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| Triplet Arp 274 [Courtesy NASA] | Bronze pseudo-magic square on exterior of La Sagrada Familia cathedral, Barcelona, Spain [Photo by DHB, (c) 2011] |
Some of the most commonly used radiometric schemes are [Dalrymple1991, pg. 111; Dalrymple2004, pg. 55]:
In mathematical terms, radioactive decay is governed by a simple exponential formula, taught in many high school math classes:
P1 = P0 e-L t
where e = 2.71828... is the well-known math constant, P0 is the original amount of the radioactive material, P1 is the amount after time t, and L is the decay constant for the radioactive isotope. This decay constant L can be expressed in terms of the half life T (the time it takes for one-half of the material to decay) as L = log(2) / T, where log(2) = 0.693147... is the base-e logarithm of 2. In other words, if we know P1 and P0, or even merely their ratio, we can solve the above equation for the time t. However, usually it is not possible to apply this formula directly, because, for instance, in many cases we do not know the original amount of the radioactive isotope when the rock was solidified. Also, such a calculation does not provide us with any statistical error margin to double-check the result.
Fortunately, scientists have developed several methods that not only circumvent the difficulty of not knowing the original amounts, but also provide a very reliable means of statistical validity checking. For example, the rubidium-strontium isochron method, one of the most widely used schemes, is based on the radioactive decay of rubidium-87 into strontium-87 by the emission of a high-energy electron. The half-life T of this decay has been measured in careful laboratory measurements as T = 48.8 billion years. On the other hand, strontium-86 is a stable isotope. By some simple algebraic manipulation of the basic radioactivity formula above, one can show that the following formula must hold at any time t:
(Sr87/Sr86)t = (Sr87/Sr86)0 + (eL t - 1) (Rb87/Sr86)t,
where (Sr87/Sr86)t is the ratio of the amounts of these two elements in the sample material after time t; (Sr87/Sr86)0 is the same ratio at time = 0; and (Rb87/Sr86)t is the ratio of these two isotopes at time t. Note that this equation is in the simple form y = b + m x, namely the formula for graph of a straight line with slope m and with y-intercept b: here y = (Sr87/Sr86)t, b = (Sr87/Sr86)0, m = (eL t - 1) and x = (Rb87/Sr86)t. If all we have is one data point, the formula above doesn't help much more than the original formula. But if we have multiple data points -- multiple measurements of different samples say within a single igneous rock, then these should all lie on a straight line, whose slope m is simply related to the age of the specimen by the formula m = eL t - 1. Note that it doesn't matter that we don't know the original ratio (Sr87/Sr86)0; instead, this original ratio actually comes out as a result of the calculation!
Of course, in real scientific research, scientists do not rely on manually drawing points on graph paper to determine a best-fit straight line or to determine the line's slope or y-intercept. Instead, they use a statistical technique known as linear regression, which computes the least-squares best fit of a straight line through a sequence of points. This technique, which is used in virtually all disciplines of modern social science, physical science and engineering, is entirely straightforward, and computer programs are widely available to do the requisite calculations. An important fact is that linear regression, in addition to giving the best fit of the slope of the line (which then leads immediately to the date), also gives a statistical confidence interval as to the possible error in the determination of the slope. Details about linear regression are available in any elementary statistics book, or online -- see [Linear2009].
Here are just four examples of isochron graphs, which are entirely typical among the tens of thousands of examples that could be mentioned. Note how breathtakingly close these points are to the fitted lines (thus confirming with high statistical confidence the validity of the resulting dates):
The data for the first graph (upper left) is a set of measurements of basaltic achondrites (meteorites) in [Basaltic1981, pg. 938]; the data for the second graph (upper right) is from early Archaean gneisses rocks near Isua, Greenland [Morbath1977]; the data for the third graph (lower left) is from ancient gneiss rocks in Swaziland [Carlson1983]; the data for the fourth graph (lower right) is from lunar dunite rocks gathered during Apollo 17 [Papanastassiou1975]. The corresponding dates obtained from these isochrons (based on the slopes of the lines), together with statistical standard deviations, are: 4.396 ± 0.18, 3.673 ± 0.014, 2.991 ± 0.15, and 4.478 ± 0.034 (each figure is in billions of years). See also [Dalrymple1991, pg. 149, 185, 248, 328].
For many years, fairly large samples were required to produce statistically reliable results. But with the advent of mass spectrometry beginning in the 1970s, even very small samples can now be accurately dated. For example, the "SHRIMP" ion microprobe now in use in numerous laboratories around the world can reliably measure U-Pb and Pb-Pb ages from spots only 0.02 mm (i.e., 20 micrometers) in size within a zircon crystal [Dalrymple2004, pg. 60-62].
It should be emphasized, though, that even relatively unsophisticated equipment can perform radiometric measurements of dates fairly well. For example, as of the present date (see above), numerous used mass spectrometers are available for sale on the auction site eBay.com. Although most items are priced at several thousand dollars, a few are available for as little as $100. Will geology-evolution skeptics continue to dismiss radiometric dating now that almost anyone can buy a basic, used mass spectrometer and perform the measurements for themselves?
One very interesting and timely application of radiometric dating in paleontology was the recent discovery, by researchers at the University of Alberta in Canada, of a fossilized dinosaur bone in New Mexico that is 64.8 million years old. According to current theories, the Cretaceous-Tertiary (C-T) mass extinction event 65.5 million years ago was caused by a giant meteorite that collided with the earth, producing a huge cloud of debris that obscured the sun for many years, killing vegetation and driving all non-avian dinosaurs quickly into extinction. A team of researchers led by Larry Heaman of the University of Alberta in Canada dated this fossil, a femur bone of a hadrosaur, using the uranium-lead method, as only 64.8 million years. This means that this hadrosaur survived for roughly 700,000 years after the C-T event. If the University of Alberta team's findings are upheld, then the meteorite theory may need to be revised [SD2011b; Fassett2011].
Another recent example is a 2011 study, by an archaeological team from the University of Tubingen in Germany, that found stone tools 127,000 years old from a site near the entrance to the Persian Gulf. The existing consensus of researchers is that modern humans emerged in Africa roughly 200,000 years ago, but did not leave Africa until roughly 50,000 years ago. The German researchers used a quasi-radiometric technique known as optical luminescence dating, which is based on the electron energy that accumulates in a sample from steady bombardment of background radiation, to date the specimen as 127,000 years old. If this finding is upheld, it may change our understanding of when and how far this first migration occurred [Wade2011a].
These two examples also underscore the futility in asserting that there is some sort of "conspiracy" or "groupthink" preventing the consideration of young-earth creationist views. Note that each of the two studies mentioned above have the potential to overthrow the beloved theories of numerous other researchers. If there are fundamental weaknesses in the general class of radiometric dating schemes (or in the particular schemes used in these two studies), why don't the researchers whose results are potentially refuted come forward to publicly identify these weaknesses or flaws? The only believable answer is that there are no fundamental flaws in these schemes -- they have withstood decades of rigorous examination within the scientific community and well deserve their reputation for reliability (although minor adjustments may be made from time to time as they are better understood). For additional discussion, see Conspiracy.
Some valuable (and generally quite readable) references on radiometric dating, including detailed responses to specific issues that have raised by creationists, are the following:
Dalrymple1991;
Dalrymple2004;
Dalrymple2006;
Isaak2007, pg. 143-157;
Miller1999, pg. 66-80; Stassen2005;
Stassen1998;
Wiens2002].
References
[See Bibliography].