By 13 October 2016 Mehdi Tayoubi already knew his ScanPyramids project was on the right track. That was the day Tayoubi and his team met with a committee of Egyptologists to tell them about the small, previously unknown cavity they’d found in the north face of the Pyramid of Khufu, also known as the Great Pyramid of Giza. The ScanPyramids project had begun just 12 months earlier, but was already yielding promising results.
Then later, in 2017, it struck gold: a huge void was detected deep within the 4,500-year-old pyramid. Although the void’s precise orientation was unknown, Tayoubi’s team was able to confirm that it was about 30 metres long and situated above the Grand Gallery – the corridor linking the Queen’s chamber to the chamber containing Pharaoh Khufu’s sarcophagus. It was the first major new structure discovered in the pyramid since the 19th Century.
“We don’t know whether this big void is horizontal or inclined. We don’t know if this void is made by one structure or several successive structures. What we are sure about is that this big void is there, that it is impressive, and that it was not expected – as far as I know – by any sort of theory,” said Tayoubi when the news broke in November 2017.
But perhaps more impressive than the two discoveries was the fact that they’d been made while the pyramid remained perfectly intact. There had been new no excavation or disassembly of the structure. No chamber walls were drilled through and no sealed corridors opened up.
The ScanPyramids team had peered deep into the limestone blocks stacked up to form the walls of the 140-metre-high tomb and identified hollows within them that nobody knew existed. And what made this astonishing feat possible was a technique known as muon tomography, which allows scientists to explore locations that have previously been out of reach.
Muon tomography is a little like space exploration in reverse. Instead of using instruments constructed on Earth to investigate space, it relies on cosmic rays produced in space to delve into things on Earth.
Cosmic rays are high-energy particles that hurtle through space at near the speed of light. They’re produced by the Sun, supernovae events outside the Solar System and even the Big Bang. They’re travelling in every direction all the time and there are so many of them that they’re constantly colliding with the oxygen and nitrogen molecules in Earth’s atmosphere. At which point, they set off a cascade of other particles, much like a white ball breaking the pack of reds in a game of snooker.
“[When] a high-energy cosmic particle hits the upper atmosphere, it produces a large shower of particles,” explains Prof Ralf Kaiser, a physicist at the University of Glasgow. “Most of these particles are stopped in the atmosphere. But some of them make it all the way down to the ground. And those are typically muons.”
A muon is an elementary particle, like an electron but 200 times heavier. Being so heavy and travelling so fast gives them a greater ability to penetrate dense material than other types of radiation, such as X-rays or gamma rays. But unlike X-rays and gamma rays, cosmic ray muons don’t damage the material they pass through.
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“[Muons can] cross tens of metres of concrete. They’ll also pass through your body without doing anything,” says Kaiser. “They’re ubiquitous, penetrating and cost-free. They’re everywhere and they’re part of the natural environment.”
In short, muons are just the thing for getting a glimpse inside structures you can’t get into, structures like sealed chambers in pyramids, closed-off caverns in archaeological sites and conduits inside volcanoes. The trick to doing that, however, is catching the muons that have passed through the structure and using them to create an image of what’s inside.
Dr Giovanni Macedonio, the principal investigator of the MUon RAdiography of VESuvius (MURAVES) project, likens the process to getting an X-ray. When there’s an object, let’s say your arm, between the source of the X-rays and the camera, your arm absorbs some of the X-rays passing through it. The different densities of the skin, muscles, blood vessels and bones determine how many of the X-rays reach the camera – the denser those things are, the more X-rays they absorb.
“[Essentially,] we see the shadows of the different parts,” says Macedonio. The lighter the shadows, the denser the part and, armed with that knowledge, it is possible to distinguish between the parts inside. The same principle applies to muon tomography and the objects, such as Mount Vesuvius, it’s used to investigate.
“Instead of X-rays, we have muons,” says Macedonio. “Muons are coming from all directions around Earth, but we’re interested in the ones that are travelling close to horizontally, so they can penetrate the volcano. The muons that pass all the way through Vesuvius produce a shadow behind it.” By placing muon detectors nearby, Macedonio and his colleagues can generate an image of that shadow, study the densities of the materials depicted in it and begin to distinguish the structures inside Vesuvius.
But studying something as big as a volcano requires patience, because muons are tiny and only about 100 of them hit any given square metre per second. So although they may be constantly bombarding Earth, collecting enough of them to provide useful information on something the size of Vesuvius takes a while.
“The flux of muons is not strong,” says Macedonio. “Most of them are absorbed by the volcano so we do need a lot of time – we need months.”
