Unlike observation-based relative dating, most absolute methods require some of the find to be destroyed by heat or other means.

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It would be like having a watch that told you day and night.” Single crystal fusion: Also called single crystal argon or argon-argon (Ar-Ar) dating, this method is a refinement of an older approach known as potassium-argon (K-Ar) dating, which is still sometimes used.

Both methods date rock instead of organic material. But unlike radiocarbon dating, the older the sample, the more accurate the dating — researchers typically use these methods on finds at least 500,000 years old.

When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.

The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.

Researchers can measure the amount of these trapped electrons to establish an age.

But to use any trapped charge method, experts first need to calculate the rate at which the electrons were trapped.

Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.

Layers of rock build one atop another — find a fossil or artifact in one layer, and you can reasonably assume it’s older than anything above it.

Sometimes only one method is possible, reducing the confidence researchers have in the results. “They’re based on ‘it’s that old because I say so,’ a popular approach by some of my older colleagues,” says Shea, laughing, “though I find I like it myself as I get more gray hair.” Kidding aside, dating a find is crucial for understanding its significance and relation to other fossils or artifacts.

Methods fall into one of two categories: relative or absolute.

Paleomagnetism: Earth’s magnetic polarity flip-flops about every 100,000 to 600,000 years.