|
Creationist's Blind Dates Excerpted
from “Abusing Science” by Philip Kitcher The standard scientific estimate is that the universe is about 15 billion years old, the earth about 4.5 billion years old. It is important to recognize from the start that there are independent procedures for obtaining each of these estimates, and that the procedures yield ranges of values that overlap. In the case of the universe, estimates can be obtained from astronomical methods or considerations of nuclear reactions. Astrophysicists can measure the rate at which galaxies are receding and use these measurements to compute the time needed for the universe to expand to its present size. A second, independent, astronomical method is to use standard techniques to measure some parameters of stars (mass, luminosity, compositor, and surface temperature), from which a well-confirmed theory of the life histories of stars enables physicists to compute their. ages. Finally, considerations of radioactive decay make it possible to calculate the time at which certain heavy elements were formed. These techniques are somewhat similar to the radiometric methods of dating rocks, which I shad consider in a little more detail. (For an excellent overview of the various ways of assigning an age to the universe, and an exposition of the radioactive decay method, see Schramm 1974.) Although the clear consensus of physical techniques is that the universe is billions of years old, and although this result controverts the claims of at least some contemporary Creationists, the principal Creationist attack has been directed against the standard geological claim that the earth is about 4.5 billion years old. Two kinds of arguments are offered. In the first place, Creationists argue that methods of radiometric dating employ false assumptions. They continue by using special techniques of their own to assign to the earth an age of a few thousand years. Excellent and exhaustive explanations of the errors in Creationist arguments about dating methods have been given by Stephen Brush (1982,1996) and Brent Dalrymple (1994). My aim in the following brief discussion is simply to hit the high spots. The basic idea behind radioactive dating is very simple. If a radioactive isotope (the parent element) was originally present in a rock at the time of its formation, then that isotope would give rise, by radioactive decay, to decay products (daughter elements). The phenomenon of radioactive decay is well understood by nuclear physicists. It is governed by the following equation: Nt = N0 e-lt (1) Here Nt represents the number of radioactive atoms present at time t, N0 is the number of radioactive atoms originally present, e is the base of natural logarithms (about 2.718), and l is a constant (the decay constant) specific to the element whose decay is being considered. Suppose that, at the time of formation of a rock, P0 atoms of a parent element and D0 atoms of one of its daughters were present. Suppose also that the rock neither gives up nor receives additional parent or daughter atoms during the ensuing years and that today atoms of the parent element and Dt atoms of the daughter element are present. Then, by the assumption that parent and daughter atoms neither entered nor exited, we know that the extra daughter atoms that are now present must come from decay of the parent. So we can conclude Dt = D0 + P0 - Pt (2) From equation (1) we get Pt = P0 e-lt (3) where t is now the age of the rock. Combining (2) and (3) gives Pt = (Pt + Dt - D0) e-lt (4) Elementary algebra will enable us to compute t from this equation, provided that we know (Pt, Dt, D0 and l. Since constants of radioactive decay are specific to elements, experimental studies of the decay of the parent element in question provide the value of l. Pt and Dt can both be calculated by measuring the amounts of parent and daughter isotopes found in the present rock. As Dalrymple (1994) points out, available techniques give us more than the accuracy we need. But there is an apparent problem with D0. How can we figure out the amount of the daughter element originally present? The answer is that in many cases (if we choose the right element for the right rock) we have excellent reasons for believing that D0 is zero (or, at least, negligibly small. In other cases, as we shall see, we can use present rock compositions to infer the value of D0. Obviously, there are two major assumptions involved in the use of radiometric dating. Scientists have to estimate D0 and they have to rule out the possibility that additional quantities of the daughter element have been added since the time the rock was formed. (Actually, the computation of the age would be affected if some of the daughter element originally present had been lost. However, since we are primarily concerned with the Creationist challenge, the main worry will concern subsequent additions. For if extra daughter element were added, then we should arrive at too large a figure for the amount of the parent element that has decayed, and thus produce too high a value for the age of the rock.) Geologists are not unaware of these assumptions, and they take great pains to construct ways of cross-checking them. Consider first the ways of computing D0. One common method of