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Thus the ratio of stable C-12 to unstable C-14, which is known in today's open environment, changes over time in an isolated specimen. As long as the tree lives, it absorbs carbon from the atmosphere in the form of carbon dioxide, both C-12 and C-14.Once the tree dies, it ceases to take in new carbon, and any C-14 present begins to decay.Though still heavily used, relative dating is now augmented by several modern dating techniques.Radiocarbon dating involves determining the age of an ancient fossil or specimen by measuring its carbon-14 content.Carbon-14, or radiocarbon, is a naturally occurring radioactive isotope that forms when cosmic rays in the upper atmosphere strike nitrogen molecules, which then oxidize to become carbon dioxide.Green plants absorb the carbon dioxide, so the population of carbon-14 molecules is continually replenished until the plant dies.
Rocks and fossils, consisting only of inorganic minerals, cannot be dated by this scheme.
Thus the earth's atmosphere couldn't be any older than this.
Efforts to salvage carbon dating are many and varied, with calibration curves attempting to bring the C-14 "dates" in line with historical dates, but these produce predictably unreliable results.
Carbon-14 has a half-life of 5,730 ± 40 years, meaning that every 5,700 years or so the object loses half its carbon-14.
Samples from the past 70,000 years made of wood, charcoal, peat, bone, antler or one of many other carbonates may be dated using this technique.
Furthermore, the assumptions on which it is based and the conditions which must be satisfied are questionable, and in practice, no one trusts it beyond about 3,000 or 4,000 years, and then only if it can be checked by some historical means.