Imaging with particles from space
High-energy particles, born from cosmic rays smashing into Earth's upper atmosphere, are helping to probe the internal structure of the 4,500-year-old Great Pyramid of Giza. These particles are called muons. Muons are fundamental particles, similar to electrons but about 207 times more massive. Every minute, around 10,000 muons rain down over every square meter of the Earth's surface, traveling at nearly the speed of light. Their high energy allows them to penetrate tens of meters into dense materials like stone before being absorbed or decaying.
This penetrative ability is the basis of muon tomography. The technique works by measuring the flux of muons that pass through a large object from different angles. Detectors placed inside or around a structure count the number of muons arriving from various directions. A higher number of detected muons indicates a path through less dense material or an empty space, while a lower number signifies solid, dense material that has absorbed more of the particles. This method allows scientists to create a 3D image of the interior of a structure non-invasively.
The international ScanPyramids project, launched in October 2015, uses this technique along with others like infrared thermography to scan the ancient pyramids at Giza. The project is a collaboration led by Cairo University and the French Heritage Innovation Preservation (HIP) Institute. By placing detectors inside and outside the Great Pyramid of Khufu, the team maps density variations within the massive limestone structure.
The ScanPyramids discoveries
In November 2017, the ScanPyramids team announced a major finding in the journal Nature: the discovery of a large, previously unknown void inside the Great Pyramid. Named the "ScanPyramids Big Void" (SP-BV), the space is located directly above the Grand Gallery, a known ascending corridor. The void has a minimum length of 30 meters (98 feet) and a cross-section similar to that of the Grand Gallery below it. The discovery was confirmed using three different types of muon detectors: nuclear emulsion films, scintillator hodoscopes, and gas detectors, ensuring the result was robust.
In March 2023, the project confirmed the details of another void, first hinted at in 2016. This feature, named the "ScanPyramids North Face Corridor" (SP-NFC), is located behind a chevron-shaped stone structure on the pyramid's north face. After initial muon detection, the team used ground-penetrating radar and ultrasonic testing to pinpoint its location with centimeter-level accuracy. An endoscope was then inserted through a tiny gap, providing the first visual confirmation of the corridor in 4,500 years. The corridor is approximately 9 meters (30 feet) long and has a square cross-section of about 2 by 2 meters.
Scanning pyramids with cosmic rays is not new. In the late 1960s, Nobel Prize-winning physicist Luis Alvarez conducted a similar experiment in the Pyramid of Khafre. He placed spark chambers in a chamber below the pyramid but found no evidence of unknown voids. The technology has advanced considerably since then, allowing the modern ScanPyramids project to achieve much higher resolution and sensitivity.