Entitled +quot;Questions and the answer for chronologists+quot;, this is taken from the mo

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Entitled "Questions and the answer for chronologists", this is taken from the most recent issue of "New Scientist" Review of "The Age of the Earth", by G. Brent Dalrymple, Stanford Univ. Press, 474 pages. $49.50 (Typos are mine!) How old is our planet? The answer, "four and one-half billion years", is in the first sentence in Brent Dalrymple's story of how scientists have deduced the age of the Earth. The events that led Dalrymple to write the book are rooted in the creationist, pseudo-science controversies of the American law courts, and this in itself makes an interesting tale. Dalrymple is an accomplished geochronologist who was retained as a witness by the State of California in 1980, defending against a civil suit brought by creationists who objected to the teaching of evolution as fact in public schools. The state had assembled a formidable panel of scientific expertise, and although ultimately never called upon, Dalrymple acquired a reputation as a scientist who was willing and able to debate the creationist issue in legal proceedings, informal seminars, and the press. Be aware, though, that this book is not a philosophical or theological debate; it is a well-informed scientific textbook based on the author's painstaking background research in a number of fields. By starting with "The Answer", Dalrymple sets out his stall. This was a wise move; it would take an extraordinary writer to maintain the suspense of a whodunit through the development of theories and techniques pertaining to a single question in earth sciences, however fundamental. The answer has been hard-won down the centuries, however, and a wide variety of disciplines have been brought to bear upon the question. Early attempts to deduce or estimate the age of the Earth from the first main section of the book, and provide some of the most entertaining reading. James Ussher's celebrated biblical chronology of 1650, which placed Creation in the evening of 22 October 4004 BC, was by no means the earliest recorded "Answer". About 2000 years before Ussher, the Hindu priesthood decided that creation had occurred 2 billion years ago. The accuracy of this guess was unsurpassed until the 20th Century. Biblical chronologies are all good fun, but the real entertainment from a modern perspective comes with the work of 19th-century scientists, such as Lord Kelvin, whose estimates were based on observational data and sound physical principles. Physicists calculated the cooling rate of the Earth and the effects of orbital forces, while geologists, who trusted their own observations rather than the mathematical juggling of the establishment, appealed to erosion and sedimentation rates. They were all spectacularly wrong because they had misunderstood processes, guessed starting conditions and overstated the certainty of their assumptions. In some cases, they were soundly criticised on these grounds by their contemporaries. Ironically, much modern scientific debate revolves around the same criticisms. Perhaps we should take a lesson from the authors who claimed their work to be no more than "wild speculation", yet whose answer is often all that remains in the popular perception about the age of the Earth. Ultimately, the age of the Earth and everything in it can only be addressed by radiometric dating. As a background to the main substance of the story, modern radiometric methods are explained in a chapter that is as clear and concise as any I have read. Here the book stops being journalism and gets its teeth into the subject; lead isotope systematics are tortuous enough for those of us who deal with them daily, and Dalrymple has probably made this process as accessible as possible. Although all the principal dating methods are described, we find out that the whole story hinges on the decay of uranium to lead. A chapter relates the oldest rocks found on the Earth, and here the diversity of techniques available to the modern geochronologist becomes invaluable, giving us a minimum age constraint that is bolstered by data from lunar samples. Another chapter introduces the meteorites, the oldest and most primitive rocks available to scientists, which are crucial to the story because the "age" of the Earth really means the time at which the matter of the Solar System first segregated into discrete bodies. The chapter devoted to lead isotopes just about wraps things up, but there follows a brief account of astrophysical methods by which the age of the Universe has been estimated, as well ant that is bolstered by data from lunar samples. Another chapter introduces the meteorites, the oldest and most primitive rocks available to scientists, which are crucial to the story because the "age" of the Earth really means the time at which the matter of the Solar System first segregated into discrete bodies. The chapter devoted to lead isotopes just about wraps things up, but there follows a brief account of astrophysical methods by which the age of the Universe has been estimated, as well ant that is bolstered by data from lunar samples. Another chapter introduces the meteorites, the oldest and most primitive rocks avail

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