II.

Origins of the Manhattan Project

The origins of the Manhattan Project can be traced to the scientific laboratories of Britain and Europe in the early 1900s. At that time, the basic unit of matter, the atom, was viewed as solid and impossible to divide. The startling discoveries of radium; the X ray; the electron, proton, and neutron; and alpha, beta, and gamma rays, however, alerted scientists to the existence of a “subatomic” world. As British physicist Ernest Rutherford and Danish physicist Niels Bohr suggested, instead of being solid, the atom resembled a “miniature solar system.” Within the atom negatively charged electrons orbited positively charged protons and electrically neutral neutrons in the atom’s nucleus.

Scientists knew well that the atoms of each chemical element differed from one another. Hydrogen, which consists of one electron orbiting one proton, is the simplest. Scientists classify hydrogen with the atomic number one. Uranium, with 92 electrons orbiting a nucleus with 92 protons, is the most complex of the natural elements. It has the atomic number 92. In addition, these elements often contain variations—called isotopes—that occur because they have different numbers of neutrons bound to the protons in the nucleus. For example, the element uranium has three isotopes, known by their atomic numbers, U²³⁸, U²³⁵, and U²³⁴. The numbers are derived by adding the number of protons in the uranium nucleus, 92, with the number of neutrons in the nucleus. U²³⁸ is the most common form of uranium; the rare U²³⁵ isotope forms only about 0.7 percent of naturally occurring uranium. Because uranium appeared to be an unstable element, scientists began to bombard it with streams of neutrons, hoping to discover a new form of energy.

The 1930s saw major breakthroughs in understanding the atom. In 1933 Hungarian-born physicist Leo Szilard, who had fled Nazi Germany for England, was standing on a London street corner waiting for the light to change. Suddenly he realized that if the right material were found, splitting the nucleus of an atom could release neutrons and cause a nuclear “chain reaction” in which the released neutrons would cause more atoms to split, or fission. The result would be a self-sustaining series of fissions, causing a continuous release of nuclear energy. Such a chain reaction could be used to produce either electricity or a bomb. The next year Szilard filed a British patent on this subject, but kept it secret out of fear that German scientists might learn it was possible to make an atomic bomb.

Meanwhile, in 1933 in Paris, French scientists Irène Joliot-Curie and her husband Frédéric Joliot discovered artificially created radioactivity. Shortly afterwards in Rome, Italian physicist Enrico Fermi created the first artificially created elements (beyond uranium). Fermi also split the atom, but at the time he did not realize what had occurred.

In Berlin, Germany, physical chemists Otto Hahn and Fritz Strassmann repeated Fermi’s experiments, bombarding uranium with neutrons. In late 1938 they were baffled when they found traces of barium in their results. Hahn wrote to his longtime scientific partner, Lise Meitner, to ask her opinion. Meitner was probably the foremost woman scientist of her generation. She had been forced to flee Germany due to the anti-Semitic laws enacted by the Nazi regime of Adolf Hitler (see National Socialism). Meitner and her nephew, physicist Otto Frisch, concluded in a December 1938 discussion that the two German scientists had split the uranium atom’s nucleus virtually in half. As a result of splitting the uranium atom, barium with the atomic number 56 and krypton with the atomic number 36 were formed. Added together they represented the 92 protons in the uranium atom’s nucleus. Frisch was the first to name this process “fission.” (See the Sidebar with this article, “The Discovery of Fission.”)

Meitner and Frisch provided a theoretical explanation for Hahn and Strassmann’s results and argued that the experiments confirmed Bohr’s model of the atom. When the uranium atom split or fissioned, it released an enormous amount of energy. How much energy could be calculated by using the famous formula of Austrian-born physicist Albert Einstein, E=mc². In this formula E is energy, m is mass, and c is the speed of light squared. Since the speed of light—300,000 km/sec (186,000 mi/sec)—is such a large number, very little mass is required to produce a great deal of energy. Moreover, if each fission released additional neutrons in the process, a nuclear chain reaction would be possible.

Meitner and Frisch raced to Copenhagen, Denmark, to inform Bohr, who was preparing to leave for a physicists’ conference in Washington, D.C., in January 1939. As soon as the scientists in Washington learned that uranium could be fissioned, several rushed to their laboratories to repeat the experiment. Within a year, more than 100 scientific papers had appeared on nuclear fission. When Szilard first heard the news of uranium fission, he predicted that the world was “headed for grief.” By 1939 a small group of scientists was well aware that a weapon of terrible power was possible—at least in theory.