How is physics used in archaeology?

Archaeology is an interdisciplinary science, and much of it – from establishing the ages of artefacts to mapping a site before excavating it – makes use of physics.

Dating
Not the saucy kind of dating. The ages of things of interest to archaeologists – including royal bones – can be estimated from the proportion of carbon-14 they contains.

The radioactive isotope of carbon exists naturally in the atmosphere, and is absorbed by plants at a predictable rate. While an organism remains alive, the ratio of carbon-14 to regular carbon-12 within it will stay roughly constant.

After it dies, the carbon-14 will undergo radioactive decay, and, without any way of replacing it, the proportion of it will decrease. Measuring the ratio of the two isotopes, and knowing carbon-14’s half-life, allows archaeologists to estimate the date on which a living organism died.

Another form of spectrometry has been used since the 1970s to improve the accuracy of radiocarbon dating even further. Accelerator mass spectrometry, a technique taken from nuclear physics, accelerates ions to high speed before analysing their mass and can count individual carbon-14 atoms. This enables very small items, or tiny parts of valuable objects, to be dated – and it’s how the Turin Shroud was proven to be from the 13th century rather than contemporaneous with Jesus.

The use of superconductivity has also been introduced to date lead. At temperatures of below 7.2 K, lead becomes a superconductor, but its magnetisation changes depending on how corroded it’s become. Because lead corrodes at a predictable rate, its age can be estimated.

What lies beneath
Physics can be used to see through the ground to buried archaeological sites.

The obvious example is using metal detectors to find objects such as coins buried beneath the soil, though they’re of more use to treasure-hunters than scientists.

Other electromagnetism-based methods are more useful. One involves measuring the electrical resistivity of the surface at various points and producing a map. Higher-than-average readings might suggest the presence of a building’s foundation stone, whereas lower-than-average resistivity is associated with organic deposits.

Magnetometers are also used to map a site’s magnetic features. A magnetic survey of the now largely buried Roman town of Viroconium Cornovinium, near present-day Shrewsbury, revealed buildings and a network of roads.

Ground-penetrating radar can also be used to build an image of whatever’s beneath the ground – objects and geological layers reflect the radio-wave signal, and depth can be determined by the time-delay involved. However it’s severely limited in soils with high conductivity, such as clay.

Uncovering even more.
Once artefacts have been found in an archaeological site they can be examined to find out more about their composition. Some of this is carried out using x-rays, which can spot features that are difficult to see in visible wavelengths. They’ve previously been used to find hidden seams in fabrics – and even contraband concealed within.

Researchers use related techniques to try to work out people looked like. CT scans of skulls create a digital model, and knowing how skeleton shapes tend to map to facial features allows faces to be reconstructed. 

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