High-altitude particle hunting
Near the summit of Mont Blanc, a series of high-altitude observatories became a natural laboratory for particle physics in the early 20th century. Scientists knew that high-energy particles, which they named cosmic rays, were arriving from space. The exact nature of these particles and what happened when they collided with Earth's atmosphere remained a mystery. To study the debris from these collisions, physicists needed to get their instruments as high as possible, placing them above the densest parts of the atmosphere that would absorb the evidence.
The effort was international, but French physicist Pierre Auger conducted a definitive series of experiments in the Alps. In 1938, he placed particle detectors at high-altitude sites, including one on the Aiguille du Midi at 3,613 meters. He observed that two detectors separated by many meters would sometimes signal the arrival of particles at the exact same moment. This could only mean one thing. The simultaneous particles were related, originating from a single, much larger event.
Auger had discovered "extensive air showers" (or gerbes cosmiques). His work demonstrated that a single primary cosmic ray, usually a proton, with energy strikes a nucleus in the upper atmosphere. This collision creates a cascade of billions of secondary particles that rain down towards the surface. Based on the spread of his detections, Auger calculated that the primary particles must have energies of 10^15 electronvolts (eV), a million times higher than any particle energy created in a laboratory at that time.
From mesons to muons
The "mesons" mentioned in early cosmic ray discoveries refer to what we now call muons. When Hideki Yukawa first proposed the existence of a "meson" in 1934, he was describing the carrier of the strong nuclear force that holds atomic nuclei together. The particle discovered in cosmic rays in 1936 had a similar mass to Yukawa's prediction, but it did not interact strongly with matter. This particle was the muon, a heavier cousin of the electron, which is a major component of the extensive air showers Auger observed. The true Yukawa meson, the pion, was not definitively identified until 1947.
The research stations on Mont Blanc were essential for this work. The first, the Vallot Observatory, was constructed by scientist Joseph Vallot starting in 1890 at an altitude of 4,350 meters. Though primarily for meteorology and physiology, it hosted a range of scientific studies. Later, the Les Cosmiques laboratory was founded in 1943 at 3,613 meters, specifically to study cosmic rays. These high-altitude outposts allowed physicists to place cloud chambers and Geiger counters in the path of these particle showers, showing the first real look at the world of high-energy particle physics.