Along the way, Libby realized, the cascade of interactions creates a trickle of carbon-14 (C14), a mildly radioactive form of carbon that over time disintegrates—decays, as scientists say—into a form of nitrogen. Libby determined that the rate at which cosmic rays create C14 is roughly equal to the rate at which it decays. As a result, a small but steady percentage of the carbon in air, sea, and land consists of C14. Plants take in C14 through photosynthesis, herbivores take it in from the plants, and carnivores take it in from them. In consequence, every living cell has a consistent, low
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