Spurred by the discovery that a substance containing uranium emitted radiation, Marie Curie began studying radioactivity in 1897. She first tested gold and copper for radiation but found none. She then tested pitchblende, a mineral that was known to contain uranium, and discovered that it was more radioactive than uranium. Acting on the hypothesis that pitchblende must contain at least one other radioactive element, Curie was able to isolate a pair of previously unknown elements, polonium and radium. Turning her attention to the rate of radioactive emission, she discovered that uranium emitted radiation at a consistent rate, even if heated or dissolved. Based on these results, Curie concluded that the emission rate for a given element was constant. Furthermore, because radiation appeared to be spontaneous, with no discernible difference between radiating and nonradiating elements, she was unable to postulate a mechanism by which to explain radiation.

It is now known that radiation occurs when certain isotopes (atoms of the same element that differ slightly in their atomic structure) decay, and that emission rates are not constant but decrease very slowly with time. Some critics have recently faulted Curie for not reaching these conclusions herself, but it would have been impossible for Curie to do so given the evidence available to her. While relatively light elements such as gold and copper occasionally have unstable (i.e., radioactive) isotopes, radioactive isotopes of most of these elements are not available in nature because they have largely finished decaying and so have become stable. Conversely, heavier elements such as uranium, which decay into lighter elements in a process that takes billions of years, are present in nature exclusively in radioactive form.

Furthermore, we must recall that in Curie's time the nature of the atom itself was still being debated. Physicists believed that matter could not be divided indefinitely but instead would eventually be reduced to its indivisible components. Chemists, on the other hand, observing that chemical reactions took place as if matter was composed of atomlike particles, used the atom as a foundation for conceptualizing and describing such reactions—but they were not ultimately concerned with the question of whether or not such indivisible atoms actually existed.

As a physicist, Curie conjectured that radiating substances might lose mass in the form of atoms, but this idea is very different from the explanation eventually arrived at. It was not until the 1930s that advances in quantum mechanics overthrew the earlier understanding of the atom and showed that radiation occurs because the atoms themselves lose mass—a hypothesis that Curie, committed to the indivisible atom, could not be expected to have conceived of. Moreover, not only is Curie's inability to identify the mechanism by which radiation occurs understandable, it is also important to recognize that it was Curie's investigation of radiation that paved the way for the later breakthroughs.