radioactive dating, the potassium-argon method, takes the radioactive potassium isotope, potassium-40, as the parent element and argon40 as the daughter element. Argon is an inert gas, so that it does not occur in chemical compounds in original rocks. In some crystalline structures it can be trapped mechanically, but for other naturally occurring minerals it can be shown that this does not occur. Hence, in the case of these minerals, we can conclude that no argon was originally present; that is, D0 = 0. The chief defect of the potassiumargon method is that, under the action of heat or compression, argon can escape from rocks, so that the estimated age is less than the true value. A second common method of radiometric dating involves the decay of uranium into lead. Here it is possible to use two decay processes, the decay of uranium-238 into lead-206 and the decay of uranium-235 into lead-207. Furthermore, the amount of lead originally present can be computed by considering another isotope of lead. Lead204 is present in small quantities in most samples of lead, and this isotope is not itself the product of a radioactive decay process. Hence, by measuring the amount of lead-204 in a rock, geologists can estimate the amount of lead originally present. Given this value of D0it is then possible to use either decay process to calculate the age of the rock. If the results agree, they are said to be concordant, and geologists are usually confident that concordant ages are the true ages of the rocks under consideration. The second worry is that extra amounts of the daughter element may enter the system after the original formation of the rock, thus giving the impression that more of the parent element has undergone radioactive decay than has actually been the case. In both the examples I have described, there are ways of checking that such intrusions have not occurred. Minerals can be tested for their capacity to absorb extra argon under experimental conditions designed to resemble their natural environment, and geologists can screen out, in this way, minerals that are liable to give erroneous results. In the second case, the existence of two separate decay processes provides a check on the assumption that the system has not been contaminated. If extra lead were to have been absorbed in the rock after the original formation, the new lead would have caused the calculated ages of the rock to diverge unless it contained the right proportion of lead-206 to lead-207. If the ratio of lead-206 to lead-207 in the newly introduced rock were greater than the ratio of lead-206 to lead-207 found in an uncontaminated system, the method of dating based on the decay of uranium-238 to lead-206 would give a relatively higher value than the method of dating based on the decay of uranium-235 to lead-207. Obviously just the opposite holds when the ratio of lead-206 to lead-207 is too small. Hence someone who supposes that concordant ages are inflated must believe that the contaminating lead contained just the right proportion of the two isotopes. I want to emphasize that I have only dealt with two of the commonly used radiometric methods, and I have only outlined the most elementary of the checks that geologists use in applying them. (More details can be found in Eicher 1968, chapter 6; and Faul 1966.) From what I have said it might seem that the assignment of ages to rocks is still a bit uncertain. However, I hope that it will help to quell anxieties when I point out that a large number of independent methods have been applied to a vast array of different rocks. The result of this enormous array of tests is a consensus. The ages assigned to various rock strata bearing distinctive types of fossils show extraordinary agreement. The many independent computations of the age of the earth during the last three decades almost invariably yield a figure between 4.2 and 4.8 billion years. Of course, there are occasional puzzling discrepancies. But geologists take these as signs that unanticipated factors have affected the system from which the result was obtained. They know that geological clocks, like other clocks, can go wrong. Frequently, further investigation dissolves the anomaly by showing what the interfering factor has been. Let us now take up some of the Creationists' attempts to criticize radiometric dating. The main lines of attack are laid down by Morris. He begins by identifying three assumptions of the use of radiometric techniques: " 1 . The system must have been a closed system. . . . 2. The system must originally have contained none of its daughter component. . . . 3. The process rate must always have been the same" (Morris 1974a, 138). We have already discussed statements akin to Morris's first and second assumptions. As will become clear shortly, the status of the third is a little different. Unsurprisingly, Morris believes that he can provide good reasons for doubting each of these assumptions in the case of every application of every method. He claims that none of the assumptions is "provable, testable, or even reasonable" (Morris 1974a, 139). Why not? Here are the reasons: 1.
There is no
such thing in nature as a closed system. 2.
It is impossible
to ever know the initial components of a system formed in prehistoric
times. 3.
