The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

Just then, in 1932, Szilard found or took up for the first time that appealing orphan among H. G. Wells’ books that he had failed to discover before: The World Set Free.63 Despite its title, it was not a tract like The Open Conspiracy. It was a prophetic novel, published in 1914, before the beginning of the Great War. Thirty years later Szilard could still summarize The World Set Free in accurate detail. Wells describes, he says: . . . the liberation of atomic energy on a large scale for industrial purposes, the development of atomic bombs, and a world war which was apparently fought by an alliance of England, France, and perhaps including America, against Germany and Austria, the powers located in the central part of Europe. He places this war in the year 1956, and in this war the major cities of the world are all destroyed by atomic bombs.

“I took a train from Berlin to Vienna on a certain date, close to the first of April, 1933,” Szilard writes. “The train was empty. The same train the next day was overcrowded, was stopped at the frontier, the people had to get out, and everybody was interrogated by the Nazis.73 This just goes to show that if you want to succeed in this world you don’t have to be much cleverer than other people, you just have to be one day earlier.”

Soddy and Rutherford had observed the spontaneous disintegration of the radioactive elements, one of the major discoveries of twentieth-century physics.142 They set about tracing the way uranium, radium and thorium changed their elemental nature by radiating away part of their substance as alpha and beta particles.

They discovered that each different radioactive product possessed a characteristic “half-life,” the time required for its radiation to reduce to half its previously measured intensity. The half-life measured the transmutation of half the atoms in an element into atoms of another element or of a physically variant form of the same element—an “isotope,” as Soddy later named it.143

Rutherford’s and Soddy’s discussions of radioactive change therefore inspired the science fiction novel that eventually started Leo Szilard thinking about chain reactions and atomic bombs.

“If the high velocity and mass of the α-particle be taken into account,” they concluded, “it seems surprising that some of the α-particles, as the experiment shows, can be turned within a layer of 6 × 10−5 [i.e., .00006] cm. of gold through an angle of 90°, and even more. To produce a similar effect by magnetic field, the enormous field of 109 absolute units would be required.”

Rutherford had found the nucleus of his atom. He did not yet have an arrangement for its electrons. At the Manchester meeting he spoke of “a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity equal in amount.” That was sufficiently idealized for calculation, but it neglected the significant physical fact that the “opposite electricity” must be embodied in electrons.183 Somehow they would have to be arranged around the nucleus.

Bohr’s contributions to twentieth-century physics would rank second only to Einstein’s.

Walking with him when he was about three years old, his father began pointing out the balanced structure of a tree—the trunk, the limbs, the branches, the twigs—assembling the tree for his son from its parts. The literal child saw the wholeness of the organism and dissented: if it wasn’t like that, he said, it wouldn’t be a tree. Bohr told that story all his life, the last time only days before he died, seventy-eight years old, in 1962.

Kierkegaard to have found: “His leading idea was that the different possible conceptions of life are so sharply opposed to one another that we must make a choice between them, hence his catchword either-or; moreover, it must be a choice which each particular person must make for himself, hence his second catchword, the individual.”

He realized in the space of a few weeks that radioactive properties originated in the atomic nucleus but chemical properties depended primarily on the number and distribution of electrons. He realized—the idea was wild but happened to be true—that since the electrons determined the chemistry and the total positive charge of the nucleus determined the number of electrons, an element’s position on the periodic table of the elements was exactly the nuclear charge (or “atomic number”): hydrogen first with a nuclear charge of 1, then helium with a nuclear charge of 2 and so on up to uranium at 92.

“On the constitution of atoms and molecules,” so proudly and bravely titled—Part I mailed to Rutherford on March 6, 1913, Parts II and III finished and published before the end of the year—would change the course of twentieth-century physics. Bohr won the 1922 Nobel Prize in Physics for the work.

work. But spectroscopy was far from Bohr’s field and he presumably had forgotten them. He sought out his old friend and classmate, Hans Hansen, a physicist and student of spectroscopy just returned from Göttingen. Hansen reviewed the regularity of line spectra with him. Bohr looked up the numbers. “As soon as I saw Balmer’s formula,” he said afterward, “the whole thing was immediately clear to me.”

Then, sensationally, with the simple formula (where m is the

Around the nucleus, Bohr proposed, atoms are built up of successive orbital shells of electrons—imagine a set of nested spheres—each shell capable of accommodating up to a certain number of electrons and no more. Elements that are similar chemically are similar because they have identical numbers of electrons in their outermost shells, available there for chemical combination. Barium, for example, an alkaline earth, the fifty-sixth element in the periodic table,

Whether a love affair or love, Oppenheimer found his vocation in Cambridge that year: that was the certain healing. Science saved him from emotional disaster as science was saving Teller from social disaster.

His work was no longer apprenticeship but solid achievement. His special contribution, appropriate to the sweep of his mind, was to extend quantum theory beyond its narrow initial ground. His dissertation, “On the quantum theory of continuous spectra,” was published in German in the prestigious Zeitschrift für Physik. Born marked it “with distinction”—high praise indeed.

Bohr and Heisenberg then went to work on the problem of reconciling the dualisms of atomic theory. Bohr

constant. So h appeared again at the heart of physics to define the basic, unresolvable granularity of the universe. What Heisenberg conceived that night came to be called the uncertainty principle, and it meant the end of strict determinism in physics: because if atomic events are inherently blurred, if it is impossible

The problem, Bohr said, was that quantum conditions ruled on the atomic scale but our instruments for measuring those conditions—our senses, ultimately—worked in classical ways. That inadequacy imposed necessary limitations on what we could know.

