Mystery in the Rocks
A physicist's discovery begins an extraordinary odyssey
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
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
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.
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 http://www.halos.com/book/mystery-in-the-rocks-01.htm on February 18, 2018.
Earth Science Associates