigator of the Enola Gay, would later recall of the 43 seconds before the plane’s cabin filled with the blinding white light of the bomb’s explosion 1,890 feet above the target.

Going critical. A chain reaction begins when a stray neutron thrown from the nucleus of an atom runs into another nucleus, causing that atom to split apart, dislodging a couple more neutrons to collide with two more atoms, which break into four pieces and so on. When atoms break apart, some of their mass (the “m” in the equation E=mc²) is converted to energy, an enormous “E” of fire and heat and light and wind.

A chain reaction only occurs when the concentration of fissile atoms is high enough. Otherwise, the reaction will fizzle, with one stray neutron failing to excite a response from its unflappable neighbors. Uranium, as the heaviest naturally occurring element on earth, is inherently unstable, with 92 positively charged protons held in tense equilibrium in its swollen nucleus, repelling each other like magnets held with their North poles head to head. To relieve this tension, uranium throws off bits of itself, little clumps of protons and neutrons, decaying into a whole host of other selves—radium, radon, polonium—before coming to rest as lead, still heavy but stable. The most common uranium isotope, uranium 238, is radioactive—the particles that go flying from its nucleus can rip through human cells, knocking things out of place and causing genetic errors and cancer when received in high doses. But uranium 238 does not explode.

Uranium 235 is a rare isotope that makes up less than 1 percent of naturally occurring uranium. Uranium 235 differs from uranium 238 by only three neutrons. Even multiplied by three, the mass of a neutron is still unfathomably tiny. But three neutrons are the difference between a slow disintegration and an explosion. Neutrons, which have no electric charge, act as a kind of buffer between the crowded, positively charged protons. Ninety-two protons squeezed into a nucleus are restless neighbors, but with three fewer neutrons, the nucleus is volatile, ready to fly apart at the slightest provocation.

To build an atomic bomb, you need uranium with a high concentration of uranium 235 atoms, also known as highly enriched uranium. In 1943, the United States government built a city in the hills of East Tennessee that was not to be listed on any maps. Oak Ridge was first known as Site X, and its purpose was to produce uranium 235.

My family tells a story about my grandfather, George Strasser, that may or may not be true. On his first day at work in Oak Ridge, in the shiny new 9203 chemical building at the Y-12 uranium enrichment plant, George’s boss instructed him not to move a certain box past a line painted on the concrete floor.

“What will happen if I do?” George asked.

“It will go critical.”

“What’s critical?”

Critical is when there is enough unstable material to sustain a nuclear chain reaction. Critical is the point of no return.

Fission. At Y-12, barrels of uranium sent from a chemical company in Missouri were separated into their violent and merely radioactive elements. Nimble-fingered girls just out of high school operated the calutrons that whirled uranium atoms in a semi-circular arc through a magnetic field; the heavier uranium 238 atoms traveled in a slightly different path than the lighter uranium 235 atoms, and the two were deposited in separate collection spots. When the desired concentration of uranium 235 had been reached, the precious green powder, in the form of uranium tetrafluoride, was packed into gold-lined cylinders and carried in briefcases by black-suited security officers from the hills of Oak Ridge to the mesas of Los Alamos, by way of passenger trains. The weekly deliveries accumulated throughout the spring and early summer of 1945, until Los Alamos had enough to form the cool gray metal of the world’s first uranium bomb.

The uranium projectile and other components of the bomb left Los Alamos on July 14 in a black truck escorted by seven security cars. From Albuquerque, they were flown to San Francisco and loaded onto a ship, the USS Indianapolis. Altogether, the bomb’s journey from Los Alamos to the Pacific island of Tinian took 12 days. The day the ship arrived, the uranium target left Albuquerque by air, divided into three pieces on three separate planes to avoid total loss of the precious material in case of accident or attack.

Three days after the USS Indianapolis delivered its mysterious cargo to Tinian, a Japanese submarine torpedoed the ship while it was en route to Leyte Gulf in the Philippines. Due to miscommunications that may have been complicated by the secret mission, the ship was not reported missing, and 900 of the crew’s 1,196 men floated in shark-infested waters for nearly four days before they were accidentally spotted by a bomber on a routine patrol. By the time the rescue ships arrived, only 317 of the crew still survived.

There were 12 men aboard the Enola Gay, and the flight from Tinian to Hiroshima took six-and-a-half hours. At just before 3:00 a.m. Tinian time (2:00 a.m. in Hiroshima), Captain William S. Parsons descended into the bomb bay of the Enola Gay mid-flight to arm the weapon while Second Lieutenant Morris R. Jeppson held a flashlight. The procedure lasted 25 minutes. Just over four hours later, they descended into the bomb bay one last time to replace the green plugs blocking the firing signal with red ones. The bomb was declared live before the target was confirmed. “Maybe I was the last one to touch the bomb,” Jeppson would later reflect. He was 23 years old at the time of the bombing.

