Each and every day and night scientists all over the world spend hours in their work rooms and laboratories, reflecting and searching, poring over technical papers, calculating, and arguing. They make measurements, record data, clarify concepts, verify results, formulate theories, look for discrepancies, struggle with puzzling phenomena: all in their attempts to gain a deeper understanding of the world we live in.
The significant discoveries of science push ever so slowly the frontiers of human knowledge, but some revolutionize our perception of the world far beyond the imagination of the discoverer and even the scientific community at large. A case in point was the discovery of radioactivity by Antoine-Henri Becquerel exactly a hundred years ago.
Antoine-Henri Becquerel came from a lineage of physicists. His grandfather, Antoine-CŽsar, had occupied the chair of physics at the MusŽe d'Histoire Naturelle in Paris, the most important science museum in France during the nineteenth century. Founded in 1793, it offered academic courses for credit and contained collections and exhibits in every field of science, from anthropology to zoology.
Henri's father, Alexandre-Edmond Becquerel, also was a physicist of stature. He had studied in depth the phenomenon of luminescence, whereby substances emit light (radiation) without being heated. In this context, he invented the phosphoroscope, an ingenious instrument that measures aspects of this phenomenon. Alexandre Becquerel became director of the MusŽe d'Histoire Naturelle, where his father had taught, and lived with his family in the residential quarters of the museum. It was here that, on December 15, 1852, the future discoverer of radioactivity first saw the light of day: He is perhaps the only physicist to have been born in a museum.First scientific interests
Young Henri attended school at the old and renowned LycŽe Louis-le-Grand, where many of France's great figures, such as Voltaire, had once been pupils. In the lore of the Becquerel family, Father Becquerel is noted as having been proud of little Henri to the point of assuring Henri's math teacher that the boy would go far.
At the age of 20, Henri went for more advanced studies at the prestigious ƒcole Polytechnique. Among his classmates here was the future astrophysicist Henri-Alexandre Deslandres, who was to study the rings of Saturn and the rotation of Uranus and who, like Henri, was to become a member of the AcadŽmie des Sciences some years later.
Soon after finishing college at the Polytechnique in 1874, Henri Becquerel married his first love, Lucie-ZoŽ-Marie Jamin, daughter of the eminent physicist Jules Celestin Jamin. Unfortunately she died in 1878, barely 20 years old, soon after their son, Jean, was born. Earlier that year Becquerel also lost his famous grandfather. Twelve years later he married again, this time to Mlle Larieux.
From the Polytechnique, Becquerel went on to study at the ƒcole des Ponts et ChaussŽes (the civil engineering school in Paris) in 1874. Here as a student he began doing scientific work. In the course of his investigations, he observed an important property of magnetic fields, which was published in the Journal de Physique and won him some scientific recognition.
Becquerel also became a laboratory assistant at the MusŽe d'Histoire Naturelle, where he worked with his father in exploring the effects of magnetic fields on light. Some of his early studies were related to the field of infrared spectroscopy.
Later positions and research
When his grandfather died in 1878, Becquerel's father became a professor at the museum and Becquerel succeeded to his father's research position. It is not often that physics professors, like royalty, succeed their fathers to the throne. Here for 10 years Becquerel taught courses while also investigating aspects of light and crystals. In 1888 his research won him a doctoral degree from the FacultŽ des Sciences at the Sorbonne. His dissertation dealt with variations in the absorption spectra of crystals.
In 1889, Becquerel was elected to membership in the AcadŽmie des Sciences, an unusual honor considering that he was only in his mid-30s.
Through his studies, Becquerel prepared himself unknowingly for the important discovery he would make later on. Like his father he was an expert on luminescence, one of the phenomena of nature that remained unexplained in the latter decades of the nineteenth century.
Ordinarily, sources of light are quite hot, be it the sun or burning firewood, glowing coal, candle flame, or the filament of an electric lamp. But some materials glow even when they are not hot. Such, for example, is the mineral fluorite. This phenomenon was therefore called fluorescence by the British physicist George Stokes. The explanation for fluorescence is that the substance in question absorbs invisible radiations (such as ultraviolet or infrared) and then emits visible light.
