Bible Chronology
Radio-Carbon Dating
The basic principle of radio-carbon dating - also known as Carbon-14 dating - are well known. Radioactive elements decay - that, after all, is what radio-activity is! Because of an imbalance in the number of protons to electrons, the atom is unstable and soon or later it will try to balance itself by expelling an alpha particle (made up of two protons and two neutrons), a beta particle (a fancy name for an electron) or gamma radiation, which is just pure energy.
Unfortunately that does not always result in a stable atom, just a different kind of atom. The new element will also go through this process of expelling alpha or beta particles or gamma radiation until finally, at the bottom of the chain, the atom settles down into one of four kinds of lead. (This is, of course, a very simplified description of a highly complex process.)
Unfortunately there is no known way of predicting when any particular atom will explode like this; what we can say is that in any cluster of atoms approximately half of them will have emitted particles after a certain length of time. This length of time, which is different for each element, is known as the "half-life". Some half-lives are measured in milliseconds, some in days or months, some in years or even tens of thousands of years.
The stable form of carbon is carbon-12; the unstable form is created as a result of cosmic radiation and is known as carbon-14. The process of creating carbon-14 and the decay of carbon-14 means that we have a balance of a certain proportion of c-14 to c-12 in the world - in the air and water. That proportion is believed to be one atom of c-14 per trillion atoms of c-12.
All living things contain carbon and obviously the carbon we contain has the same proportion of c-12 to c-14 as everywhere else in the world. When a living organism dies, however, it ceases to take in carbon and the c-14 it already contains continues to decay. That means that after a while the proportion of c-14 to c-12 in the dead organism is less than is found in the world at large. Because we know what the proportion should be, we can calculate how long ago the organism was alive.
To take a very simple example: the half-life of carbon-14 is 5730 years (plus or minus 40), so if you found a piece of wood with exactly half the normal amount of c-14, you would know that the wood was exactly 5730 years old.
When Dr Libby discovered this method of dating he at first thought that age determination was a simple matter of measuring the proportion of c-14 to c-12, and dividing by the half-life. He quickly discovered, however, that when he tried this on samples from the real world the dates he produced differed wildly from those accepted by the historians. He and others then realised that in fact the proportion of c-14 varies significantly over time; a nearby supernova can bombard the earth with cosmic rays and increase the production of c-14 or the earth's magnetic field can strengthen temporarily and decrease the production of c-14.
Another source of variation which will cause problems for archaeologists of the future is the atmospheric tests of nuclear weapons, which have almost doubled the concentration of c-14, particularly in the northern hemisphere. Curiously, the burning of fossil fuels has also increased the level of c-14 in the air. In theory foil and coat were laid down so long ago that all the carbon-14 in the original plants should have decayed away to nothing. In practice, however, there are still traces of c-14 in oil and coal. Scientists attribute this to later contamination, but of course it could be an indication that coal and oil were laid down more recently than is commonly believed.
It was also discovered that the proportion of c-14 in living creatures varies from species to species. For example, there are certain sea creatures which have so much c-14 in them that they appear to come from the future!
The result was the Dr Libby had to draw up a table of corrections for different dates and different creatures. Initially this table was based on the chronology of Egypt: Dr Libby was given artefacts to test whose age was "known". He produced a radioactive age and adjusted it to match the age told to him by the Egyptologists. This adjustment was then used in all subsequent radioactive ages of that date.
Since then considerable work has been done to perfect this table of corrections. Dendro-chronology has allowed some refinement, but in a way that is a circular argument because frequently the age of a particular piece of wood has been first determined by c-14 dating!
As I understand it, Professor Lord Colin Renfrew is arguing for greater reliance to be placed on the raw carbon-14 dates rather than the adjusted dates from the table of corrections.
