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The Particles behind the Pyramid

Nov 9 2017 • 7 min read • Tags: physics, science, writing

Last week, a team of physicists and engineers announced they had discovered a large ‘void’ in the oldest of the Seven Wonders of the World: The Great Pyramid of Giza. Published in an article in Nature, the void, found by the ScanPyramids team, is the first major structure found in the Great Pyramid since the 19th century. It is estimated to be the same size the Grand Gallery, a passage roughly 50 metres long that connects the two largest chambers inside, the Queen and King’s chambers. Understanding this new void may help Egyptologists distinguish between the competing theories that aim to explain how the pyramids were constructed 4,500 years ago (although the void’s relevance to this issue remains contentious, see below).

How exactly does one go about making such a discovery? The first option is to simply dismantle all six million tonnes of limestone that comprise the pyramid, carefully noting the position of each part, as you destroy the UNESCO world heritage site. The advantage here is that, in principle, you could map out the entire structure of the pyramid, or as much as you could take apart before the Egyptian authorities stopped you

The alternative, is to work alongside the Heritage Innovation Preservation (HIP) Institute and find a non-invasive way to look inside the pyramid that reduces the chances of you and your colleagues being arrested. This was the approach, tactfully, adopted by the ScanPyramids team, aided by the help of tens of thousands of subatomic particles streaming in from the edge of space called cosmic ray muons.

I’d like to give these new, unsung heroes of archaeology a bit more of the limelight they deserve and explain how they were used to peer into the pyramid. The first thing to note is that every second, dozens of these muons, created at edge of the atmosphere, come showering down (harmlessly) through your body. Bear with me for this next paragraph, it’s a little jargon heavy, but the payoff is worth it.

Energetic protons (a subatomic particle found in the centre of atoms) come streaming in from supernovae, quasars and gamma ray bursts in the far reaches of the universe and collide with the atoms in the upper atmosphere of Earth. The resulting collision creates a plethora of particles called pions, each of which last only a fraction of a second before ultimately decaying into muons, charged particles that are nearly identical to *electrons, *the particles that orbit on the perimeter of atoms, giving every element its chemical properties.

What’s the upshot of all of this? When a particle like an electron bumps into matter, it gets jostled around by all the electrically charged atoms in the object and slows down. In other words, put something in an electron’s way and it’ll lose its energy and eventually stop. Muons, however, are 207 times heavier than their electron cousins! That means it takes a lot more matter to slow down a muon. Even for objects the size of buildings, most muons pass straight through, unimpeded. Neat!

Back to the pyramids. When we want to look inside something without cutting it open, what do we normally do? It’s a question anyone who’s fractured a bone might be familiar with. Medical imaging works because when we illuminate an object (like a human body) with a source like X-rays, soft matter, like flesh, doesn’t absorb very much of the rays’ energy, so they pass through with relative ease. Denser parts like bone absorb more of the X-rays. By measuring the X-ray energies coming through the other side of a human body, we can look for regions with a relative* lack *of X-rays to see the densest parts of the body, because that’s where they will have been absorbed.

The principle of using muons to image the inside of the pyramids is virtually identical. Although most muons will pass straight through the pyramids, the many tonnes of limestone will absorb a sizeable fraction of the muons passing through. The more limestone in their way, the more will be absorbed. So by measuring the flux (the number passing through an area per second) of muons from different angles, we can infer the densities in different parts of the pyramid. This was an idea suggested, and even roughly implemented by Nobel Prize winning physicist  Luis Alvarez back in the 1970s, albeit with less accurate detectors.

For the voids that were already known of, the King and Queen’s chambers, the researchers anticipated measuring more muons when looking at the paths of particles that passed through them, since there was less limestone there to absorb them. Indeed, looking at the number of muons measured through those chambers, they saw an increase in the flux, in line with the predictions of computer-generated models. However, that’s not all that was found.

As well as the expected increase in muons from the previously discovered chambers, three independent measurements found an unexpected excess of muons in the same region of the pyramid, located above the Grand Gallery.

The first came from nuclear emulsion, a special kind of photographic film that can record three dimensional tracks left by muons, placed in the Queen’s chamber alongside a second set of detectors called scintillator hodoscopes, which give off a flash of light when a muon passes through them. In addition to these, the team positioned gas detectors outside the pyramid as another independent measurement. Gas detectors operate on the principle that passing muons give up a tiny bit of their energy and kick electrons out of atoms in a gas as they pass. Ordinarily, these electrons would jump back into their host atoms, but if you apply an electric field between two plates surrounding the gas, the electrons will instead accelerate towards the positively charged plate (anode) where they can be detected, indicating that a muon has passed through.

Putting all these images together, the muon imaging of the pyramid revealed that there was much less material than expected in a sizeable region inside the pyramid, likely indicating a new structure inaccessible by exploration. Previous attempts had gone as far as using snake-like robots to tunnel through and explore the pyramid but were unable to navigate into voids such as the one announced by ScanPyramids.

While archaeology is a fascinating use of muons, for the curious minded this is only scraping the surface of the potential these particles have. When muons were first discovered in 1936, they blew the lid off our understanding of the natural world. The world was thought to consist solely of atoms, made up of protons, neutrons and electrons and the light that interacted with them. When muons were discovered in the upper atmosphere, they helped usher in the dawn of particle physics as we know it today.

Furthermore, like electrons, muons have a ghostly partner particle called a neutrino, whose nature is mysterious and not entirely understood. Muons provide a handle on detecting and investigating the neutrino’s behaviour. Over at the Large Hadron Collider in Geneva, both experiments involved with the Higgs Boson discovery have enormous chambers dedicated solely to measuring the muons that may emerge from exotic, previously undiscovered particles. Back over in Illinois USA, a new experiment G-2 (G Minus Two) is under construction specifically to measure the “magnetic dipole moment” of the muon, a measure of how much the muon acts like a little bar magnet on subatomic scales. Studying this seemingly abstract number might yet again upend the foundations of physics if a sizeable disagreement with the best theoretical prediction is found.

As for our prized pyramid, the story isn’t quite as clear cut as it seems. The number of muons measured is relatively low and the resolution with which the detectors can spatially resolve their paths (tens of centimetres to a few metres depending on the detector) isn’t accurate enough to fully map out the shape of the void. In fact, it isn’t even clear if it’s one continuous entity or several, smaller cavities that happen to clump together. As for what archaeologists think? Zahi Hawass, the former Minister of State for Antiquities Affairs in Egypt went as far as to say the new discovery offers virtuallynothingof value to Egyptologists. In fact, some voids were already known of in other pyramids.

So was this all for nothing? Well, for starters, the ScanPyramids team aims to collect more data on the void to produce a higher resolution model of it’s shape and size, which will narrow down the ideas about what exactly it *is *(the fact it slopes at such a strange angle seems to indicate it probably isn’t a secret chamber full of treasure…). Even if it turns out simply to be a constructional quirk, I would argue that finding such a gaping hole in such a famous monument is fascinating in its own right (and the media attention in the last week seems to corroborate this).

Even if the archaeologists are less than overwhelmed, the physicists certainly have reason to be excited. As I mentioned, this is just another notch in the belt for the muons’ discovery potential, and trialling this sort of technology on a structure so embedded in the public consciousness is an excellent demonstration. Personally, I look forward to seeing what else we can discover using these subatomic, skyborn tomb-raiders.