While Rutherford and Curie experimented with radiation, other physicists were taking a mathematical approach to these newly discovered phenomena. In 1901 German researcher Max Planck published his "quantum hypothesis" which defined energy behavior in terms of a theoretically established constant. In 1905 Albert Einstein reinterpreted Planck’s hypothesis, imagining light as individual packets of energy - packets which later came to be known as "photons."
Einstein's theory put forth the founding tenets of quantum mechanics, a branch of physics which was then expanded tremendously by individuals like Max Planck, Wolfgang Pauli, Erwin Schrödinger, and Werner Heisenberg.1 2 By the mid-1920s, quantum mechanics was a fully established field used to describe and predict particle and wave behavior.
James Chadwick demonstrated, in 1932, the existence of the neutron, a particle with no charge residing in the nucleus of the atom.3 4 Later that year, John Cockroft and Ernest Walton bombarded a sample of lithium with protons which transmuted it into various elements, including helium. In so doing, the duo demonstrated conclusively that the atom could be split.
In 1934 Frédéric and Irène Joliot-Curie found that radioactivity could be created in non-radioactive elements through exposure to alpha particles. In the same year, Leó Szilárd expanded on recent findings, theorizing that a chain reaction could be created through neutron bombardment.5 This lead Szilárd to conceive of an atomic bomb, an idea he patented and transferred to the British Royal Navy in 1936.