The Making of the Atomic Bomb

tradition on their necks.

That Wednesday Kapitza wined and dined the exhausted Chadwick into what Mark Oliphant calls “a very mellow mood,” then brought him along to a Kapitza Club meeting. “The intense excitement of all in the Cavendish, including Rutherford,” Oliphant remembers, “was already remarkable, for we had heard rumors of Chadwick's results.” Oliphant says Chadwick spoke lucidly and with conviction, not failing to mention the contributions of Bothe, Becker, Webster and the Joliot-Curies, “a lesson to us all.” C. P. Snow, who was also present, remembers the performance as “one of the shortest accounts ever made about a major discovery.” When tall and birdlike Chadwick finished speaking he looked over the assembly and announced abruptly, “Now I want to be chloroformed and put to bed for a fortnight.”

He deserved his rest. He had discovered a new elementary particle, the third basic constituent of matter. It was this neutral mass that compounded the weight of the elements without adding electrical charge. Two protons and 2 neutrons made a helium nucleus; 7 protons and 7 neutrons a nitrogen; 47 protons and 60 neutrons a silver; 56 protons and 81 neutrons a barium; 92 protons and 146 (or 143) neutrons a uranium.

And because the neutron was as massive as a proton but carried no electrical charge, it was hardly affected by the shell of electrons around a nucleus; nor did the electrical barrier of the nucleus itself block its way. It would therefore serve as a new nuclear probe of surpassing power of penetration. “A beam of thermal neutrons,” writes the American theoretical physicist Philip Morrison, “moving at about the speed of sound, which corresponds to a kinetic energy of only about a fortieth of an electron volt, produces nuclear reactions in many materials much more easily than a beam of protons of millions of volts energy, traveling thousands of times faster.” Ernest Lawrence's cyclotron, spiraling protons to million-volt energies for the first time the same month that Chadwick made his fateful discovery, fortunately proved to be adaptable to the production of neutrons. More than any other development, Chadwick's neutron made practical the detailed examination of the nucleus. Hans Bethe once remarked that he considered everything before 1932 “the prehistory of nuclear physics, and from 1932 on the history of nuclear physics.” The difference, he said, was the discovery of the neutron.

Word of the discovery reached Copenhagen in the midst of preparations for an amateur theatrical, a parody of Goethe's Faust, to celebrate the tenth anniversary of the opening of Bohr's Institute for Theoretical Physics. The postdoctoral dramatists gave the new particle the last word. They had cast Wolfgang Pauli, a corpulent man with a smooth, round face and protuberant, heavy-lidded eyes who resembled the actor Peter Loire, as Mephi-stopheles, Bohr as The Lord. Eclectically they cast Chadwick in absentia as Wagner and an anonymous illustrator drew him into the script, “the personification of the ideal experimentalist” according to the stage directions, balancing a vastly magnified neutron on his finger:

In Copenhagen, as before in Cambridge, Chadwick reports his discovery briefly and succinctly:

The Neutron has come to be.
Loaded with Mass is he.
Of Charge, forever free.
Pauli, do you agree?

Pauli steps forward to dispense his Mephistophelean blessing:

That which experiment has found —
Though theory has no part in —
Is always reckoned more than sound
To put your mind and heart in…

And a chorus of clowning, friendly physicists, Bohr's brilliant young crew, dances out to sing a finale and bring the curtain down:

Now a reality,
Once but a vision.
What classicality,
Grace and precision!
Hailed with cordiality,
Honored in song,
Eternal Neutrality
Pulls us along!

It was the last peaceful time many of them would know for years to come.

7 Exodus

“Antisemitism is strong here and political reaction is violent,” Albert Einstein wrote Paul Ehrenfest from Berlin in December 1919. The letter coincides with Einstein's discovery by the popular press, the beginning of his years of international celebrity. “A new figure in world history,” the Berliner Illustrirte Zeitung described him under a cover photograph on December 14, “…whose investigations signify a complete revision of our concepts of nature, and are on a par with the insights of a Copernicus, a Kepler, a Newton.” Immediately the anti-Semites and fascists set to work on him.

Einstein was already, at forty-three, respected in the first rank of theoretical physicists. He had been nominated for the Nobel Prize in all but two years since 1910, the secondings increasing in number after 1917; Max Planck, who was not given to exaggeration, wrote the Nobel Committee in 1919 that Einstein “made the first step beyond Newton.” The award might have come sooner than in 1922 (belatedly for 1921: the 1922 prize was Bohr's) had relativity been less paradoxical a revelation.

Physically Einstein was not yet the amused, grandfatherly notable of his later American years. His mustache was still dark and his thick black hair had only begun to gray. C. P. Snow would observe “a massive body, very heavily muscled.” The Swabian-born physicist's friends thought his loud laugh boyish; his enemies thought it rude. “A powerful sensuality,” Snow suspected, suspecting also that Einstein took his sensuality to be “one of the chains of personality that ought to be slipped off.” Nor had he yet learned, in the psychoanalyst Erik Erikson's words, “to look into cameras as if he were meeting the eyes of the future beholders of his image.” In the past year Einstein had endured a stomach ulcer, jaundice and a painful divorce; he had lost and partly regained fifty-six pounds; his mother was dying of cancer: fatigue stained his expressive face. Leopold Infeld, a young Polish physicist who knocked at his door in postwar Berlin seeking a letter of recommendation, found him “dressed in a morning coat and striped trousers with one important button missing.” Infeld knew Einstein's face from magazines and newsreels. “But no picture could reproduce the shining glow of his eyes.” They were large and dark brown, and the diffident young visitor was one of many — Leo Szilard was another — who found comfort in those cold days in their honest warmth.

The immediate occasion for world notice was an eclipse of the sun. Einstein had presented a paper to the Prussian Academy of Sciences in Berlin on November 25, 1915, “The field equations of gravitation,” in which, he