To illustrate, suppose there is a burning candle sitting on the table. This can be calculated if the candles burn rate and original length is known.However, if the original length is not known, or if it cannot be verified that the burning rate has been constant, it is impossible to tell for sure how long the candle was burning.The most commonly used radiometric dating methods are potassium-argon, uranium-lead, and rubidium-strontium. Half-life is simply the time required for half of the atoms in a pound of uranium, for example, to disintegrate into lead.The concept of how these methods work is simple: one element decays into another at a rather predictable rate. If these assumptions are correct, then the radiometric dates are correct.The geochronologist must also be sure that the rate of decay, from uranium to lead for example, has remained constant in the rock over the past 4.5 billion years.When dating a rock, the geochronologist (scientist who performs the dating procedure) must first assume the rocks age before it is dated.For example, if a scientist believes a piece of rock is 4.5 billion years old, he or she may then use the uranium-lead dating method because it has a half-life of about 4.5 billion years.However, there is no way to independently test these assumptions.
This involves circular reasoning, as is clearly evident in the article on dating in the Encyclopedia Britannica: Most geologists must rely on geochronologists for their results.
In turn, the geochronologist relies on the geologist for relative ages.
Francis Crick (co-discoverer of one of the most important discoveries of 20th century biology) arrived at the theory that life could never have evolved by chance on planet earth."One need only look in virtually any reference text to quickly find that the earth is thought to be some 4.5 billion years old.
As the Encyclopedia Britannica notes, methods such as measuring radioactive decay (radiometric dating) make it possible to estimate the time period when earths rocks and associated fossils were formed. All three of these decay processes have half-lives measured in billions of years.
A similar problem occurs with radiometric dating of rocks.Since the initial physical state of the rock is unknowable, the age can only be estimated according to certain assumptions.