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Mystery in the Rocks

A physicist's discovery begins an extraordinary odyssey through
pride and prejudice in the scientific world.

By Dennis Crews

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The early 1960s was a time of unclouded promise for many American college students. Industry was booming, the infamous war in Indochina had not yet ground itself into public consciousness and civil rights uprisings were the concern of only a principled few. Young professionals ascended by thousands into the American dream, while visions of a home in suburbia, a new car in the driveway and the promise of a comfortable retirement beckoned still more thousands of new graduates into the mainstream.

In this setting, quests for truth and justice seemed the stuff of history and Hollywood hype; the melodrama of moral odyssey paled beside the lure of financial success and professional recognition. But every age is redeemed by its own few who are driven by something other than the urge to get ahead, and Robert Gentry was one of those individuals. Not that he saw himself as any kind of hero—such people never do—but his feet were destined for the high and lonely path where truth and trial intertwine.

Gentry was a graduate student in physics. His analytical mind thrived on certainties. Though he considered himself a Christian he was not much troubled by the strident war between creationists and evolutionists. All through his schooling he had reconciled the seemingly incompatible concepts of science and religion by crediting God as the force behind the big bang, that primal explosion scientists believe started the motor of the universe. Was it really possible to know with any certainty what else might have happened so long ago?

One factor above all others seemed to place the time frame of evolution beyond serious doubt. That factor was radioactive dating—a technique scientists use to determine the age of objects in the natural world. The principle behind radioactive dating is simple. Many rocks contain traces of radioactive elements, which are in the continual process of decaying into lead, a non-radioactive end product. It is possible to measure both the amount of a given radioactive element and the amount of lead resulting from that element in a rock. Scientists correlate the ratio of these two amounts with the known decay rate of that element, to find the period of time that has elapsed since the rock was formed. (Decay rates are calculated by the half-life—the time it takes for half the atoms in a given element to decay.) Many scientists rest their proof of the earth's age upon radioactive dating of rocks that are thought to be associated with the formation of the earth itself.

After acquiring his master's degree in physics Gentry staked out a promising career in the defense industry, working first for Convair (later to become General Dynamics) and then for Martin-Marietta Corporation, researching nuclear weapons effects. By now he was married to an intelligent, pretty math major named Pat, and had a good slice of the American dream within sight. In the summer of 1962 he was awarded a National Science Foundation Fellowship to attend the Oak Ridge Institute of Nuclear Studies in Tennessee. Fall of the same year found him working toward his Ph.D. at Georgia Institute of Technology.

Gentry's fascination with nuclear physics kept bringing one question persistently to his mind. It had always been assumed that decay rates of various radioactive elements had remained constant since the beginning of time—since the big bang, as his fellow scientists believed. Was this a valid assumption? Nobody even knew if physical laws prevailed before that event. Did they spring into existence fully stabilized? His university physics courses treated the uniformity of decay rates as self-evident truth, but nobody had seriously examined that assumption. If the decay rates had ever fluctuated, Gentry realized, the earth might not be as old as scientists believed. Could past uniformity of decay rates be proven?

In graduate school Gentry began studying radioactive dating techniques more closely. As he reviewed past work in the field, he was fascinated by a specific area of research that once seemed to hold much promise in the field of radioactive dating, but had received little attention for the past two decades. It had begun in the late 1800s when improved microscopes became available. When thin, translucent slices of certain minerals were examined under high magnification, some of them were discovered to have tiny dots imbedded in them, surrounded by concentric, colored rings. Further study revealed that each set of rings was actually the cross-section of a series of spherical shells, like the layers of an onion, surrounding a tiny grain of a different mineral. Scientists first called the rings pleochroic halos, after their property of exhibiting different colors when viewed from different directions by transmitted light.

For a time mineralogists thought that an organic pigment might have been trapped in the rocks when they were formed, eventually diffusing out into the surrounding matter to form the halos. But nobody knew what that pigment might be, or could explain how it formed multiple halos. The phenomenon remained a minor scientific mystery until around the turn of the century, when uranium and certain other elements were discovered to be radioactive.

The man who unlocked the secret of pleochroic halos was Professor John Joly of Trinity College in Dublin. Joly had done extensive study on halos in biotite (a type of mica commonly found in granite), and realized that diffusion of pigments was not adequate to explain the sharply defined edges of the concentric rings, or the regularity of their sizes. In 1907 he began to consider an origin for the halos that could never have been postulated only a few short years before—radioactivity. By that time scientists knew uranium to be the initial member in a series of radioactive elements. Uranium eventually decays into another element (called a daughter element), which in turn decays into another daughter element, and so on down the line until finally only lead remains as a stable end product.

Joly understood that uranium and its radioactive daughter products decay in one of two ways: by emitting either beta particles, which are very light; or alpha particles, which are much heavier. Emitted beta particles harmlessly bounce around the molecular interior of matter like tiny ping-pong balls until they finally come to rest, but the heavier alpha particles blast their way through matter like bullets. A single alpha particle will ionize about 100,000 atoms along its line of travel before being spent, leaving a microscopic damage trail behind it. Single particles firing off from uranium atoms dispersed randomly through a rock would have little discernable effect on it, but billions of atoms clustered in a grain of uranium enclosed within another rock could, Joly realized, leave a distinct signature within the host rock.

Alpha particles emitted from the uranium would all come to rest about the same distance from the center of the inclusion in all directions, Joly believed, producing a spherical damage field. Could this be the cause of pleochroic halos? Several crucial bits of information would resolve the question, and Joly set out to find them. Did the halo sizes correspond to the distances alpha particles would travel in mica?

His research was fruitful, for it demonstrated not only that the sizes were correct, but that the number of rings surrounding certain of the particles corresponded with the number of alpha-emitting members in the uranium series decay chain.

In the years following Joly's discovery many more scientists began studying radiohalos, as they began to be called. Physicists believed they held information that could lead to a better understanding of radioactive phenomena—of decay rates in particular. Geologists studied them in hopes of finding an accurate method by which to determine the age of geologic formations.

Since there seemed to be several distinct halo types, Joly believed radiohalos had ring sizes that varied with age, which implied that radioactive decay rates had once been different from the present rate. Later researchers found that various alpha emitters in the decay chain created rings of different sizes, depending on their alpha energy. Yet many unanswered questions about radiohalos remained. Unfortunately, two world wars and other more pressing kinds of research intervened, sweeping radiohalos off to the periphery of scientific inquiry. For many years they received little further study.

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For readers interested in a more comprehensive treatment of this story, Robert Gentry's book, Creation's Tiny Mystery, is available for $18 (U.S.) + S/H.

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The above page was found at on September 28, 2023.

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