Recall that light consists of electromagnetic waves spread over a specific range of wavelengths, where shorter wavelengths are more energetic than longer ones. Invisible ultraviolet radiation has a shorter wavelength and hence more energy than visible light, whereas invisible infrared waves are longer. The fluorescent material merely transforms invisible wavelengths of greater energy into visible ones of lesser energy, somewhat as a person might change a quarter into dimes and nickels.
When such a transformation occurs right away, it is referred to as fluorescence. Sometimes the emitted radiation persists even after the external source (normally sunlight) is removed. This phenomenon, in which the absorbed invisible radiation continues to be re-emitted as visible light even after some time rather than ceasing right away, is called phosphorescence. Glowworms and certain dyes display the property of phosphorescence.
Fluorescence and phosphorescence together are known by the common name luminescence.
Scientific context of the discovery
In November 1895, Wilhelm Conrad Ršntgen, a professor of physics in WŸrzburg, Bavaria, discovered mysterious radiation that he called X rays. His research on electrical discharges through gases revealed the existence of "rays" that penetrate chunks of matter that are opaque to light.
The news of Ršntgen's discovery spread fast through scientific journals, newspapers, and magazines. Not everyone understood its scientific import. The phenomenon had to be explained.
In January 1896, at least three popular Parisian journals reported about the discovery of X rays [see "Lifting the Veil," The World & I, November 1995, p. 174]. People talked about it in the streets and cafŽs, but also in universities and research centers. On January 20, 1896, photographs of a person's bones, taken by two Parisian doctors with the aid of X rays, were displayed in the august assembly of the AcadŽmie des Sciences.
Mathematician Jules-Henri PoincarŽ presented to the leaders of French science all the available details on the Ršntgen discovery. The X-ray photograph that he passed around seemed like an eerie exhibit, for it was a ghastly peep into the interior of a living human being. More important for the men of science gathered there, the matter needed further investigation and cried for a rational explanation. Here was a challenge for anyone who was interested in any aspect of light at all.
Henri Becquerel was surely such a person. Recall that he had studied aspects of fluorescence and phosphorescence with his father. Quite naturally, Becquerel was intrigued by what he witnessed. He chatted further with PoincarŽ on the matter. The X ray discovered by Ršntgen emanated from a gas-discharge tube. In fact, Ršntgen's observation had been that when kept in the vicinity of such tubes, some salts begin to fluoresce. Now PoincarŽ noted that the X rays normally emanated from the area where the cathode rays in the discharge tube struck the glass surface. Since the glass had been made fluorescent by the cathode rays, he surmised that perhaps X rays were intimately associated with fluorescence and phosphorescence. The conversation with PoincarŽ stimulated Becquerel's interest in the phenomenon even more.
On his way back to the museum, Becquerel racked his brains about X rays and luminescence. He then consulted his father's treatise on the causes and effects of light, browsing through its sections on phosphorescence and fluorescence, its history and many fascinating aspects. After all, his father and grandfather had both studied the phenomena for many long years. His father had also examined with him the properties of certain uranium compounds. Father and son had once noted that uranium salts displayed the phosphorescent property.
Becquerel decided to examine minerals from his father's collection. His conversation with PoincarŽ led him to inquire what, if any, was the connection between Ršntgen's X rays and luminescence. Could it be, he thought, that X rays were a kind of invisible luminescence? He wanted to find out if any of those minerals in his laboratory emitted some X rays also when they phosphoresced.
January 21, 1896, a particularly bright sunny day in Paris, was a good day for testing phosphorescence phenomena. At his laboratory, Becquerel carefully wrapped a silver bromide photographic plate with two thick black papers, then exposed the package to sunlight for several hours. Upon unwrapping the plate in the darkroom, he verified that the plate was in no way affected by solar radiation--the wrapping was light-tight.
