Locating hidden spaces within large earth and stone constructions like pyramids presents unique challenges. Even X rays, which the dentist uses to detect cavities, or other methods we are familiar with, cannot be used to peer into a six million tonne stone pyramid. An international team from institutes in France, Japan and Egypt, report, in the journal Nature that while X rays or conventional radiation cannot penetrate the material of the pyramid, elementary atomic particles, created in the upper atmosphere by cosmic rays, are uniquely suited.

Muon showers hence helped a group of archaeologists, physicists and civil engineers to show that the Great Pyramid of Giza, famous for its internal passages and chambers, has yet another chamber, comparable in size and unknown so far. The Great Pyramid, also known as the Pyramid of Khufu or the Pyramid of Cheops, nearly 140 metres tall and 230 metres at the base, is a marvel of accurate engineering and sheer scale, for something built over 4,500 years ago.

A remarkable feature about its measurements is that the ratio of the perimeter to the height (170/280 royal cubits) is within 0.05 per cent of the value of two times Pi (or the ratio of the perimeter of a circle to its diameter). The pyramid consists of 2.3 million blocks, with 5.5 million tonnes of limestone and 8,000 tonnes of granite, some of the granite blocks being 80 tonnes in weight. As shown in the picture, the entrance to the pyramid is through a descending, interior passage and then an ascent, leading to the Queen’s Chamber, the Grand Gallery and the King’s Chamber.

There has been much research and conjecture about the purpose and intention of these different passages and spaces and the suspicion and belief that there were yet more cavities to be found. As most parts are inaccessible, different kinds of probes and robotic devices have been used to follow narrow channels and to probe for cavities in the rock.

There has been probing with radar and there have been studies of microgravity to detect voids, in the same way that gravity scans show deposits of ores underground. But they happened with little success till the present studies with penetrating radiation using muons — elementary particles that arise from cosmic rays.

The muon is an elementary particle that is like the electron in its charge and spin but has over 200 times the mass. It is unstable and rapidly decays, usually into a normal electron and a pair of very light, neutral particles, called neutrinos, which carry away the energy that represents the difference in mass. Because of its greater mass, however, the muon suffers less deflection by other charges or fields and hence does not lose energy by radiation during deflection, which happens with lighter charged particles.

The muon hence penetrates deeper into dense materials than other charged particles. Radiation like radar, X rays or gamma rays is anyway rapidly absorbed. Muons arise in high energy nuclear reactions or as secondary products of the interaction of cosmic rays with atomic nuclei in the earth’s atmosphere. As the interactions are of high energy, the heavy muon particles created continue to move towards the earth at nearly the speed of light. Relativistic effects make time to pass more slowly for the muon and it covers large distances before it decays.

Muons are thus found not only to reach the surface of the earth, but survive to a good depth within, and down to the beds of oceans. In this way, muons can pass through much of the rock that composes the pyramid and when they emerge, they can be detected with the help of their decay products. Nevertheless, muons do get deflected or absorbed in their passage through the rock and the numbers detected vary according to the mass of rock that the muons have passed through. This is the principle behind using muons to look for voids in rock, and cosmic ray muons for scanning a large structure like a pyramid.

The quest for cavities in the Grand Pyramid with muons was first undertaken in the 1960s and the detector was the spark chamber, a stack of metal plates filled with a gas that gets ionised if a charged particle passes through. The effort, however, failed to detect cavities, perhaps due to inadequacies of the equipment.

Current muon detectors have been effective in investigating volcanoes, in the inhospitable interior of the Fukushima reactor, for detecting contraband, interiors of heritage buildings and in archaeology. In using muons with a structure like Cheops’ Pyramid, of course, only cosmic ray muons can be used. Cosmic ray muons are limited to one direction, and there is need for long periods for data collection.

The present search for cavities, known as the Scan Pyramids mission, started in 2015 with “nuclear emission films” — developed by the Nagoya University — as the detector. These are photographic films on both sides of a plastic sheet and provide fine grained detection with directional information.

Sheets, about a foot square, were laid on in eight-square-metrewide panels in the Queen’s Chamber and an adjoining niche, with the axis of the King’s Chamber and the Grand Gallery running in between.

The positions were changed periodically and observations were made over several months. The results showed areas of higher muon flux in places where expected because of the voids of the King’s Chamber and the Grand Gallery.

While this validated the procedure, it was interesting that the muon flux indicated yet another void, nearly the same size as and parallel to the Grand Gallery.

A second detection method consisted of four layers of panels of scintillator bars, developed by the KEK particle accelerator establishment in Japan. This method was deployed for many months, the Nature report says, and it identified the same void, consistent with the results of the first method.

A third method, developed by the Atomic Energy Research organisation, CEA, at Saclay, near Paris, employs a pattern of gas detectors whose output is amplified and processed by built-in electronics. The 3D image created by this method also confirms the new cavity discovered by the Nagoya University arrangement.

A large, new void detected with a high confidence by three different muon detection technologies and three independent analyses constitutes a breakthrough in the understanding of Khufu’s Pyramid and its internal structure, says the paper in Nature.

“While there is currently no information about the role of this void, these findings show how modern particle physics can shed new light on the world’s archaeological heritage,” it says.

(The writer can be contacted at response@ simplescience.in)