Radiometric dating - RationalWiki
Radiometric dating breakthroughs by Carl Wieland A few years ago, some The samples were sent (without any hint that it was a creationist project) to a. Radiocarbon dating can easily establish that humans have been on the earth for So, if we measure the rate of beta decay in an organic sample, we can calculate how old the sample is. ICR creationists claim that this discredits C dating. Simply stated, radiometric dating is a way of determining the age of a sample of .. A classic example of this tactic is a claim by creationist geologist Steve Austin .
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.
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.
- How Good Are Those Young-Earth Arguments?
- Answers to Creationist Attacks on Carbon-14 Dating
He begins by identifying three assumptions of the use of radiometric techniques: The system must have been a closed system. The process rate must always have been the same" Morris a, 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 a, Here are the reasons: There is no such thing in nature as a closed system. The concept of a closed system is an ideal concept, convenient for analysis but non-existent in the real world. The idea of a system remaining closed for millions of years becomes an absurdity.
It is impossible to ever know the initial components of a system formed in prehistoric times. Obviously no one was present when such a system was first formed. Since creation is at least a viable possibility, it is clearly possible that some of the "daughter" component may have been initially created along with the "parent" component. Even apart from this possibility, there are numerous other ways by which daughter products could be incorporated into the systems when first formed.
No process rate is unchangeable. Every process in nature operates at a rate which is influenced by a number of different factors. If any of these factors change, the process rate changes. Rates are at best only statistical averages, not deterministic constants. Morris a, 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.
How Good are those Young-Earth Arguments: Radiocarbon Dating
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.
Refuting “Radiometric Dating Methods Makes Untenable Assumptions!”
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. Clearly, it is required only that D0 be known, not that it be zero. 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 4from which rock ages can be computed, I employed equation 1the 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.
Extensive attempts to modify these rates under a variety of physicochemical conditions have produced no effects. 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 a, 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: However, argon 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 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.
Creationists Blind Dates
The picture thus presented is that radiometric dating methods compute the ages of rocks by applying equation 4assuming 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? 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.
Radiometric dating requires that the decay rates of the isotopes involved be accurately known, and that there is confidence that these decay rates are constant. Fortunately, this is the case. The physical constants nucleon masses, fine structure constant involved in radioactive decay are well characterized, and the processes are well understood. Careful astronomical observations show that the constants have not changed significantly in billions of years—spectral lines from distant galaxies would have shifted perceptibly if these constants had changed.
In some cases radioactive decay itself can be observed and measured in distant galaxies when a supernova explodes and ejects unstable nuclei. Indirect observations can allow us to infer radioactive decay rates over time scales that are quite long. For example, we can measure gamma radiation rates at specific frequencies from distant supernovae and compare this to the rate expected for the mass of the star.
This has given rates for supernovae as distant aslight years which are consistent with those measured today. Thus it would seem decay rates have been the same for at least the pastyears.
Some people, perhaps in support of a Creationist viewpoint, have suggested that decay rates have changed significantly because "energy levels" have changed significantly. Whether electron energy levels or nucleon energy levels are being referred to, this is simply not true.
There are a few effects that can alter radioactive half-lives, but they are mostly well understood, and in any case would not materially affect the radiometric dating results. That is, the analysis of the isotopic and chemical composition of the sample has far greater uncertainty than any uncertainty in the decay rate itself.
The major reason that decay rates can change is that the electric field, from the atom's electron cloud, can change due to chemical changes. That is, electrons can move closer to or farther away from the nucleus depending on the chemical bonds.
This affects the coulomb barrier involved in Alpha decayand therefore changes the height and width of the barrier through which the alpha particle must tunnel.
The effect of this on alpha decay, which is the most common decay mode in radiometric dating, is utterly insignificant. There is another effect that takes place in the "electron capture" type of Beta decay. This is an example of the Weak forceand is fairly rare.
Electron capture requires that there be an electron in the vicinity of the nucleus, so its activity depends strongly on the configuration of the electron cloud, which depends on the chemical state. In fact, it is possible to shut down electron capture completely—simply ionize the substance so that there are no electrons nearby.
There is a fairly well-known example of chemical state affecting electron capture activity. The 7Be nucleus Beryllium-7 is an electron capturer with a half-life of about 53 days, turning into Lithium The variation is about 1. While this half-life is way too short to be useful for radiometric dating, the effect of the chemical state is noticeable. The reason is that, because the atomic number is only four, the 2s valence electrons are very close to the 1s electrons involved in capture.
It appears that some radioactive decays are affected the Sun, and fluctuate over a period of about 33 days as the Sun rotates. The variation is about one tenth of a percent. It has been observed in silicon and chlorine While it is not understood why this happens, it would average out over long time periods and therefore not affect the final result. Creationists cast doubt on this analysis, and some apparently believe that decay rates could be so far off by a factor of millions or more that it could explain observations pointing to an old-Earth time-frame while young-Earth cosmology was actually taking place.
They cite large numbers of articles on Creationist web sites. It uses the phenomenon of ionization, described above, to shut down electron capture decay, which could indeed cause a billion-fold discrepancy.
However, this ionization would not have taken place under real-world circumstances. The Walker and Knapp articles refer to the noticeable discrepancies in Beryllium-7, described above. Creationists also suggest that decay rates were almost certainly not constant near the creation or beginning of the universe. However, the billions of years of Uranium decay, for example, did not take place near the beginning of the universe.
See Half-life for an explanation of the exponential decay involved in radioactivity, and the meaning of the term "half-life". The exponential decay pattern is the same for all kinds of nuclear radiation— alpha decaybeta decayand gamma decay. This governs what is known as the "decay rate. This makes different elements useful for different time scales of dating; an element with too short an average lifetime will have too few particles left to reveal much one way or another of potentially longer time scales.