The Making of the Atomic Bomb Summary and key ideas

The Making of the Atomic Bomb, by Richard Rhodes.

Atomic energy became thinkable only when the atom ceased to be a philosophical possibility and became an object that experiments could interrogate. Ancient atomism imagined invisible particles and void. Newton made atoms hard, permanent units in a mechanical universe, while Maxwell added electromagnetic fields without abandoning mechanical particles. Yet by the late nineteenth century, atoms remained contested. Chemistry and spectral lines could be explained with them, but their reality, structure, and divisibility were unsettled. The question was no longer simply whether atoms existed, but what kind of atom could account for the evidence.

Experiments made matter stranger. Thomson showed that cathode rays were electrons, common particles far lighter than hydrogen. Röntgen’s X rays and Becquerel’s uranium radiation revealed that matter could emit penetrating energy without light or other stimulation. Rutherford and Soddy then showed that radioactive elements could change into other elements, with decay measured by characteristic half-lives. Radioactive change released vastly more energy per mass than chemical reactions. This revealed an enormous reserve inside matter, though not yet a way to control or multiply it.

Rutherford supplied the structural breakthrough. He used fast, massive alpha particles as probes of the atom. In Geiger and Marsden’s gold-foil experiments, most passed through, as a diffuse charge would predict. A few scattered through very large angles, and some returned toward the source. Such rebounds could not come from spread-out positive charge. They required nearly all the atom’s mass and positive charge to be concentrated in a minute nucleus, with the rest mostly empty space. The nuclear atom explained the scattering and located the atom’s concentrated energy at its center.

It also created a new problem. Classical mechanics said that orbiting electrons should radiate energy continuously and spiral into the nucleus. Repelling electrons in a ring would destabilize one another as well. Yet ordinary matter was stable, and Rutherford’s atom matched the experimental evidence. The experiment and the theory had parted company. A workable atom required a break with classical continuity.

Niels Bohr made that break in 1913. Planck had proposed that radiation came in discrete amounts, and Einstein had extended the idea by treating light as energy quanta. Bohr applied this discontinuity to Rutherford’s atom. Electrons, he proposed, could occupy distinct stationary states in which they did not radiate. They could move between states only in jumps, emitting or absorbing a photon whose energy matched the difference between the states. There was no calculable continuous path between them.

This gave physical meaning to the sharp lines seen in atomic spectra. Balmer’s and Rydberg’s formulas were no longer merely mathematical patterns; they could be understood as signatures of electron transitions, and Bohr’s calculations closely matched measured values. His postulates initially looked arbitrary, and they did not answer every causal question. But they made the atom stable while preserving the evidence. Classical mechanics remained useful at larger scales, yet it could no longer describe atomic processes completely. At the atomic level, matter and change were both discontinuous.

That new picture supplied the logic Leo Szilard needed. In 1933, after reading a newspaper account of Rutherford dismissing useful atomic energy as moonshine, Szilard asked whether a nuclear process might produce more energy than its initiating particle supplied. The relevant particle was the neutron: because it carried no electric charge, it could approach a nucleus without first being repelled by the electrical barrier. Borrowing an analogy from chemical chain reactions, Szilard imagined an element that absorbed a neutron, split, and released two more neutrons. If enough material could be assembled, each generation might produce the next, turning one initiating event into a multiplying process. He immediately saw two consequences: energy on an industrial scale and the possibility of an atomic bomb.

But the idea remained conditional. Szilard had not identified the element, specified the experiment, or shown that the multiplication could occur. The bomb was not yet an engineering project or an inevitable result of atomic science. Rutherford had revealed the nucleus, Bohr had replaced continuous atomic mechanics with quantum discontinuity, and Szilard had supplied a hypothesis for neutron multiplication. Together, those pieces made immense nuclear energy conceivable. Whether nature provided the required material and behavior belonged to the experimental work still ahead.