Difference between revisions of "Radiometric dating"

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=== Rate of decay ===
 
=== Rate of decay ===
  
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As far as is known, the rate of decay of a radioactive material is largely constant.<ref>At least one example of a change in the rate has been observed in laboratory experiments.  See ''[http://www.creationontheweb.com/content/view/1608/ Radioactive decay rate depends on chemical environment]''.</ref>  This is based upon knowledge of [[quantum mechanics]] and the structure of the atom.<ref>''Principles of Quantum Mechanics'' page 89</ref><!-- I cannot find any YEC articles that suggest otherwise with any supporting evidence beyond conjecture.  If any change is to be made, please refute with www.talkorigins.org/indexcc/CF/CF210.html www.talkorigins.org/faqs/hovind/howgood-c14.html#R2 -- Furthermore, remember there is more than C14 out there.--> Atoms consist of a heavy central core called the [[nucleus]] surrounded by clouds of lightweight particles (electrons), called [[electron shell]]s. The energy locked in the nucleus is enormous, but fortunately it cannot easily be released. The phenomenon we know as heat is simply the jiggling around of atoms and their components, so in principle a high enough temperature could cause the components of the core to break out. However, the temperature required to do this is in in the millions of degrees, so this cannot be achieved by any natural process that we know about. The second way that a nucleus could be disrupted is by particles striking it. However, the nucleus has a strong positive charge and the electron shells have a strong negative charge. Any incoming negative charge would be deflected by the electron shell and any positive charge that penetrated the electron shells would be deflected by the positive charge of the nucleus itself. Non-charged particles such as neutrons can penetrate to the nucleus and disrupt it (this is how a nuclear bomb works) but such a process will either cause more neutrons to emerge at a rapidly escalating rate&mdash;which would be very obvious to say the least&mdash;or it will fizzle out in a tiny fraction of a second. It thus seems very unlikely that radioactive decay rates can be altered without this being obvious.
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As far as is known, the rate of decay of a radioactive material is largely constant.<ref>At least one example of a change in the rate has been observed in laboratory experiments.  See ''[http://www.creationontheweb.com/content/view/1608/ Radioactive decay rate depends on chemical environment]''.</ref>  This is based upon knowledge of [[quantum mechanics]] and the structure of the atom.<ref>''Principles of Quantum Mechanics'' page 89</ref><!-- I cannot find any YEC articles that suggest otherwise with any supporting evidence beyond conjecture.  If any change is to be made, please refute with www.talkorigins.org/indexcc/CF/CF210.html www.talkorigins.org/faqs/hovind/howgood-c14.html#R2 -- Furthermore, remember there is more than C14 out there.--> Atoms consist of a heavy central core called the [[nucleus]] surrounded by clouds of lightweight particles (electrons), called [[electron shell]]s. The energy locked in the nucleus is enormous, but cannot easily be released. The phenomenon we know as heat is simply the jiggling around of atoms and their components, so in principle a high enough temperature could cause the components of the core to break out. However, the temperature required to do this is in in the millions of degrees, so this cannot be achieved by any natural process that we know about. The second way that a nucleus could be disrupted is by particles striking it. However, the nucleus has a strong positive charge and the electron shells have a strong negative charge. Any incoming negative charge would be deflected by the electron shell and any positive charge that penetrated the electron shells would be deflected by the positive charge of the nucleus itself. Non-charged particles such as neutrons can penetrate to the nucleus and disrupt it (this is how a nuclear bomb works) but such a process will either cause more neutrons to emerge at a rapidly escalating rate&mdash;which would be very obvious to say the least&mdash;or it will fizzle out in a tiny fraction of a second. It thus seems very unlikely that radioactive decay rates can be altered without this being obvious.
  
 
=== Outside influences ===
 
=== Outside influences ===

Revision as of 18:16, June 29, 2007

Radiometric dating is a method of determining the age of an artifact by measuring the amount of radioactive decay that has occurred.[1] Radiometric dating is mostly used to determine the age of rocks, though a particular form of radiometric dating - called Radiocarbon dating - can date wood, cloth, skeletons, and other organic material. It relies on the well-established function for exponential decay <math>F</math><math>t</math><math>=e</math><math>-kt</math>.

Principles

The basic principle in any dating method is to find a process that is occurring at measurable rate and which is causing a change, measure the rate of that process, work out what state the artifact was in at the beginning of the process, observe what state it is in now, and to calculate how long the process at the measured rate would need to occur to effect that change.

For example, to work out how long a candle has been burning, the following steps would be needed:

  1. Measure how long it takes the candle to burn down a given amount.
  2. Find out how long the candle was when it started burning.
  3. Measure how long the candle is now.
  4. Calculate the difference between the two lengths.
  5. Calculate how long it would need to burn in order to burn that length.

