By comparing how much carbon-14 there is in the dead organism with the amount in a living one, the age of the dead organism can be estimated.The half-life of uranium-238 is 4500 million years.Prior to 1905 the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state.Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential (Energy) barrier which bonds them to the nucleus.It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.
His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.It might take a millisecond, or it might take a century. But if you have a large enough sample, a pattern begins to emerge.It takes a certain amount of time for half the atoms in a sample to decay.The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.When it decays it forms thorium-234 which is also unstable.