The Manhattan Project

The Letter

On August 2, 1939, Albert Einstein signed a letter to President Franklin D. Roosevelt. The letter, drafted by physicist Leo Szilard, warned that recent advances in nuclear physics made it possible to build a weapon of unprecedented destructive power — and that Nazi Germany might already be working on one.

"A single bomb of this type," the letter said, "carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory."

Einstein later called signing this letter "the one great mistake" of his life. But in 1939, the threat seemed real. German physicists had discovered nuclear fission just months earlier. The race was on.

Fission

In December 1938, German chemists Otto Hahn and Fritz Strassmann had bombarded uranium atoms with neutrons and found something impossible: barium. Barium has an atomic number of 56. Uranium has an atomic number of 92. Where had the barium come from?

The answer, worked out by physicists Lise Meitner and Otto Frisch over Christmas 1938, was that the uranium nucleus had split in half. A neutron had struck the uranium-235 nucleus, made it unstable, and it had divided into two smaller nuclei — barium and krypton — releasing a burst of energy and two or three additional neutrons.

This was nuclear fission.

The energy released was enormous. When a uranium-235 nucleus fissions, it converts about 0.1% of its mass into energy, according to Einstein's equation E = mc². That 0.1% doesn't sound like much, but because c² is such an enormous number (the speed of light squared: 9 × 10¹⁶ m²/s²), the energy from fissioning a single kilogram of uranium-235 equals the energy from burning 3,000 tonnes of coal.

But fission alone isn't a weapon. A single fission event releases energy equivalent to about 200 million electron volts — powerful at the atomic scale, but invisible to the human eye. What makes a bomb possible is the chain reaction.

The Chain Reaction

When a uranium-235 nucleus fissions, it releases 2-3 neutrons. Each of those neutrons can strike another uranium-235 nucleus, causing it to fission and release 2-3 more neutrons. Each of those neutrons can cause another fission. And so on.

In mathematical terms, this is exponential growth: 1 → 3 → 9 → 27 → 81 → 243... After just 80 generations of fission (which takes about one microsecond in a bomb), a single neutron has multiplied into 6 × 10²³ fission events — roughly Avogadro's number, about one kilogram of uranium fissioned.

But there's a catch. For a chain reaction to sustain itself, each fission must produce, on average, at least one neutron that goes on to cause another fission. Many neutrons escape from the surface of the material without hitting another nucleus. Many are absorbed by uranium-238 (which doesn't fission) or by impurities.

The critical mass — the minimum amount of fissile material needed for a self-sustaining chain reaction — depends on geometry and purity. A sphere has the smallest surface-area-to-volume ratio of any shape, which means fewer neutrons escape. For a sphere of pure uranium-235, the critical mass is about 52 kilograms — roughly the size of a grapefruit.

The Secret City

In 1942, the US Army Corps of Engineers established the Manhattan Engineer District — the bureaucratic name for what became known as the Manhattan Project. The military director was General Leslie Groves. The scientific director was J. Robert Oppenheimer, a theoretical physicist from the University of California, Berkeley.

Oppenheimer chose a remote boys' school on a mesa in New Mexico — Los Alamos — as the site for the weapons laboratory. Over the next two years, he assembled the greatest concentration of scientific talent in history: Enrico Fermi (who had built the world's first nuclear reactor under a squash court at the University of Chicago), Richard Feynman (22 years old, a genius at computation), Hans Bethe (who had figured out how stars generate energy), Niels Bohr (the father of quantum mechanics), and dozens more.

They faced two problems. First: getting enough fissile material. Natural uranium is 99.3% uranium-238 (which doesn't fission easily) and only 0.7% uranium-235 (which does). Separating these two isotopes — chemically identical, differing only by three neutrons — required industrial processes on a staggering scale. The enrichment plants at Oak Ridge, Tennessee employed 75,000 workers and consumed more electricity than New York City.

Second: making it explode. Simply bringing two pieces of uranium-235 together wouldn't work — the chain reaction would start before the pieces were fully assembled, producing a "fizzle" rather than an explosion. The solution for the uranium bomb ("Little Boy") was a gun-type design: one piece of uranium was fired down a gun barrel into another piece at high speed, assembling the critical mass in a fraction of a millisecond.

For the plutonium bomb ("Fat Man"), the problem was harder. Plutonium's fission rate is so high that even a gun-type assembly would fizzle. The solution was implosion: a sphere of plutonium was surrounded by precisely shaped explosive charges that, when detonated simultaneously, compressed the plutonium from all sides, increasing its density and reducing the critical mass. The explosive charges had to detonate within one microsecond of each other — a precision that required months of testing and the invention of new detonator technology.

Trinity

On July 16, 1945, at 5:29 AM, the world's first nuclear device — a plutonium implosion bomb called "The Gadget" — was detonated at the Trinity test site in the Jornada del Muerto desert, New Mexico.

The flash was visible 290 kilometres away. The mushroom cloud rose to 12 kilometres. The steel tower that held the device was vaporized — not melted, not destroyed, but converted to gas. The sand beneath the tower was fused into a glassy substance called trinitite — green, mildly radioactive, and found nowhere else on Earth.

Oppenheimer, watching from a bunker ten kilometres away, recalled a line from the Hindu scripture, the Bhagavad Gita: "Now I am become Death, the destroyer of worlds."

Test director Kenneth Bainbridge turned to Oppenheimer and said something less poetic but equally true: "Now we are all sons of bitches."

The Aftermath

Three weeks later, on August 6, 1945, a uranium bomb was dropped on Hiroshima. Three days after that, a plutonium bomb was dropped on Nagasaki. Together, they killed approximately 200,000 people — most of them civilians.

Many of the scientists who built the bomb had signed a petition — organized by Leo Szilard — urging the government to demonstrate the weapon to Japan rather than use it on a city. The petition never reached President Truman.

After the war, many Manhattan Project scientists became vocal advocates for nuclear arms control. Einstein, Szilard, and others founded the Bulletin of the Atomic Scientists, which established the Doomsday Clock — a symbolic measure of how close humanity stands to self-destruction. In 1947, it was set at 7 minutes to midnight. As of 2024, it stands at 90 seconds — the closest it has ever been.

The Manhattan Project proved that physics is not neutral. The same equation — E = mc² — that explains how stars shine also explains how cities burn. The science doesn't choose sides. The people who wield it do.

The end.