For most radiometric dating methods, one radioactive element changes by a process of nuclear decay into another element (often through a number of intermediate steps. For example, uranium will eventually decay into lead. So to measure how old a specimen containing some uranium and some lead is, the following steps are required:

  1. Measure the decay rate of uranium.
  2. Find out how much uranium was in the specimen to start with (this might be done by assuming that all the lead was originally uranium).
  3. Find out how much uranium is in the specimen now.
  4. Calculate how much uranium has turned into lead.
  5. Calculate how long it would take that much uranium to turn into lead, given the measured rate.

Key assumptions

There are a number of assumptions involved in radiometric dating. They are assumptions because they are unable to be proved one way or the other.

Initial quantities

One key assumption is that the initial quantity of the parent element can be determined. With uranium-lead dating, for example, you must be able to determine how much uranium was in the sample to start with. One assumption that can be made is that all the lead in the sample was once uranium, but if there was lead there to start with, this assumption is not valid, and any date based on that assumption will be incorrect (too old).

In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C14 to C12, but the same principle otherwise applies.

Rate of decay

As far as is known, the rate of decay of a radioactive material is largely constant.[2] This is based upon knowledge of quantum mechanics and the structure of the atom.[3] Atoms consist of a heavy central core called the nucleus surrounded by clouds of lightweight particles (electrons), called electron shells. The energy locked in the nucleus is enormous, but cannot easily be released. The phenomenon we know as heat is simply the jiggling around of atoms and their components, so in principle a high enough temperature could cause the components of the core to break out. However, the temperature required to do this is in in the millions of degrees, so this cannot be achieved by any natural process that we know about. The second way that a nucleus could be disrupted is by particles striking it. However, the nucleus has a strong positive charge and the electron shells have a strong negative charge. Any incoming negative charge would be deflected by the electron shell and any positive charge that penetrated the electron shells would be deflected by the positive charge of the nucleus itself. Non-charged particles such as neutrons can penetrate to the nucleus and disrupt it (this is how a nuclear bomb works) but such a process will either cause more neutrons to emerge at a rapidly escalating rate—which would be very obvious to say the least—or it will fizzle out in a tiny fraction of a second. It thus seems very unlikely that radioactive decay rates can be altered without this being obvious.

Outside influences

It is important that the sample not have had any outside influences. One example of this can be found in metamorphic rocks.[4] This does not mean that all rock samples are unreliable, but it is possible to account for a process which throws off the data for metamorphic rocks.

For example, with Uranium-lead dating with the crystallization of magma, this remains a closed system until the uranium decays. As it decays, it disrupts the crystal and allows the lead atom to move. Likewise, heating the rock such as granite forms gneiss or basalt forms schist. This can also disrupt the ratios of lead and uranium in the sample.

Calibration

In order to calibrate radiometric dating methods, the methods need to be checked for accuracy against items with independently-known dates.

This is a problem because most of the dates of artifacts are only known from radiometric dating methods.

Carbon dating, with it's much lower maximum theoretical range, is often used for dating items only hundreds and thousands of years old, so can be calibrated in its lower ranges by comparing results with artifacts who's ages are known from historical records.

Scientists have also attempted to extend the calibration range by comparing results to timber which has its age calculated by dendrochronology, but this has also been questioned because carbon dating is used to assist with working out dendrochronological ages.

Otherwise, calibration consists of comparing results with ages determined by other radiometric dating methods.

However, tests of radiometric dating methods have often shown that they do not agree with known ages of rocks that have been seen to form from volcanic eruptions in recent and historic times, and there are also examples of radiometric dating methods not agreeing with each other.

Young-Earth creationists therefore claim that radiometric dating methods are not reliable and can therefore not be used to disprove Biblical chronology.

Some major methods of radiometric dating

There are several major types of radiometric dating in use:[5],[6]

  1. Radiocarbon dating, also called carbon dating
  2. Potassium-argon dating
  3. Uranium-lead dating
  4. Uranium-thorium
  5. Rubidium-strontium dating

References

  1. ↑ Radiometric Time Scale USGS
  2. ↑ At least one example of a change in the rate has been observed in laboratory experiments. See Radioactive decay rate depends on chemical environment.
  3. ↑ Principles of Quantum Mechanics page 89
  4. ↑ Radiometric Dating Course notes for EENS 211 at Tulane University
  5. ↑ Quarternary Dating Methods, by M. Walker (Wiley & Sons, 2005).
  6. ↑ Isotopes: Principles and Applications, by G. Faure and T. Mensing (Wiley & Sons, 2005).
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