The nucleus’s makeup had been a puzzle ever since that surprising discovery. It was known that an oxygen atom, for example, had eight electrons surrounding a nucleus containing eight protons, but the atom’s mass seemed to indicate the presence of 16 protons–twice the expected number. It was commonly believed that nuclei contained additional protons tightly bound to the very much lighter electrons, thus neutralizing their charges. But this didn’t seem to make much sense: how was it possible that electrons were sometimes inside the nucleus, if ordinarily they resided well outside it? An alternative explanation, long suspected by Chadwick and Rutherford, was the existence of a particle with a mass very close to the proton’s but with no electric charge. As expected on the basis of mass estimates, the oxygen nucleus contained eight of these newly named neutrons, alongside the eight protons.
Chadwick’s discovery beat out the cross-Channel competition of Madame Curie’s daughter Irène, who had formed a formidable research duo with her husband, Frédéric Joliot. Irène and Frédéric had the Nobel Prize in physics within their grasp twice, having achieved first sightings of both the positron (the electron’s antiparticle) and the neutron. Each time, they misidentified their observation and saw the prize go to others. Forging ahead despite these disappointments, in January 1934 the Joliot-Curies announced the first instance of artificially induced radioactivity, a result that would have immense repercussions for medicine as well as pure science. All were satisfied when, in 1935, Chadwick received the Nobel in physics and the chemistry prize went to the young French couple.
The literal family ties hardly end with the Curies. William Lawrence Bragg, Rutherford’s successor as Cavendish Professor, had shared the 1915 Nobel Prize in physics with his father, William Henry Bragg, for their study of crystal structure by means of x-rays. Rutherford’s predecessor, too, saw his son receive a Nobel, albeit 31 years after his own: curiously, Joseph John Thomson was cited for discovering that the electron is a particle, while George Paget Thomson received the award for proving that the electron is a wave. Cognoscenti recognize this apparent contradiction as a confirmation of one of the central tenets of quantum mechanics: that an electron (as well as a photon) is both a particle and a wave, though the two manifestations cannot be detected simultaneously. The particle nature of radiation explains the photoelectric effect; the wave nature of electrons has enabled the development of the short-wavelength microscopes that bear their name.
The man principally responsible for developing the theory of wave-particle duality is Niels Bohr, a theoretical physicist whose career was critically shaped by a 1912 stay with Rutherford in Manchester. A deep bond of affection was forged between the established scientist and the young Dane, who later referred to Rutherford as a second father and even named one of his sons Ernest. Although Rutherford tried more than once to have Bohr join his professional family, first in Manchester and later in Cambridge, Bohr’s commitment to his native Denmark could not be broken. Yet the two maintained a tie grounded in their common physics interests and their complementary areas of expertise. In approaching problems of first the atom and later the nucleus, Rutherford looked to Bohr for guidance in theoretical matters and Bohr to Rutherford for the significance of experiments (though as their frequent correspondence attests, neither shied away from criticizing the other’s conclusions).
In Copenhagen, Bohr modeled his style of work on Rutherford’s, tailoring it to the pursuit of theoretical problems. As in Cambridge, the ideal was to surround yourself with young people and follow their work at an almost daily level while pursuing your own. To that end, Bohr founded the Institute for Theoretical Physics in 1921. Carrying the notion of family even further than Rutherford’s lab, it was housed in a single three-story building comprising a lecture hall, a library, work space for the young physicists, a cafeteria, and an apartment on the top floor for Niels and Margrethe Bohr and their children. One of the children who grew up there, Aage Bohr, succeeded his father as director of the Institute for Theoretical Physics and, in 1975, won a physics Nobel of his own.
Out of this institution came the Copenhagen interpretation of quantum mechanics, the set of rules for what is probably the 20th century’s greatest physical-science revolution.