How is Dating Performed?

• Universe: assume Big Bang, measure Hubble constant

• Earth: radiometric dating - decay of one radioactive isotope to another at known rate; works on igneous rocks (lava/granite) but not sedimentary rocks

• Earth/solar system: radiometric dating of meteorites

• Earth: global processes (oceans, atmosphere, magnetic field, erosion of continents, etc.)

• Sedimentary rock: assume evolution has occurred, look for presence of specific "index fossils"

• Recent history, once living things: carbon-14

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The dating of the universe is performed based on the Big Bang assumption that the universe is expanding, or inflating from a single point. Hubble's law (velocity = Hubble constant X distance) combined with the Doppler effect (the velocity-dependent shift in receding light towards the red end of the spectrum) are used to estimate how long the inflation of the universe has been taking place. The difficulty is that both the velocity and distance of far away objects are hard to measure accurately since local motions of stars and galaxies must be accounted for.

Radiometric rating methods such as uranium-lead or potassium-argon use the known decay rate of one radioactive isotope into another as the basis for measuring age. These methods assume that the initial ratios of the two isotopes can be estimated, and that the sample being measured has been undisturbed so that none of either the parent or daughter isotope has been added to or subtracted from the sample. Dating of meteorites is assumed to give the age of our solar system (and therefore also the age of the formation of the Earth).

Before the advent of radiometric dating, global processes were used to estimate an age for the Earth, such as the flow of various mineral sediments into the ocean, the cooling of the Earth or Sun, or the radioactive decay of elements in the Earth's crust. These typically gave values in the millions to a few billions of years. None of these estimates are regarded today as reliable, compared with radiometric dating.

Sedimentary (water deposited) rocks are dated by looking for the presence of so-called "index fossils" of organisms that supposedly lived only during a certain period of evolutionary time.

Carbon-14 dating is only valid for ages up to 30,000 years because of its short half-life, and is used to measure the age of once-living things, but not the age of the Earth.