Those people who know me will tell you that I hate traipsing round museums. I have had two brilliant jobs at museums (the V & A and the Science Museum) but I would much rather read about something in a book, in fact I have been known to buy the museum guide without actually going round the museum.
However I did make an exception when the British museum had an exhibition about Egyptian mummies because physics was involved.
A mummy is a deceased human or animal whose skin and organs have been preserved by either intentional or accidental exposure to chemicals, extreme cold, very low humidity, or lack of air, so that the recovered body does not decay further if kept in cool and dry conditions. Some authorities restrict the use of the term to bodies deliberately embalmed with chemicals, but the use of the word to cover accidentally desiccated bodies goes back to at least 1615 AD.
Mummies of humans and other animals have been found on every continent, both as a result of natural preservation through unusual conditions, and as cultural artefacts. Over one million animal mummies have been found in Egypt, many of which are cats.
The exhibition looked at eight mummies in detail, a Christian woman from the Sudan, a young child from the Roman period, an unusual mummy from the Roman period, a young temple singer, a temple doorkeeper, a priest’s daughter, a man embalmed for the afterlife, and a man preserved in the sand.
The British museum staff used the most up-to-date technology to investigate the structure of each mummy. The advantage of this technology is that you don’t have to make any physical changes to the mummy to find out what is inside it.
Physics has played a considerable part in archaeology for over a hundred years. The first use is probably radiometric dating. Ernest Rutherford invented it as a method of determining the age of the Earth.
Ernest Rutherford, 1st Baron Rutherford of Nelson, OM FRS (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist who became known as the father of nuclear physics. Encyclopædia Britannica considers him to be the greatest experimentalist since Michael Faraday (1791–1867).
Dating of organic matter such as dead things can be done with radiocarbon, a radioactive isotope of carbon. The method was invented by Willard Libby in the late 1940s.
Willard Frank Libby (December 17, 1908 – September 8, 1980) was an American physical chemist noted for his role in the 1949 development of radiocarbon dating, a process which revolutionized archaeology.
Radiocarbon dating depends on the fact that radiocarbon, or carbon 14, is constantly being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting radiocarbon combines with atmospheric oxygen to form radioactive carbon dioxide. This is then incorporated into plants by photosynthesis, and animals acquire carbon 14 by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point the amount of carbon 14 it contains begins to reduce as the carbon 14 undergoes radioactive decay. Measuring the amount of carbon 14 in a sample from a dead plant or animal such as piece of old wood or a fragment of bone (or a mummy) provides information that can be used to calculate when the animal or plant died. The oldest dates that can be reliably measured by radiocarbon dating are around 50,000 years ago, though special preparation methods occasionally permit dating of older samples.
The equation for the radioactive decay of carbon 14
The equation governing its decay is
Where N is the number of carbon 14 atoms now, N0 is the number of carbon 14 atoms t years ago and l is a constant. Knowing the constant l, N0 and N you can work out the time t.
Other forms or radio dating include:
Dendrochronology — for dating trees; also very important for calibrating radiocarbon dates
Thermoluminescence dating — for dating inorganic material (including ceramics)
Optically stimulated luminescence (OSL)/optical dating — for absolutely dating and relatively profiling buried land-surfaces in vertical and horizontal stratigraphic sections, most often by measuring photons discharged from grains of quartz within sedimentary bodies (although this technique can also measure feldspars, complications caused by internally induced dose-rates often favour the use of quartz-based analyses in archaeological applications)
Electron spin resonance, as used (for example) in dating teeth
Potassium-argon dating — for dating (for example) fossilized hominid remains by association with volcanic sediments (the fossils themselves are not directly dated).
Mass spectrometry is used to find the chemical composition of any remains.
Provided the atom is first turned into an ion, becoming an electrically charged particle, it will be affected by a magnetic field.
The ions are accelerated by an electric field so that they all have the same kinetic energy. They are deflected by the magnetic field and the amount of deflection depends on the mass of the ion and the amount of charge.
Lighter ions are deflected more than heavier ones and ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.
The amount of deflection can allow the original atom to be identified.
Isadore Perlman and Frank Asaro in the mid-1960s realised that they could use the gamma ray emissions of trace radioactive elements in a pottery sample to measure very accurately the abundances of elements in the sample, pinning the sample’s origin to a unique location where the materials were quarried or dug.
Researchers from Israel have developed a new way to date archaeological objects that is based on superconductivity. The new technique relies on measuring the magnetic signal from lead – which was widely used in antiquity – in samples that have been cooled to cryogenic temperatures. The method could be used to date pipes, coins, bottles and other objects (S Reich et al. 2003 New J. Phys 5 99)
Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden.
Heike Kamerlingh Onnes (21 September 1853 – 21 February 1926) was a Dutch physicist and Nobel laureate.
Archaeologists also use physics to locate artefacts.
Magnetometers are measurement instruments used for two general purposes: to measure the magnetization of a magnetic material like a ferromagnet, or to measure the strength and, in some cases, the direction of the magnetic field at a point in space.
Magnetometers can be used as metal detectors: they can detect only magnetic (ferrous) metals, but can detect such metals at a much larger depth than conventional metal detectors; they are capable of detecting large objects, such as cars, at tens of metres, while a metal detector’s range is rarely more than 2 metres. Magnetometers can also be used to map an archaeology site’s magnetic features.
