Does the Pyramid of the Sun harbor any tombs? What might such tombs reveal about the society that two millennia ago built one of Mesoamerica’s largest pyramids? In an experiment à la Luis Alvarez, who in the late 1960s concluded that there are no tombs in Egypt’s Chephren pyramid, a collaboration of physicists and archaeologists hopes to glean answers to these questions by monitoring the passage of muons through the Pyramid of the Sun in Teotihuacan, 50 kilometers northeast of Mexico City.
In the 1970s, nuclear physicist Arturo Menchaca and archaeologist Linda Manzanilla each independently discussed with Nobel prizewinner Alvarez the idea of conducting such an experiment in the Teotihuacan pyramid. “[Alvarez] wrote me that the muon approach could be applied wherever you have a hole underneath the pyramid,” recalls Manzanilla. But it wasn’t until three years ago, when another physics Nobelist, Leon Lederman, asked if the “Alvarez test” could be applied to the Pyramid of the Sun, that broader interest was sparked. Manzanilla, a researcher at the Institute of Anthropological Research at the National Autonomous University of Mexico (UNAM), and Menchaca, who heads UNAM’s Institute of Physics, teamed up to do the experiment.
Detecting muons
Fortuitously, an ancient tunnel runs 8 meters below the pyramid, which is about 225 meters on a side and 65 meters tall. The tunnel was discovered in the early 1970s, “though it was later realized that the pyramid was built as a monument to the tunnel,” says Menchaca. “For physicists, it means we can do experiments. For archaeologists, the tunnel is interesting in its own right.”
This spring, a detector in the tunnel will begin a year of muon counting. Created when cosmic protons hit the atmosphere, muons rain down uniformly and are absorbed when they interact with matter. In hunting for a tomb, the researchers are looking for a surplus of the charged particles. “If you find more muons than you expect, the difference is an indication that in that particular direction you have less matter,” says Menchaca. “That is the secret of the technique.” (See Physics Today, Physics Today 0031-9228 56 5 2003 19 https://doi.org/10.1063/1.1583524 May 2003, page 19 .)
“It’s easily said, but the experiment is complicated,” Menchaca adds. “You have to make a model and run code to simulate the flow of muons.” The simulations are more difficult than for the larger Chephren, he says, because the Pyramid of the Sun has a more irregular shape and is less dense and less homogeneous.
The detector was made by Menchaca’s group, which built, for example, part of ALICE for the Large Hadron Collider in Switzerland. In the Teotihuacan experiment, muons are tracked in three dimensions with a traditional multiwire chamber, in which passing muons generate electrical signals by ionizing gas. The one-meter-cubed chamber is sandwiched between scintillators to identify muons by coincidence signals. The data are sent via a cellular phone connection to Menchaca’s lab for analysis. The detector, says Menchaca, “is the largest particle physics detector in Mexico.” The Mexican government is picking up the $500 000 tab for the experiment.
Decoding muons
The key question that archaeologists hope to help resolve is, What type of government did Teotihuacan have? The name means “City of the Gods,” and was bestowed by the Aztecs when they discovered the city centuries after its fall. At its peak, Teotihuacan was home to an estimated 125 000 people. Some scholars believe there was a dynasty with a single ruler, says Manzanilla, who, for her part, is “working with a co-rulership hypothesis, a corporate model with perhaps two to four rulers.”
Co-rulership is known to have been common in central Mexico in the early postclassical period (around 900 AD) and could have been in practice earlier in Teotihuacan, says Manzanilla. “Teotihuacan was divided into four parts. The palace of Xalla has four buildings, so it might be that four individuals went to the palace to make decisions.” The tunnel under the Pyramid of the Sun has a four-petaled chamber at the end. What’s more, she adds, “until now, no royal tombs have been found in Teotihuacan. It’s strange not to have royal tombs.” Recent findings in the neighboring Pyramid of the Moon of “a rich burial with three persons and a fourth represented by a jade statue” further support the co-rulership hypothesis, she says. “There could have been many four-ruler groups.” Co-rulership would win support if evidence of more than one elite burial is found in a single chamber.
But teasing answers out of the data may be tricky. “What you can tell is where there is less density than you expect,” says Menchaca. Such a spot might indeed be a tomb or other empty chamber. But it could be that the soil in the pyramid has settled over time to create caves. Or stone walls might surround a cave, canceling out any muon effect. Or a tomb’s contents may have been stolen. “Another possibility,” Menchaca says, “is a stone-filled tomb, which seems likely based on the recent findings in the Pyramid of the Moon. That would show up as a region with fewer muons than expected, rather than more. If we do find a compact localized region with more—or less—muons, we will perceive it as the end of work for my group. Once we understand the topology, we will pass things on to the archaeologists.”
“If the Pyramid of the Sun has a void, a chamber, we will detect it with the muon experiment,” says Manzanilla. “Then we will excavate.”