The carbon-14 clock does not start when an organism is born but the moment it dies and stops exchanging carbon with the atmosphere. From then on the surviving nuclei follow N = N₀·(½)^(t/T½) with a half-life of 5,730 years, and after about ten half-lives too little C-14 remains to measure, which is why the method stops near 50,000 years. Dating rock takes a different isotope: uranium-238 decaying to lead-206 with a 4.47-billion-year half-life inside zircon, the very pair that puts Earth’s age at 4.54 billion years.
The carbon-14 clock does not start when an organism is born but the moment it dies and stops exchanging carbon with the atmosphere. From then on the surviving nuclei follow N = N₀·(½)^(t/T½) with a half-life of 5,730 years, and after about ten half-lives too little C-14 remains to measure, which is why the method stops near 50,000 years. Dating rock takes a different isotope: uranium-238 decaying to lead-206 with a 4.47-billion-year half-life inside zircon, the very pair that puts Earth’s age at 4.54 billion years.
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