The uranium in the Hiroshima bomb was about 80 percent uranium 235. One metric ton of natural uranium typically contains only 7 kilograms of uranium 235. Of the 64 kilograms of uranium in the bomb, less than one kilogram underwent fission, and the entire energy of the explosion came from just over half a gram of matter that was converted to energy. That is about the weight of a butterfly.

Complicated chemistry. To achieve the necessary 80-percent uranium 235, uranium had to be fed through two sets of calutrons at Y-12: the alpha, then the beta. Ideally, no uranium would be lost in the process. In reality, though, a lot of uranium got stuck on the insides of the alpha calutrons, and the machines had to be taken apart and cleaned with acid. This acidic solution, containing uranium polluted by bits of stainless steel, was given to the chemists in the 9203 building to purify and reprocess into a form that could be fed into the beta calutrons, where a separate process was used to recover uranium.

In the fall of 1943, the 9203 lab was small—just six or eight young chemists, some with only bachelor’s degrees, a few with master’s. My grandfather was one of these chemists, I learned on a stormy day in mid-June of 2011, while sitting in the formal living room of Bill Wilcox, one of his former colleagues. Bill did not seem to notice when a particularly violent roar of thunder shook the house and a strike of lightning momentarily transformed the branches outside into bright white bones against a gray sky.

As a boy fresh out of college, Bill Wilcox was the first of the group to arrive in October 1943; he found a lab still smelling of paint and equipped with fume hoods and little else. He spent his first few weeks ordering equipment from lab catalogs. My grandfather and the others joined him about a month later, and the small group, just a few among the thousands at Y-12, set about testing chemical solutions and processes, preparing to receive the first of the uranium from the alpha calutrons.

Bill was the first person who told me what my grandfather did during the war, and the only person I’ve met who actually worked beside him, breathing the same air and handling the same tools. I hungered for all of the little details that bring the urgency of the past alive, and mark my grandfather’s place against the vast swirl of history and imagination.

Bill called their work “complicated chemistry stuff” but humored my curiosity by describing how, in their hands, a brilliant blue acidic solution became a soft yellow substance “just like New York cheesecake” that, when heated in an oven, decomposed into a “very pretty” bright orange powder. He remembered how the smell of ether that pervaded the lab for the first month or so, until they switched to a less dangerous solvent.

Despite his nearly 90 years, Bill pulled numbers and dates out of his head with the easy confidence of an expert, lining them up as a cavalry of the truth alongside his own memories, which had become fixed through many tellings. But while Bill could tell me that he arrived in Oak Ridge on October 26, 1943, he could not remember any workplace jokes exchanged in moments of irreverence or exhaustion. He could not conjure my grandfather for me. He urged me instead toward historical rigor and scientific precision.

Yet as I fill in the puzzle pieces of numbers, dates, and atomic masses, I hope to zoom in on the outline of what I do not know, to brush skin in the dark, a hairy arm, a warm laugh. My grandfather’s job was necessary because, in practice, the numbers did not always come out right. His task lay in the tension between people and science. In a world that worked by the numbers, my grandfather would not have had a job.

The beginning. Bill Wilcox showed me two basketballs. One of them had five pennies glued to the outside and the other had nothing. “This weight,” he told me, “is the only difference between uranium 238 and uranium 235.” Bill carries these basketballs to local schools and explains the basics of nuclear science to bored fourth graders. What do they see in the apparent innocence of five pennies?

We can go back and back in search of a beginning. The Earth’s uranium was formed in the deaths of stars more than 6 billion years ago. It has been there, buried deep in the earth, scattered through our hair and fingernails even, just waiting. Five pennies, three neutrons, 43 seconds, the weight of a butterfly, a few men in a lab putting uranium through its paces—when did it all become critical?

What happened in the end was not the product of a single decisive moment or a brilliant equation, but a slow accumulation of many hands, tedious hours, mistakes, cleanup, decisions made and not made. It is the neutrons, those bland and chargeless particles, that make the difference. Most people in Oak Ridge knew very little of the project that was the purpose of their city; they pulled the lever, cleaned the pipe, stuck a cake of uranium in the oven, and went home to feed their children. At the height of production, there were 75,000 people living, working, and attending school in the hilly corner of East Tennessee that would claim responsibility for a butterfly’s weight of uranium.

I never had the chance to ask my grandfather how much he knew, and whether he regretted his part.

Bill Wilcox was honest and unapologetic: “Hell yes, I knew exactly what I was doing—damn right. I knew what my job was … I didn’t know I was building a bomb but I knew I was building some kind of military weapon for sure and hoped like hell it was gonna do something to shorten the darn war.”

By the end of 1944, the chemical operations had outgrown the 9203 lab, and Bill and my grandfather parted ways. Bill, who excelled in the technical areas, moved to a small experimental trouble-shooting laboratory. George, recognized as a natural leader, was put in charge of about 100 people in the new 9206 chemistry building. My grandfather, I’m told, was better with people than he was with science. Still—I suspect that he knew what critical meant after all.