So when you do eventually get a picture, what can you do with it? Can you use it to predict eruptions? No, not exactly. But what you can do is understand the relationship between the geometry of the volcanic conduits and the style of eruptions.
In particular, what conditions may cause ash clouds (that can ground planes and collapse roofs) or pyroclastic flows (fast-moving, super-heated mixes of rock fragments and gases capable of burning anything in their path) if Vesuvius were to erupt. And if you combine this information with seismic and meteorological data, you can alert or evacuate anyone who might be in harm’s way when an eruption is due.
Recent advances in imaging technology are enabling muon tomography to find a growing range of applications, but the technique isn’t new. The engineer EP George used it to check the amount of material above a mine in Australia in 1955, fewer than 20 years after the muon had been discovered (by Carl Anderson and Seth Neddermeyer in 1936).
And before the end of the 1960s the renowned American physicist Luis Alvarez was using muon tomography to look for hidden chambers in pyramids. “If you look at the original paper by Alvarez, and his measurements of the pyramid, he did absolutely everything right,” says Kaiser. “It was very cleverly done. He didn’t find any cavities, but he was just unfortunate to be looking in the wrong pyramid.”
Alvarez was looking inside the Pyramid of Khafre. Had he set his detector up next door, at the Pyramid of Khufu, he might have beaten the ScanPyramids project to the punch by almost 50 years.
All of this goes some way towards explaining why muon detectors are appearing at a growing number of archaeological sites. With improving imaging processes offering higher resolution pictures and cheaper, more portable detectors being developed, muon tomography is expanding our scope for exploration by providing us with a window – a window that gives us a glimpse into places we can’t go.
And there’s no shortage of such places. Mount Echia, in Italy, for example, is a 60-metre-high rocky headland that extends into the Gulf of Naples. It’s a built-up part of the city today, but almost 3,000 years ago, in the 8th Century BC, it was the site of Parthenope, the Ancient Greek colony that would later become Naples.
The headland largely consists of tuff, a soft, yellow rock made from volcanic ash, that’s often used in ancient constructions. As such, a complex system of tunnels and caves exists beneath Mount Echia, where generations of people have excavated the tuff to use as building material.
Investigations of the tunnels and caves have been underway for years, but in 2017 a team of physicists from Naples and Florence realised Mount Echia’s characteristics would make it the ideal location to test the muon detector they’d been developing – partly because so many of the cavities are already known (so the team would have something to verify their results against), but also because it’s not just ground the cavities are buried under.
“Mount Echia is not an isolated hill; it’s completely covered by buildings,” says Prof Giulio Saracino of the University of Naples Federico II and Italy’s National Institute of Nuclear Physics (INFN). “So it was not an easy test. But it was a very interesting one because it wasn’t clear at the beginning if all the buildings would interfere with the measurements.”
Nevertheless, the test was successful: not only was the team able to identify a selection of the known cavities, they also found signs of a new, previously hidden one. “We discovered the new cavity, reconstructed it in three dimensions and were able to give the speleologists [cave experts] a sense of its position underground, because there’s no way to reach it at the moment,” says Saracino.
From Mount Echia, the team moved on to another cavernous archaeological site in Cuma, a town near Naples believed to be the location of the first Greek colony on mainland Italy. Work there was interrupted by the COVID-19 pandemic, which is just one of the obstacles to muon tomography investigations – because not only are the right geographical and topological characteristics required, but the political situation needs to be amenable too, as Prof Nural Akchurin from Texas Tech University explains.
“We were trying to get our first prototype [muon detector] into Turkey to image an archaeological site in Limyra. But the politics in Turkey were messy; there was a coup attempt [in 2016] and a lot of things came to a screeching halt for a year or two … So we said, ‘Okay, let’s just work on a second prototype,’ because we need to improve things.
“But we haven’t given up on deploying our instruments somewhere in Turkey and there are a couple of candidate sites. Right now, we’re testing things in the lab. But, in short order, we could deploy our detectors – maybe this summer, if COVID allows.”
COVID has also affected the ScanPyramids project. Prior to work being suspended in 2020, continuing muon tomography at the Pyramid of Khufu had revealed more of the smaller cavity discovered in 2016 (suggesting it’s a corridor extending at least five metres into the pyramid, possibly angled upwards) and refined the estimated dimensions of the big void discovered in 2017 (it’s now thought to be at least 40 metres long).
If the global rollout of COVID vaccines goes according to plan, it’s possible work could resume on the ScanPyramids project, and the others, soon. And when it does, more of the secrets hidden inside some of the world’s oldest natural and human-made structures could begin to reveal themselves.
- This article first appeared in issue 362 of BBC Science Focus Magazine – find out how to subscribe here
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