No process rate is unchangeable. These rejoinders make it apparent that Morris's formulations of the assumptions underlying radiometric dating are only akin to the assumptions examined above. When geologists calculate the ages of rocks, they do assume that the system under consideration has remained closed in one particular respect. They suppose that none of the daughter element has been added or subtracted. However, this does not commit them to the idea that the system was completely closed, that it engaged in no exchange of matter or energy with the environment. Like his memorable argument about the evolving junkyard, Morris's first reply only demonstrates his lack of understanding of basic concepts of physics. The crucial question is whether we can ever be justified in believing that the system was never contaminated by extra amounts of the daughter element. I have tried to explain how geologists can sometimes obtain good evidence for this conclusion. Similarly, the second point is misguided. Geologists do not have to suppose that the system originally contained none of the daughter element. What is important is that they be able to compute the amount of the daughter element originally present. [Equation (4), let us recall, enables us to compute the age of the rock if we know Pt, Dt, l, and D0. Clearly, it is required only that D0 be known, not that it be zero. However, for some methods we can calculate D0 precisely because we can be sure that none of the daughter element could originally have been present; that is, D0 = 0.] Furthermore, Morris is overstating his case when he declares that we can know nothing about the elements originally present. It is perfectly possible to have excellent evidence for statements about events and situations that no human has observed. Geologists draw conclusions about the composition of original rocks by applying claims about the possibilities of incorporating elements into minerals, claims that can be tested in the laboratory. So, for example, the thesis that certain minerals would have contained no original argon rests on a perfectly testable (and well-confirmed) claim. While those minerals were in the molten state, prior to the solidification of the rock, argon would have diffused from them. It is only after the molten rock has solidified that the argon formed through radioactive decay becomes trapped within it. Obviously, what is being applied in this case is our knowledge of the physical and chemical interactions of minerals and elements. Morris's third assumption, and his attempt to undermine it, raises a new issue. In deriving equation (4), from which rock ages can be computed, I employed equation (1), the equation of radioactive decay. I asserted that l, which measures the rate of decay, is a constant. Morris suggests that the assertion is unwarranted. However, the claim that l is a constant does not descend out of thin air. It is the result of our knowledge of nuclear physics. Although the sciences sometimes teach us that the rate at which a process occurs can be affected by a number of factors, as when we learn that the rate at which water boils is affected by the pressure or that the rate at which mutations occur varies with X-ray irradiation, what we sometimes discover is that a process is impervious to outside influence. Precious little affects the time of passage for a light ray between two points. Similarly, nuclear physics tells us that radioactive decay is well insulated against external interference. The reason is that the emission of particles from an atomic nucleus is under the control of forces that are enormously more effective at short distances than the forces at work in most physicochemical reactions. Moreover, the theoretical predictions that radioactive decay rates are extraordinarily hard to alter are experimentally well confirmed. Extensive attempts to modify these rates under a variety of physicochemical conditions have produced no effects. When Morris attempts to support his points in the context of particular dating methods, he fares no better. For example, his chief weapon in arguing for the possibility of variable decay rates is a vague proposal that the capture of free neutrons or the impact of neutrinos could affect decay constants (Morris 1974a, 142-143). (The latter idea is linked to a paragraph quoted from a "Scientific Speculation" column.) But neither of these processes would affect rates of decay; even granting the possibility of change by neutrino impact or the practical likelihood of neutron capture, the result of these processes would be a modification not of the decay rate, but of the decaying nucleus. (The old nucleus, which had been decaying at its specific rate, would be changed to a new nucleus, which would then change at its specific rate. Note that if processes like these were to occur, they would be detectable since two separate sets of daughter elements would be produced.) Morris's speculations are based on confusion. Morris then goes on to ignore the methods that geologists employ to ascertain the original amount of daughter element present in the rocks they attempt to date. His discussion of uranium-lead dating contains no mention of the simple technique for computing the initial abundance of lead that I described above. (Needless to say, nothing is said about more sophisticated methods.) His treatment of potassium argon dating includes the statement: "Since argon-40 is a gas, it is obvious that it can easily migrate in and out of potassium minerals" (Morris 1974a, 145). However, argon-40 