WHEN Arturo Menchaca was studying physics at the University of Oxford, he never imagined his passion for subatomic particles would lead him into archaeology. But next month, Menchaca, who heads the Institute of Physics at the National Autonomous University of Mexico (UNAM), will venture under tons of mud and rock in search of a kings鈥� tomb that vanished centuries ago. And there he will install a cosmic ray detector 鈥� a bizarre device even for this era of high tech archaeology. Menchaca believes that the high energy particles that bombard Earth from space will help unearth the secrets of the 2000-year-old Pyramid of the Sun at Teotihuacan, near Mexico City.
Teotihuacan鈥檚 origins stretch as far back as 800 BC, when the first inhabitants settled in the area. It was the first large metropolis in the Americas, the capital of a complex civilisation that thrived for longer than Rome did. But that seems to be about all we know. Nobody knows, for instance, whether the city had absolute monarchs like most ancient cultures or whether it was governed by a coalition of the most influential families.
Even the real name of the city has been lost. For some unknown reason, the metropolis was abandoned around the 7th century. Later on, when the Aztecs found it completely desolate, they thought the whole area was inhabited by supernatural beings. It was the Aztecs who named it: Teotihuacan means 鈥淧lace of the Gods鈥� in their Nahuatl native tongue.
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鈥淯nfortunately, Teotihuacan has no history. We do not have any known language or written sources to decipher the past of the city,鈥� says Linda Manzanilla, an archaeologist at UNAM鈥檚 Institute of Anthropological Research.
One of the most important of the unanswered questions about Teotihuacan concerns its origins: who founded and ruled this sacred city? Archaeologists believe that royal chambers, hidden under the city, might contain the answer, but decades of excavations have so far unearthed no conclusive evidence. So is the answer about to come to them from space? Menchaca thinks it might.
After all, he points out, it has been done once before. In the 1960s, a Nobel prizewinner, the late Luis Alvarez of the University of California, Berkeley, used a cosmic ray detector to analyse the Pyramid of Kephren, located 10 kilometres from Cairo, Egypt. Although one of the many wars in the area prevented a complete exploration, Alvarez still managed to report in the journal Science that he had found no undiscovered rooms.
Years later, as a postdoc Menchaca met Alvarez when he was a professor at the University of California, Berkeley. They talked about the Kephren experiment, but Menchaca forgot all about the idea until his return to Mexico 鈥� which was when he met Manzanilla. Together they agreed that recreating Alvarez鈥檚 Egyptian experiment under the Mexican pyramid might just give the archaeologists the breakthrough they so desperately needed.
When cosmic rays reach Earth鈥檚 atmosphere there is a violent but invisible collision that creates millions of muons, tiny particles that live for only a millionth of a second. During that brief time, the muons travel for dozens of kilometres, passing almost unhindered through most kinds of material 鈥� almost but not quite. Every obstacle the muons pass through will absorb a tiny fraction of them. The denser the object, the bigger the absorption, so a cosmic ray detector installed below a structure like the Pyramid of the Sun will provide an indirect measure of the density of what is above it. 鈥淚f we detect more particles than expected coming from one part of the building, it means that there must be a hole in that direction,鈥� explains Menchaca.
The detector looks like a cube with sides measuring 1 metre. The top and bottom of the cube are covered with plastic 鈥渟cintillators鈥�, which produce light and an electrical signal when they are hit by high energy particles. Since muons take less than a nanosecond to go across the detector, if both plastic layers detect a signal within this time-window then there is definite proof of a muon passing by.
According to Menchaca鈥檚 estimates, if the Pyramid of the Sun has no hidden compartments the detector should count 100 particles every second. If this count rate turns out to be higher, the researchers think that will indicate the presence of a hidden chamber.
But detection is only half of the job; the researchers also need to know which direction the muons came from. This is done by 1200 electrical wires stretched across the cube at varying orientations. As muons pass by, they ionise the air within the detector, and the resulting electric field affects the currents flowing in the wires.
Since the effect is strongest at the wire closest to the muon鈥檚 trajectory, the researchers can use the location of the varying currents to pin down the muon鈥檚 trajectory and trace back their route through the temple. If they see an unexpectedly high number of muons from a particular direction, the researchers will infer the presence of a hidden chamber.
In theory, the system sounds like a winner. But there鈥檚 an obvious problem. How do you get a cosmic ray detector under the city? Digging out a chamber is just too invasive, as well as expensive, and could destroy the archaeological heritage.
Here, the Teotihuacan researchers have struck lucky. The original inhabitants of the city excavated a tunnel that almost reaches the heart of the Pyramid of the Sun. The entrance to the gallery is pretty small, but once inside there is plenty of space. The corridor winds for more than 100 metres until it reaches a big cave that opens out into a number of chambers. 鈥淭he place is just perfect for the experiment,鈥� says Menchaca. He installed a prototype detector last year in the middle of the cave. After all the hardware and software tests turned out fine, Menchaca and his UNAM colleagues built the final version of the machine.
Next month the detector at Teotihuacan will be left in the shadows beneath the pyramid, and will stay there for a year, counting millions of muons. Gigabytes of data will travel continuously to a computer located 40 kilometres away at UNAM, where researchers will crunch numbers in search of hidden rooms, with a precision of about half a metre.
Once they spot a good candidate for a royal chamber, then comes the hardest part of the job: digging towards the 3D coordinates obtained from the cosmic ray data. But since they will know the precise location, the excavation will have a minimal impact compared to a blind exploration.
Should the researchers finally locate and enter a royal chamber, what might they find? Gold and emeralds consecrated to the glory of ancient kings? Or just dirty bones?
Based on numerous paintings found at Teotihuacan 鈥� none of which obviously depict monarchs 鈥� Manzanilla suspects the original inhabitants of the city did not glorify their authorities. Indeed, she believes they operated under a collective leadership. 鈥淭he whole city was divided in four main sectors. The same number, four, is a recurrent leitmotif in Teotihuacan art, as well as in the main plaza of a palace which I am excavating now near the Pyramid of the Sun. All this evidence may be an indicator of a stable political coalition formed by four rulers,鈥� she suggests. By this time next year cosmic rays will have played their part and we should know if she is right.