The next day he repeated this experiment with a slight difference: He placed a small sample of a uranium mineral (a chunk of potassium uranyl disulfate) on the light-protected dark-paper-wrapped photoplate. After the plate had been exposed to sunlight for four hours, he unwrapped and developed it. Lo and behold, there was a small spot on the plate right under where the mineral sample had been placed! He was excited by this observation, which seemed to confirm his suspicion that phosphorescent materials sometimes emitted X rays too. He repeated the experiment several times to be sure of what he had observed.
On February 24 Becquerel presented a formal communication of his finding to the AcadŽmie des Sciences. This paper said in effect that the phosphorescent uranium mineral he had examined, when exposed to sunlight, also emitted X-radiation.
The following week he repeated his experiments. However, on February 26 and 27 it was rather cloudy, and Becquerel felt that there was not enough sunlight for the experiment to be successful. So he left the uranium mineral sample on a fully protected photoplate inside a table drawer.
March 1, 1896, was again a sunny day in Paris. So Becquerel went to his laboratory intending to repeat his experiment. Before doing so, however, he decided to verify his assumption that the mineral had not emitted anything while in the drawer.
To his utter surprise, the photoplate had been affected right under the spot where the uranyl had been sitting in pitch dark for a few days. Clearly, the uranium minerals were giving out penetrating rays, even though they had not seen the light of day for quite some time. He did not realize it then, but he had discovered radioactivity on that sunny Sunday in Paris.
Becquerel pursued his research on the matter and soon found that these invisible radiations given out by the uranium salts could discharge an electroscope (a device for detecting small charges of electricity). This was a unique property of great significance, for it was to reveal the electrical nature of some of the emanations from radioactive materials. Furthermore, another series of experiments put into evidence the highly penetrating nature of the uranic rays. Then Becquerel showed that the emissions from the mineral never ceased in intensity, whether day or night or under varying conditions. He also discovered that this property was displayed by different uranium salts. Clearly, the metal uranium was the source of the radiation. On November 23 of that year he dubbed this radiation uranic rays.
Early interpretations
When news of uranic rays spread to the scientific community, ingenious ideas were proposed to explain the phenomenon. The physics community understood that the uranium minerals were emitting energy, for any form of radiation is energy. But how could energy be seeping out from a chunk of inert matter, enclosed in pitch darkness?
To explain this intriguing state of affairs, it was suggested, for example, that the minerals were perhaps sucking up heat from the air molecules surrounding them and spewing out this energy as invisible rays. This hypothesis had to be abandoned when it was shown that the uranium ore displayed the property even when it was placed in a vacuum. Another suggestion was that perhaps there existed some hitherto unknown, all-pervading, invisible radiant energy in empty space that the mineral somehow captured and re-emitted. Such explanations turned out to be on the wrong track.
It took two more years of sustained study by many physicists before some light could be shed on the unceasing outpouring of energy from the very core of certain materials. Inspired by Becquerel's discovery, Marie Sklodowska Curie, a brilliant physicist in Paris, undertook a thorough investigation of the material that emitted this new radiation. Her husband, Pierre, also a noted physicist, soon joined her research effort. Their tireless investigation revealed the existence of two new elements (polonium and radium) that display the phenomenon with even greater vigor. In January 1899 Madame Curie proposed the more general name radioactivity for the phenomenon discovered by Becquerel.
But it was not until the dawn of the twentieth century, in 1902, to be exact, that the notion of radioactive transformation was formally enunciated as a scientific principle.
Honors
In the very year that Becquerel discovered radioactivity, Alfred Nobel, the wealthy Swedish inventor of dynamite, died. Just prior to his death he had bequeathed his immense fortune to a foundation that would give out prizes "to those who, during the preceding year, shall have conferred the greatest benefit on mankind."
The first physics prize went in 1901 to Ršntgen for his discovery of X rays. The 1903 prize for physics was awarded to the discoverer and further explorers of radioactivity, Henri Becquerel, and Marie and Pierre Curie.