is an inert gas, which does not become chemically bound to potassium minerals. Moreover, the crystalline structure of some minerals makes them impermeable to argon. Hence the suggestion that the minerals that geologists date are easily contaminated is simply false. My brief discussion has only looked at a sample of the objections that Morris and his colleagues (notably Slusher; see Slusher 1973) offer against radiometric dating. The errors I have identified are typical. No attempt is made to criticize the techniques that geologists carefully employ to determine the value of D0 or to test whether the system has been contaminated. Instead, those techniques are ignored. The picture thus presented is that radiometric dating methods compute the ages of rocks by applying equation (4), assuming dogmatically that D0 is zero and that the system is uncontaminated. Add to this distortion some vague speculations about changing decay rates (perhaps based on a revisionist nuclear physics under development at the Institute for Creation Research?) and the usual ploy of emphasizing professional disagreements. The result is the typical Creationist mélange - something that appears authoritative to the inexpert, but can be unmasked by even the briefest account of the standard geological practices. I shall deal with the positive arguments for a young earth in much less detail. The reason for this is that once one has appreciated the radiometric dating techniques and their overwhelming evidence for the claim that the earth is more than 4 billion years old, it is clear that there must be some flaw in the attempts to show that the earth was created a few thousand years ago. In addition, the Creationist arguments most commonly trotted out share a simple flaw. Creationists assume that certain processes, which we have independent reason to believe to be irregular and sporadic, take place at uniform rates. Two examples will suffice. Thomas Barnes (1973) argues that the earth's magnetic field is decaying, and he uses the observed rate of decay to compute that prior to about 10,000 B.C. the earth's magnetic field would have been impossibly strong. However, there is overwhelming geophysical evidence for the claim that the earth's magnetic field fluctuates both in intensity and direction, so that Barnes's extrapolation from the present is simply misguided. A similarly erroneous argument is given by Morris (Morris 1974a, 167-169; Morris 1974b 150-154; Wysong 1976, 147-148). He uses the current rate of growth of the human population to calculate the time required for the present population size to be reached from an original pair of individuals. It should be fairly obvious that this is a blunder. There is every reason to believe that the rate of growth of the human population has not been constant, but has fluctuated quite wildly in the past. Indeed, it is surely an over simplification to consider the growth of the human population except during the last few centuries. If we think about the distribution of humans at the dawn of recorded history, then it is far more realistic to conceive the human race as consisting of a number of relatively small populations. Some of these were fairly successful and were able to expand until they reached the maximum size that their local environments could bear. Others were wiped out by disease, dwindling resources, or competition with other groups. The entire human race may be regarded as a single population only for the most recent past; that is, it has only been very recently that humans have had the power, though not necessarily the desire, to redistribute the earth's resources so as to overcome local limits imposed by the local environment. Barnes and Morris both choose processes that we know to operate at different rates at different times, and then use the observed rates to estimate the time at which the process began. Dating the past is a complicated and technical business, and one cannot ignore the technical details simply to generate the ages one wants. Without a thorough understanding of which rates are constant over time and which rates fluctuate wildly, Creationist dates are bound to be stabs in the dark. However, Creationists know what they want the age of the earth to be. So just as in the case of the second law of thermodynamics, important parts of science are abused. By carefully picking a process on the basis of its ability to give the desired result, without attending to the question whether it is reasonable to think that it happened at a constant rate, Creationists attempt to convince the uninitiated that their blind dates have scientific references. Nobody should be taken in. Creation "science" is spurious science. To treat it as science we would have to overlook its intolerable vagueness. We would have to abandon large parts of well-established sciences (physics, chemistry, and geology, as well as evolutionary biology, are all candidates for revision). We would have to trade careful technical procedures for blind guesses, verified theories for motley collections of special techniques. Exceptional cases, whose careful pursuit has so often led to important turnings in the history of science, would be dismissed with a wave of the hand. Nor would there be any gains. There is not a single scientific question to which Creationism provides its own detailed problem solution. In short, Creationism could take a place among the sciences only if the substance and methods of contemporary science were mutilated to make room for a scientifically worthless doctrine.
|