Newspapers soon published the discovery in plainer words: Sir Ernest Rutherford, headlines blared in 1919, had split the atom.

By the time Aston returned to Cambridge in 1918 he had worked the problem out theoretically; he then began building the precision instrument he had envisioned.514 It charged a gas or a coating

Aston called his invention a mass-spectrograph because it sorted elements and isotopes of elements by mass much as an optical spectrograph sorts light by its frequency. The mass-spectrograph was immediately and sensationally a success. “In letters to me in January and February, 1920,” says Bohr, “Rutherford expressed his joy in Aston’s work,” which “gave such a convincing confirmation of Rutherford’s atomic

Oppenheimer looked into the subtleties of the invisible cosmic margins, modeled the imploding collapse of dying suns and described theoretical stellar objects that would not be discovered for thirty and forty years—neutron stars, black holes—because the instruments required to detect them, radio telescopes and X-ray satellites, had not been invented yet.550 (Alvarez believes if Oppenheimer had lived long enough to see these developments he would have won a Nobel Prize for his work.) That was originality not so much ahead of its time as outside the frame.

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.”603 The difference, he said, was the discovery of the neutron.

That prepared him for that cool, humid, dull day in September when he would step off the Southampton Row curb and begin to shape the things to come.

The Joliot-Curies reported their work—“one of the most important discoveries of the century,” Emilio Segrè says in his history of modern physics—in the Comptes Rendus on January 15, 1934, and in a letter to Nature dated four days later.748 “These experiments give the first chemical proof of artificial transmutation,” they concluded proudly.

quickly nicknamed Fermi “the Pope” for his quantum infallibility; Corbino,

In the meantime, two months after the Solvay Conference, Fermi completed the major theoretical work of his life, a fundamental paper on beta decay. Beta decay, the creation and expulsion by the nucleus of highenergy electrons in the course

That Szilard saw beyond “energy for industrial purposes” to the possibility of weapons of war is evident in his next patent amendments, dated June 28 and July 4, 1934.

Surface tension and deforming forces work against each other in complex ways; the molecules of the liquid bump and collide; the drop wobbles and distorts. Eventually the added energy dissipates as heat, and the drop steadies again. The nucleus, Bohr proposed, was similar. The force that stuck the nucleons together was the nuclear strong force.

856 The liquid-drop model proved useful, however, and Frisch in Copenhagen and Meitner in Berlin, among others, took it to heart.

Lise Meitner and Hans Geiger. James Chadwick was “quite convinced that she would have discovered the neutron if it had been firmly in her mind, if she had had the advantage of, say, living in the Cavendish for years, as I had done.”874 “Slight in figure and shy by nature,” as her nephew Otto Frisch describes her, she was nevertheless formidable.

Fermi took the Stockholm call. The Nobel Prize, undivided, would be awarded for “your discovery of new radioactive substances belonging to the entire race of elements and for the discovery you made in the course of this work of the selective power of slow neutrons.”950 In security the Fermis could leave the madness behind.

They had discovered the reason no elements beyond uranium exist naturally in the world: the two forces working against each other in the nucleus eventually cancel each other out.

They pictured the uranium nucleus as a liquid drop gone wobbly with the looseness of its confinement and imagined it hit by even a barely energetic slow neutron. The neutron would add its energy to the whole. The nucleus would oscillate.

“Lise Meitner

Meitner or Frisch calculated that energy to be about 200 MeV: 200 million electron volts.

Frisch remembers him scoffing; to think that it could be made to burst as well “was like dissecting a man killed by a falling brick and finding that he would have died of

Later that day Frisch looked me up and said, “You work in a microbiology lab. What do you call the process in which one bacterium divides into two?” And I answered, “binary fission.” He wanted to know if you could call it “fission” alone, and I said you could.

However energetically interesting a reaction, fission by itself was merely a laboratory curiosity. Only if it released secondary neutrons, and those in sufficient quantity to initiate and sustain a chain

Francis Aston had found only U238 when he first passed uranium through his mass spectrograph at the Cavendish. In 1935, using a more powerful instrument, physicist Arthur Jeffrey Dempster of the University

“I have often been asked,” Otto Frisch wrote many years afterward of the moment when he understood that a bomb might be possible after all, before he and Peierls carried the news to Mark Oliphant, “why I didn’t abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer was very simple. We

Not until 1942 would they officially propose a name for the new element that fissioned like U235 but could be chemically separated from uranium. But Seaborg already knew what he would call it. Consistent with Martin Klaproth’s inspiration in 1789 to link his discovery of a new element with the recent discovery of the planet Uranus and with McMillan’s suggestion to extend the scheme to Neptune, Seaborg would name element 94 for Pluto, the ninth planet outward from the sun, discovered in 1930 and named for the Greek god of the underworld, a god of the earth’s fertility but also the god of the dead: plutonium.

light. Arnold Sommerfeld hailed the Compton effect—elastic scattering of a photon by an electron—as “probably the most important discovery which could have been made in the current state of physics” because it proved that photons exist, which hardly anyone in 1923 yet believed, and demonstrated clearly the dual

“The next morning,” Leona Woods remembers—the beginning of the fateful day, December 2, 1942—“it was terribly cold—below zero.

“Jim,” I said, “you’ll be interested to know that the Italian navigator has just landed in the new world.” Then, half apologetically, because I had led the S-1 Committee to believe that it would be another week or more before the pile could be completed, I added, “the earth was not as large as he had estimated, and he arrived at the new world sooner than he had expected.”1706 “Is that so,” was Conant’s excited response. “Were the natives friendly?” “Everyone landed safe and happy.”

No essence was ever expressed more expensively from the substance of the world with the possible exception of the human soul.

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