I Spy With My Little Muon: Peering Inside Pyramids With Cosmic Rays

我们通过发表论文来表彰学生 STEM 写作比赛的前 10 名获奖者。这是 Andrei Li 的作品。

2017 年,研究人员在吉萨大金字塔北面前架设了一个 μ 子望远镜。借用粒子物理学的一项技术,他们在金字塔内部发现了一个 100 英尺的“空洞”。

这篇来自多伦多 Monarch Park Collegiate 的 18 岁的 Andrei Li 撰写的文章是 The Learning Network 第四届年度 STEM 写作比赛的前 10 名获奖者之一,我们收到了 3,000 多份参赛作品。

Have you ever wanted to X-ray the pyramids? Peer into their bowels and search, à la Indiana Jones, for hidden chambers?

In the 1960s, a United States-Egypt joint archaeological expedition arrived at the pyramids of Giza with that exact goal. To accomplish this, they called upon an unexpected ally, a subatomic particle that had been discovered only three decades prior: the muon.

Leading this expedition was Luis Walter Alvarez, a physicist at the University of California Berkeley. In 1965, Dr. Alvarez thought of using muons to scour the insides of the Egyptian pyramids for hidden structures.

The muon is, in a sense, the electron’s larger sibling: Both have the same negative charge, but the muon is 207 times more massive. Born from the violent collisions between cosmic rays and atoms in the upper atmosphere, muons exist for a little over two microseconds before decaying into simpler particles.

Despite their short life spans, the highly energetic muons travel fast and far. Muons can “cross tens of meters of concrete. They’ll also pass through your body without doing anything,” explained Ralf Kaiser, a professor at the University of Glasgow. “They’re ubiquitous, penetrating and cost-free.”

For Dr. Alvarez, the muon’s abundance, safeness and penetrative properties made it the perfect candidate for peering inside the pyramids.

Dr. Alvarez and his team set up spark detectors around the Pyramid of Khafre. Dr. Alvarez’s spark detectors consisted of two metal plates with a small gap between them. Whenever a muon passed through the plates, it disturbed the gas molecules along its path and formed a visible spark across both plates. This allowed the researchers to extrapolate how many muons had passed through the pyramid from each direction.

In turn, they could deduce density differences inside the pyramid: Less dense regions, like hidden chambers, would let more muons through than denser regions, those completely filled with limestone.

What we see is “the shadows of the different parts,” said Giovanni Macedonio, who leads the Muon Radiography of Vesuvius project. He likened the process to X-ray imaging, which sees inside your body by projecting X-rays onto a film. The denser parts of your body, such as the bones, block more radiation and therefore appear as dark regions on the resulting image.

Dr. Alvarez presented his findings to the American Physical Society in 1969. Though no hidden chambers were detected in the Pyramid of Khafre, his work proved the viability of muon tomography, or muography, to the wider scientific community.

Subsequent technological advances, like the replacement of bulky spark chambers with compact plastic scintillators, catapulted muography into commonplace use. Muography has been applied in predicting volcanic eruptions and tracing illicit nuclear materials, and proposed in probing the tomb of China’s first emperor. Detectors now even fit into your pocket.

In a twist of fate, researchers announced the discovery of a hidden chamber within the Pyramid of Khufu in 2017. Had Dr. Alvarez not set up camp at the neighboring Pyramid of Khafre, he might have made this discovery himself.

Nonetheless, Dr. Alvarez’s work lit a spark for future scientists — and muons — to kindle.

Works Cited

Alvarez, Luis Walter, et al. “Search for Hidden Chambers in the Pyramids.” Science, 6 Feb. 1970.

Andrews, Robin George. “How Do You See Inside a Volcano? Try a Storm of Cosmic Particles.” The New York Times, 10 Nov. 2021.

Banino, Rob. “How Scientists Are Using Cosmic Radiation to Peek Inside the Pyramids.” BBC Science Focus Magazine, 10 June 2021.

Chen, Stephen. “Could Cosmic Rays Unlock the Secret Tomb Guarded by China’s Terracotta Army?” South China Morning Post, 15 Dec. 2021.

Chu, Jennifer. “Physicists Design $100 Handheld Muon Detector.” MIT News, 20 Nov. 2017.

Conover, Emily. “Muons Spill Secrets About Earth’s Hidden Structures.” Science News, 22 April 2022.

“DOE Explains … Muons.” U.S. Department of Energy.

Leone, Giovanni, et al. “Muography as a New Complementary Tool in Monitoring Volcanic Hazard: Implications for Early Warning Systems.” Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 10 Nov. 2021.

Morishima, Kunihiro, et al. “Discovery of a Big Void in Khufu’s Pyramid by Observation of Cosmic-Ray Muons.” Nature, 2 Nov. 2017.

“Spark Chamber.” Encyclopaedia Britannica.