Some months after he received his award, Becquerel gave a public lecture on radioactivity in the huge amphitheater of his museum. It is worth recalling this event because it gives us a glimpse of the high regard in which science was held in those days. Becquerel's lecture was a grand event in Paris. Hundreds of people, both lay and scientific, gathered in the hall. High officials were present to listen to a lecture by a physicist on the subject of his research. Even ƒmile Loubet, the president of the French Republic, was in the audience. After all, here was the first French Nobel laureate in history. It is difficult to imagine, in our own days, the president attending a scientific lecture at the Smithsonian, for example.
On August 25, 1908, less than two months after he was elected a permanent secretary of the AcadŽmie des Sciences, Henri Becquerel took ill and died while resting in a country home in beautiful Brittany. Many dignitaries attended Becquerel's funeral in Paris on the 29th. They recalled his life, referred to his achievements, and paid homage to him in various ways. Telegrams of condolence came pouring in from many countries, and from institutions like the Royal Society of London. The eminent mathematician Jean-Gaston Darboux, who had been Becquerel's mathematics teacher in high school as well as a colleague at the academy, spoke eloquently about him, though he described him somewhat imprecisely as the "inventor of spontaneous radioactivity."
Among the family members he left behind was his son Jean Becquerel, who was to become a physicist in his own right, completing a lineage of physicists that may well be without parallel in history.
Reflections
Henri Becquerel's discovery of that small smudge on a photographic plate was a discovery of no small significance, leading as it did to a drastic reformulation of our understanding of the ultimate core of the material world. Thanks to radioactivity we have come to know that the deep recesses of matter are not made up of static entities in mute inertness like lifeless sand grains, but rather throbbing microcosms that are eerie replicas of the world at large. Radioactivity is merely an externally observable manifestation of the turmoil in the heart of matter.
We have known for ages that the sun and the stars put forth endlessly radiations of all kinds. But who could have surmised that such emissions occur from the incredibly tiny centers of minuscule atoms, too? Yet that, in effect, is what radioactivity is all about: the ceaseless outpouring of energy from within atoms.
This is not all. Our very model of a mechanically evolving universe had to be revised as a result of radioactivity. By the close of the nineteenth century, the supreme successes of classical mechanics had led physicists to picture the world as a complex clock that was functioning in accordance with exact mathematical laws in strictly calculable ways. But this, reveals radioactivity, is not quite so. For there is no telling which atomic nucleus will break up next, though one may be able to say how many in a given sample will decay in a given time interval. Thus, atoms degenerate through radioactivity with a randomness that is only statistically uniform: not unlike the deaths of individuals in a community.
Then it turns out that there is some truth to the alchemists' ancient fantasy of turning lead into silver and copper into gold. During the eighteenth and nineteenth centuries, after the rise of modern science, only charlatans and imposters would claim transmutation was possible. However, another revelation of radioactivity is that matter does undergo elemental transformation. We call such changes transmutations.
Furthermore, quantitative studies of radioactivity led to dramatic revisions of our estimates of the age of our planet. Before the impact of radioactivity on geology, physical scientists had been gradually increasing the estimates of the age of the earth from just a few thousand years, through a few hundred thousand years, to some twenty-four million by the close of the nineteenth century. But all of a sudden, it was becoming clear that the planet has been here for at least a few billion years!
Finally, as if to remind us that no knowledge is without some negative impact, radioactivity has become a threat to human life and civilization. For, as a result of our successes in harnessing nuclear energy, we have created huge amounts of radioactive wastes, whose eliminataion from our environment, it appears, is a superhuman task. This is one of the horrendous challenges facing technological man.
Truly radioactivity is the sleeping genie whose existence was unexpectedly, and perhaps appropriately, discovered in a dark room by a lone scientist in the seclusion of his laboratory when the rest of his community was probably attending church services. The genie has unveiled for us many secrets of this mysterious universe, but he has also aroused much fear and concern.