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.
Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can be used in a variety of media, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and voids and cracks.
So how does physics tell us what makes up a mummy?
The first modern scientific examinations of mummies began in 1901, conducted by professors at the English-language Government School of Medicine in Cairo, Egypt. The first X-ray of a mummy came in 1903 when professors Grafton Elliot Smith and Howard Carter used the only X-ray machine in Cairo, at the time, to examine the mummified body of Tuthmosis IV.
Howard Carter (9 May 1874 – 2 March 1939) was an English archaeologist and Egyptologist who became world famous after discovering the intact tomb of 14th-century BC pharaoh Tutankhamun (colloquially known as “King Tut” and “the boy king”) in November 1922.
In 1928, the Field Museum X-ray photographed the mummy of a 7th century B.C. Egyptian boy — based on the irregular development of some of his bones, the researchers surmised that the child had a calcium deficiency and was malnourished.
Professor Rosalie David has spent over forty years investigating mummies and her use of X rays has enabled her to identify diseases and other health problems such as dental attrition (wearing away of the teeth) caused by eating bread with bits of stone in it.
In 1975 radiography done by Professor David showed evidence of Guinea worm infestation in a mummy by revealing the calcified remains of a male Guinea worm in its abdominal wall. Her lower legs had been amputated probably about two weeks before death, possibly because they were ulcerated as the result of trying unsuccessfully to extract the female Guinea worms which sometimes try to break out and emerge through the skin on the legs.
The investigation of mummies has gone even further since the advent of X-ray computed tomography (CT scans).
X-ray computed tomography (x-ray CT) is a technology that uses computer-processed x-rays to produce tomographic images (virtual ‘slices’) of specific areas of the scanned object, allowing the user to see what is inside it without cutting it open. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional radiographic images taken around a single axis of rotation. Medical imaging is the most common application of x-ray CT. Its cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines.
X-ray slice data is generated using an X-ray source that rotates around the object; X-ray sensors are positioned on the opposite side of the circle from the X-ray source.
CT scans of mummies have revealed a number of other interesting finds such as the fact that ancient Egyptians suffered from atherosclerosis (which is typically considered a modern disease attributed to a sedentary lifestyle).
Atherosclerosis (also known as arteriosclerotic vascular disease or ASVD) is a specific form of arteriosclerosis in which an artery wall thickens as a result of invasion and accumulation of white blood cells (termed “fatty streaks” early on because of appearance being similar to that of marbled steak) and containing both living active WBCs (called inflammation) and remnants of dead cells, including cholesterol and triglycerides, eventually calcium and other crystallized materials, within the outer-most and oldest plaque. These changes reduce the elasticity of the artery walls but do not affect blood flow for decades because the artery muscular wall enlarges at the locations of plaque. However, the wall stiffening may eventually increase pulse pressure and cardiovascular disorders such as stroke or heart attack can occur.
Within the last decade, scientists have begun using MRI machines on mummies. The technique can typically only be used on hydrated tissues, but that limitation apparently wasn’t a problem. Last year, researchers examined a dry Egyptian mummy using a newly developed technique for MRI scans and found evidence of the rare Hand-Schueller-Christian’s disease.
The CT scans of the eight mummies found the following information:
The Christian woman was about 158cm tall; her skeleton was almost intact and most of her internal organs were preserved. The state of her bones indicated she was in her twenties or thirties. Her teeth showed moderate wear with some of her teeth missing.
The young child still had his milk teeth and his permanent teeth were only half-formed. Most of his internal organs were removed but his brain was not. The cause of his death could be ascertained.
The unusual mummy was about 167cm tall and had terrible dental health. Teeth were worn down or missing. He also had severe dental disease with five abscesses present. The mummy was conventionally preserved and the CT scans showed how the body was fashioned to look life-like.
The temple singer was shown to be a child of about seven years of age and that the body was remarkably preserved. Her permanent teeth were only just coming out when she died. She was about 125cm when she died but her cause of death could not be ascertained.
The temple doorkeeper’s skeleton showed wear-and-tear and the joints of his pelvis indicated that he was at least thirty five when he died. He was about 170cm and he had poor dental health, with dental abscesses. He also had atherosclerosis. The CT scan also shows that the head became detached, probably during the embalming process and was re-attached using rods. It also showed he was too tall for his coffin.
The priest’s daughter was difficult to image due the presence of a metal covering over her pelvis. She suffered from dental attrition, was about 157cm long and she wore her hair short. The state of her teeth indicated she was older than 30 when she died. She had an abscess and was suffering from atherosclerosis. The CT scan showed she was very carefully embalmed.
The middle-aged man embalmed for the afterlife was about 171cm and well preserved. He had poor dental health with several dental abscesses and organs had been removed during the embalming process. He was well preserved with an almost intact skeleton and there were no detectable signs of illness or trauma.
The man preserved in the sand had several broken bones but lack of wear on some joints indicates he was in his early twenties when he died. His teeth showed little dental wear. He was about 160cm with most of his organs and partially digested food was found where his intestine and colon had been,