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How cosmic rays revealed a new, mysterious void inside the Great Pyramid

This tantalizing finding raises more questions than it answers.

It’s not ancient aliens.
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Today, the journal Nature has published a finding that sounds like the setup to a Nicolas Cage movie. A team of physicists and engineers has discovered a previously unknown “void” in the Great Pyramid at Giza in Egypt. And they did it with the help of cosmic rays created at the edge of space.

The “ScanPyramids Big Void,” as scientists are calling it, is around 98 feet long and about 50 feet high. The investigators don’t know what’s inside this void or what its purpose is. Nor do they have any way to currently access it.

But it’s a huge finding. “No very big structure has not been discovered inside the Khufu pyramid since the Middle Ages,” Mehdi Tayoubi, co-founder of the nonprofit that led the research, told reporters Wednesday.

The Great Pyramid, also known as Khufu’s pyramid, was built around 2560 BC to ensure the immortality of the Pharaoh Khufu after his death. The pyramid is one of the wonders of the ancient world, and it’s one of three at the site at Giza (along with the Great Sphinx).

Khufu wasn’t just a king — he was thought to be a god. And so his death commanded something spectacular. At 455 feet, his pyramid stood as the world’s tallest man-made building until the year 1300. That’s 3,800 years. The pyramid was about as old to the ancient Romans as the Romans are to us. And throughout the rise and fall of civilizations, the pyramids have remained a fascination. Even today, they still contain mysteries — like the newly identified void.

But archaeologists say that while it’s still hard to say how archaeologically significant the void is, it’s likely an intentional design element.

“When you’re dealing with creating a monument to house the immortal remains of an individual who bridges heaven and Earth, and whose ascendance to the stars helps assure the perpetual prosperity of Egypt, I don’t think [this void] was a cost-cutting measure,” Adam Maskevich, an archaeologist who was not an author on the paper, explains. (The New York Times explains the void could have been an engineering necessity to lessen the weight of the structure.)

And scientists suspect it’s there because of cosmic rays.

How cosmic rays found the void

An illustration depicts where the research team thinks the void is located in the pyramid.
ScanPyramids mission

For two years, the ScanPyramids project, a collaboration between the Heritage Innovation Preservation project in Egypt, Cairo University, and the Egyptian government, has been using advanced techniques from the world of particle physics to learn more about the pyramids.

“Just because a mystery is 4,500 years old doesn’t mean it can’t be solved,” the ScanPyramids project explains.

Subtle techniques from physics are particularly helpful for exploring priceless ancient treasures like pyramids. You can’t knock out walls in the pursuit of a new discovery. So the ScanPyramids team takes a nondestructive approach.

Here’s the simplest way to describe what they did: It’s like they took an X-ray of the structure with cosmic rays.

Recall that when you go for an X-ray, radiation passes through your body. But this radiation gets partially stopped by the denser parts of your body (i.e., bones), while the soft tissue lets most of them pass on through. The X-ray machine is essentially picking up on the shadow of X-rays cast by your bones.

With the pyramid scan, the scientists used not X-rays but cosmic rays. These are sprays of high-energy subatomic particles that shower us every day.

Kyle Cranmer, a particle physicist at New York University, explains how cosmic rays form. It starts with huge energetic events like exploding stars. These events shoot jets of atomic nuclei across the universe at speeds nearing the speed of light. When these high-energy particles reach Earth, they slam into our atmosphere like shotgun pellets and “hit the nuclei of other atoms” in our atmosphere, Cranmer says.

When those atomic nuclei from space hit those atoms in our atmosphere at near the speed of light, they burst open, spilling out the subatomic particles that make up all the matter in the universe: electrons, positrons, neutrinos, muons, and so on. (This is exactly what scientists try to replicate with particle accelerators like the Large Hadron Collider.)

Some of these particles are very short-lived: They fall apart in a fraction of a second. But muons — which are a heavy version of the electron — are heavy and stable enough to reach the ground.

“They [muons] are going through us right now — thousands going through us every second,” Cranmer says. (You can actually build your own cosmic ray detector at home, and it really doesn’t seem all that hard. Even your smartphone can be turned into a cosmic ray detector.)

Those muons rocket down to Earth at 98 percent the speed of light — so fast they experience the time dilation predicted by Einstein’s theory of special relativity. They’re supposed to decay in just 2 microseconds, which would mean they’d barely get 2,000 feet down from the top of the atmosphere before dying. But because they’re moving so fast, relative to us, they age much more slowly. (A similar thing happens to Matthew McConaughey’s character in Interstellar.)

When they hit objects on the ground, they act exactly like X-rays: Dense objects absorb them; less dense objects let them pass through. The ScanPyramids team used photographic plates sensitive to muons. These photographic plates were placed inside the already-explored chambers of the pyramid and around the outside of the structure. The data from each plate was then combined to make a map of the void.

A muon detector set up outside the Great Pyramid.
Muon emulsions film is set up in Khufu Queen’s Chamber.

And the researchers found that the muon pattern observed in these photographic plates looked a lot like the pattern of muons from the pyramid’s grand gallery. That makes them confident that what they are observing is truly an empty space and not just a region of less dense rock.

The team previously had success using these techniques to map the (already known) internal structure of the Bent Pyramid, a smaller pyramid in Egypt. No new voids were found on that investigation.

Okay, so what might be the purpose of this void?

The Great Pyramid is a wonder of the ancient world built around 2400 BC; experts still don’t know exactly how it was constructed. The void is completely sealed off from the known passageways in the pyramid. There’s no way to currently get to it. And so Nature’s finding opens more questions than it provides answers.

“This is an exciting new discovery, and potentially a major contribution to our knowledge about the Great Pyramid,” Peter Manuelian, a Harvard Egyptologist not involved in the research, says in an email.

A new chamber could provide clues to how the pyramid was constructed. Or, more tantalizingly, it could contain treasure. The Great Pyramid was ransacked and looted millennia ago. This void could represent the last untouched portion of the structure.

But it’s too soon to say any of this. “Most people want to know about hidden chambers, grave goods, and the missing mummy of King Khufu. None of that is on the table at this point,” Manuelian says.

The ScanPyramids team currently has no concrete plan to get inside the void, and they have much more work to do to pinpoint its location in the pyramid. The muons only give a blurry, rough sketch.

“For the time being we cannot allow ourselves [to start drilling bore holes into the void],” says Hany Helal, vice president of the Heritage Innovation Preservation Institute, which runs the ScanPyramid project. “We need to continue the research with nondestructive techniques, which will allow us to have a complete picture of what is inside.”

Once there’s consensus on the exact dimensions of the void and its location, then a team can drill a small hole and deploy a robot drone to explore it.

“We can’t allow for trial and error,” Helal says.

After all, this is perhaps the most famous building on Earth.

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