The role of an archaeologist is to uncover the truths about past civilisations. When an artefact is discovered, they must piece together how it fits into history; where it is found, when it was made, what it was used for. In this essay, I will be investigating the various ways in which physical methods are used to aid this process in order for us to learn about our history and past cultures.
Estimating the age of a fossil was a difficult task before the 1950s. Techniques such as comparing the typology of pottery [1] could be unreliable and provide time scales that vary over thousands of years. Moreover, absolute dating techniques like dendrochronology — using tree ring cross-sections to determine the age of wood— despite being more precise, require a substantial piece of wood with many years worth of rings to be effective. This means that age determination was largely inaccurate as dates were more likely to be based on estimates rather than facts. However, in 1949, this changed due to the discovery of radiocarbon by W. F. Libby while he was researching neutron production, for which he later received a Nobel Prize. He found that cosmic rays from outside of the solar system were colliding with atoms in the earth’s upper atmosphere to form secondary showers of particles, including energetic neutrons at a rate of 2 neutrons produced per second per square centimetre. [2] These neutrons would then go on to collide with atoms of Nitrogen-14 in the air, resulting in neutron capture and forming the radioactive isotope of carbon-14 and a hydrogen atom, as shown in equation (1):
(1)
These carbon-14 atoms combine with oxygen in the usual way to form ‘heavy’ carbon dioxide, which mixes with the CO2 containing the more common isotope of carbon-12 and is distributed randomly throughout the atmosphere. In the carbon cycle, CO2 enters plants through photosynthesis and enters animal bodies through ingestion when the plants are eaten, meaning all living things contain a small fraction of carbon-14 and therefore are slightly radioactive. If we assume this process has been happening for thousands of years (longer than the half-life of C-14 which is 5730±40yrs [3]) and that the rate of cosmic showers has stayed the same throughout time, we can assume that the percentage of C-14 in the carbon cycle has also remained constant. Libby found these assumptions to be the case [2] and so concluded that plants and animals contain the same percentage of C-14 as the atmosphere (one for every million atoms of C-12 [5]). When a plant or animal dies it becomes cut off from the carbon cycle so that while the C-14 continues to decay, it cannot be replenished. This means that if we measure the amount of radiation from fossils we can calculate how much C-14 has decayed over time and find how old it was when it died, giving an absolute and accurate time frame for a find.
While radiocarbon dating is indubitably very useful, it has several drawbacks. Firstly, it cannot be applied to inorganic material that does not play a part in the carbon cycle such as pottery or stone tools. Organic matter older than 60,000 years old can also not be dated using this method as there an insufficient amount of carbon-14 left in the sample to provide an accurate reading. However, another example of the importance of nuclear radiation in dating is thermoluminescence, a technique that, unlike radiocarbon, can be applied to minerals such as flint tools used by Palaeolithic man dating back to 500,000 years ago.[9] This technique is developed around luminescence, which is a phenomenon exhibited by some crystalline materials when they release stored energy in the form of light.
Radioactive impurities in a crystal can generate defects in the structure, which can cause negative ions to be absent from their original places. In these areas, there are negative ion vacancies that are called luminescence centres or ‘electron traps’ due to the deficit of negative charge.[9] When the impurities decay and give off radiation, it is absorbed by the material itself which excites electrons to ionisation and leaves them able move around the structure to diffuse into the traps where they remain stuck in a potential well. The depth of this potential is a measure of how easily the electron can escape, so small potential wells give an electron a high probability of escaping naturally, whereas deep wells may be capable of trapping electrons for millions of years. When heated to temperatures of around 500C, all trapped electrons gain enough energy to oscillate out of these potential wells and are free to move around the structure. They will then recombine with luminescence centres and as they fall back down the energy levels they emit this energy as light. The amount of light given off is proportional to the number of trapped electrons, and the number of trapped electrons is proportional to the amount of radiation they have been exposed to, and so the amount of time since the traps were last emptied by heating. This means that if immediately reheated, the sample will not give off an emission because the “thermoluminescence clock” has been reset from the first heating. Thermoluminescence dating is most useful for artefacts which were exposed to high temperatures the past, such as ceramic pottery which has been baked in a kiln, or metal weapons forged with fire, because the act of heating will have caused electron traps to empty, and then fill again over time. If you can measure the rate of acquisition of the stored energy, and establish a relationship between absorbed energy and emitted light, then measuring the light output of the sample allows us to measure the amount of time the sample has been buried, allowing accurate dating of materials.
