Oppenheimer is unusual among films about physics in that the physics is mostly correct and mostly load-bearing. It also compresses, reorders, and occasionally invents. The compressions are more interesting than the inventions, because they show which parts of the science are hard to dramatize — and those tend to be the parts that mattered most.

The Problem the Film Is Actually About

The film frames the Manhattan Project as a race, which it was, but the technical problem is not the one most people take away. Building a fission weapon is not conceptually difficult. Fission was demonstrated in 1938, and the basic idea — assemble enough fissile material fast enough for a chain reaction to run before the assembly blows itself apart — was clear by 1940.

Nearly all the difficulty was in materials. Natural uranium is 99.3% U-238, which does not sustain a fast chain reaction, and 0.7% U-235, which does. Separating two isotopes that differ by 1.3% in mass and not at all in chemistry is a problem in physical engineering, and it consumed most of the project's money and workforce. Oak Ridge is where the war effort actually lived. The film gives it a few shots.

What Fission Actually Requires

A U-235 nucleus absorbing a neutron splits into two fragments and releases two or three additional neutrons. If more than one of those goes on to cause another fission, the reaction grows exponentially. The generation time is on the order of 10 nanoseconds, so a full chain runs to completion in under a microsecond.

The constraint is geometry. Neutrons that reach the surface before hitting a nucleus escape and are lost. Fission scales with volume, escape scales with surface area, so there is a size below which the reaction cannot sustain itself — the critical mass. Roughly 52 kg for a bare sphere of U-235, less with a neutron-reflecting tamper.

This gives the design its central tension. Subcritical is safe and useless; supercritical detonates. The weapon has to move from one state to the other faster than the released energy can push the material apart. Fail, and you get a fizzle: a small explosion that scatters the fissile material before meaningful yield.

Two Designs, and Why the Film Only Explains One

The uranium weapon used a gun assembly — fire one subcritical slug into another down a barrel. Crude, slow by weapon standards, and reliable enough that it was never tested before Hiroshima.

Plutonium, which the reactors at Hanford produced far more cheaply than U-235 could be separated, would not work this way. Reactor-bred plutonium contains Pu-240, which fissions spontaneously at a high rate. In a gun assembly, taking milliseconds, a stray neutron will almost certainly initiate the reaction while the pieces are still approaching — predetonation, and a fizzle.

The solution was implosion: surround a subcritical plutonium sphere with shaped explosives and compress it to a higher density, where the same mass becomes supercritical. Assembly time drops to microseconds. But the compression has to be nearly perfectly spherical. Detonation waves from separate charges must arrive simultaneously and converge inward without turbulence, or the core squirts out sideways.

This required explosive lenses — shaped charges of fast and slow explosive arranged to convert a diverging spherical wave into a converging one, which is optics done with detonation fronts. Neddermeyer proposed implosion, von Neumann worked the mathematics, Kistiakowsky built the lenses. The film compresses this into a montage. It is the single hardest technical problem the project solved, and the reason Trinity was necessary at all: they tested plutonium because they were not confident it would work.

The Atmospheric Ignition Question

The film treats this well, and it is worth being precise about what it was.

Teller raised the possibility in 1942 that a fission explosion might ignite fusion in atmospheric nitrogen, propagating a self-sustaining reaction. Bethe worked the problem and showed it could not happen — radiative cooling removes energy from the reaction zone faster than nitrogen fusion can supply it, so there is no runaway. The margin is large, not marginal.

The film keeps the calculation as an unresolved anxiety through Trinity, with Oppenheimer offering "near zero." This is dramatic license, but a defensible kind. The physics was settled well before the test; what the film preserves is that a probability derived from theory is not the same as a probability you feel, and the distinction was real to the people involved.

What the Film Gets Wrong

The chronology is deliberately scrambled — Oppenheimer's conversations with Bohr, Heisenberg's German program, and the Los Alamos timeline are reordered for structure. Most of this is standard biographical compression.

The more substantive distortion is the shape of discovery. The film has physics happen in conversations: two people at a chalkboard, an insight, a cut. Real theoretical work at Los Alamos meant enormous quantities of hand computation, done largely by the Theoretical Division's computing group and by human computers — many of them women, including the wives of staff scientists — running desk calculators and later IBM punch card machines through hydrodynamic simulations that took weeks.

Feynman's actual documented role was running that computing operation and dramatically improving its throughput. The film uses him mostly as a bongo player.

Some Thoughts

The thing the film cannot dramatize, and does not really try to, is that the Manhattan Project was not primarily a physics achievement. The physics was largely complete by 1942. What followed was three years of metallurgy, chemical engineering, isotope separation, explosive hydrodynamics, and industrial construction at a scale that consumed about 0.4% of American GDP.

This is a general problem with how discovery gets narrated. The insight is legible and compressible; the implementation is neither, and the implementation is usually where the years go and where most of the people are. A film about Oak Ridge's gaseous diffusion barriers would be more representative and unwatchable.

What the film does capture, and what is genuinely hard to convey, is the position of a theorist whose work becomes an object. Oppenheimer spent his career in a field where the output was papers, and then spent three years in one where the output was a device that would be used on cities. The Trinity sequence handles this by removing the sound, which is a better choice than any dialogue would have been — the shockwave arrives late, and for a few seconds the film is a physics demonstration and a moral one simultaneously.