APS News

Session Marks Centenary Of Discovery Of Radioactivity

February 24th marked the 100th anniversary of the first scientific presentation by French physicist Henri Becquerel that led to the discovery of radioactivity. Human use of radioactivity not only led to such obvious results as nuclear energy, nuclear weapons, medical X rays and cancer therapy; it also provided tools that helped launch studies of the subatomic world and making it possible to determine the three-dimensional structure of important biomolecules.

According to Erwin Hiebert, a science historian at Harvard University who spoke at the March APS Meeting, the discovery of radioactivity was one of the first milestones in terms of toppling the inertia of causality in physics. Until the end of the 19th century, chance and probability were taken by most scientists and philosophers as expressions of ignorance, and not as basic components in the structure of the world. With the discovery of radioactivity in 1896, such views began to be questioned. "Here was an event, obeying an exponential decay law, in which chemical elements were known to disintegrate and transmute into other elements in a process that could not be made to alter its course by external changes; fluctuations in the radioactive decay process were real and acausal," he said.

Radioactivity was also the first event in the development of nuclear physics. Although the neutron, positron, deuterium atom, and so forth weren't discovered until 1932 and beyond, radioactivity was the first reaction in which a nuclear phenomenon was taking place at a subatomic level. And to some extent, Hiebert believes, the discovery of radioactivity as a nuclear fluctuating phenomenon was the first example of a quantum phenomenon, in which quantum notions were put to the test. It wasn't until 1928 that Werner Heisenberg put forth his uncertainty principle, which implies that one cannot simultaneously precisely determined the position and the momentum of a particle, implying that there is some indeterminancy in the way that nature behaves.

Scientists were soon using radioactive materials as projectiles to bombard atoms, resulting in the first scattering experiments. On the basis of those, it was recognized in 1913 that the atom had to be extraordinarily compact, and that the nucleus was very much smaller than the atom itself. This early use of projectiles has become modern particle physics. "Before this, it was believed that elements were the basic building blocks of nature," said Helena Pycior (University of Wisconsin, Milwaukee). "Now we understand that they are not; they're all made up of the same particles. Our whole view of the universe was transformed by this."

In addition, radioactive materials were used as early as the 1930's to destroy cancer cells, and diseases, again as a source of subatomic projectiles. Some of the most important applications of radioactivity have been in nuclear medicine, as well as nuclear reactors, power and nuclear weapons, carbon dating, and other radioactive dating, such as potassium argon dating of geologic specimens.

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Editor: Barrett H. Ripin