While the ideas of atomic theory and nuclear physics have revolutionised age determination in archaeology, other concepts from physics are routinely employed to develop different areas. For example, Ground Penetrating Radar (GPR) is a method of data collection that uses radar waves to form a high definition spacial map of the ground, allowing archaeologists to discover and plot archaeological features before they begin an excavation. This works by applying an oscillating electric current to an antenna and producing an oscillating magnetic field (from Ampere’s law) causing electromagnetic waves to propagate perpendicularly to both the magnetic and electrical components. If either of these two components were to be absorbed in the ground then the wave would cease its propagation. Electric currents that oscillate at high frequencies produce waves with short wavelengths, while lower frequencies correspond to longer wavelengths, like the radio waves used in GPR. Discontinuities in the earth will cause the waves to partially be reflected back up to the surface to be detected again while the rest will eventually dissipate into the earth. Frequencies can be adjusted if necessary, as longer wavelengths usually propagate deeper into the ground and are reflected less off small objects, while shorter wavelengths are more easily attenuated. [6] The relative dielectric permittivity of the soil is an important factor to consider, as it relates to the speed at which the waves propagate underground in equation (2):
(2) where c=0.3m ns-1 is the speed of light and v is the wave’s velocity. This dielectric constant is not only related to the wave velocity, but it also determines the amplitude of the waves which are reflected. Wave reflections occur when changes in the dielectric constant occur over a short distance, like a boundary between two materials, which is how ruins are detected underground. The amplitudes of the reflected waves can be calculated with equation (3):
(3)
where R is the coefficient of reflectivity at the interface between two materials with different values for . If the coefficient is negligible, this means there are weakly reflected waves because the dielectric constants between two surfaces are so similar that most of the wave is transmitted and not reflected. The relative dielectric permeability of the soil can often vary with depth [7] which needs to be taken into account when calculating the distance of objects below the surface of the ground as it can change the speed at which the waves have travelled, but the changes in the dielectric constant of the soil are not abrupt and since the changes are small every few centimetres, then the reflected waves will be very weak or won’t exist. However, reflected waves detected with high amplitudes must have been produced at an interface where the coefficient of the soil is very different to the object the wave is reflecting off and could indicate something significant buried underground.
Magnetometry is another technique used to detect buried features derived from concepts in electromagnetism. It works by measuring the magnetism of the ground at certain points and comparing it to other samples to check for abnormalities. As Aspinall, Gaffney and Schmidt explain [8] soil is rich in iron oxides, the three most common being hematite (-Fe2O3) which is anti-ferrimagnetic so has low magnetic susceptibility, and magnetite (Fe3O4) and maghemite (-Fe2O3) both of which are ferrimagnetic and so have high magnetic susceptibility. There are multiple ways in which past human activity can alter magnetism on earth, one of which is fire. Burning has long been associated with human occupation, and fire creates a reducing atmosphere with a deficiency of oxygen so under these conditions with a temperature over 200C the hematite iron oxides in the soil are reduced, and form magnetite. When the fire is out and oxygen becomes available this can then re-oxidise to form maghemite, which means that the majority of the soil is made up of ferrimagnetic material with high magnetic susceptibility. Bacteria is another cause of this; micro-organisms are found everywhere, but especially on or near organic waste which is another indicator of human activity, and they create the reducing and oxidising environment necessary for decay to take place which also happens to increase the magnetic susceptibility of the soil. Whenever a ditch or a pit is filled with soil that is magnetically enhanced, the difference between magnetic susceptibilities of this soil compared to the surrounding soil causes induced magnetisation, and this magnetic field can be easily measured on the surface. Therefore, in areas where we find these abnormalities in the soil, it is likely that the land was used for human habitation in the past, and is an indicator as to where archaeologists should continue to search for artefacts.
In conclusion, modern developments in physics have led to groundbreaking applications in archaeological fields. Physicists have developed the tools with which archaeologists have created a reliable timeline of events through history, and which have allowed new methods of discovering ancient ruins and artefacts. There is little doubt that the physical sciences will play a vital role in the future in discovering more about our ancestor’s passage through time, and improvements in methodology will continue to increase the speed and accuracy of our findings.
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