Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay.
Radioactive decay occurs at a constant rate, specific to each radioactive isotope.
We find the bones of the people who lived and were buried in these tombs. But primarily we date the pyramids by their position in the development of Egyptian architecture and material culture over the broad sweep of 3,000 years.
So we're not dealing with any one foothold of factual knowledge at Giza itself.
Familiar to us as the black substance in charred wood, as diamonds, and the graphite in “lead” pencils, carbon comes in several forms, or isotopes.
One rare form has atoms that are 14 times as heavy as hydrogen atoms: carbon-14, or C ratio gets smaller.
We study the pottery and how it changes over the broad sweep, some 3,000 years.
Obviously, this works only for things which were once living.
Carbon-14 is also passed onto the animals that eat those plants.
After death the amount of carbon-14 in the organic specimen decreases very regularly as the molecules 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.