cosmic rays

History

In 1912, Victor Hess ascended in a balloon with an electroscope and discovered that radiation increases with altitude. Thus cosmic rays—visitors from beyond Earth—were discovered.

How it works

Upon reaching the atmosphere, a particle strikes an atomic nucleus, triggering a cascade of secondary particles—an extensive air shower. Like a billiard ball breaking a rack, it creates an avalanche of millions of 'fragments' that reach the surface.

History

In 1912, Victor Hess ascended in a balloon with an electroscope and discovered that radiation increases with altitude. Thus cosmic rays—visitors from beyond Earth—were discovered.

How it works

Upon reaching the atmosphere, a particle strikes an atomic nucleus, triggering a cascade of secondary particles—an extensive air shower. Like a billiard ball breaking a rack, it creates an avalanche of millions of 'fragments' that reach the surface.

History

In 1912, the Austrian Victor Hess proved in a balloon that the ionizing radiation had an extraterrestrial origin. In the 1930s, at the Pic du Midi, Pierre Auger discovered extensive air showers—cascades of secondary particles. Fritz Zwicky suggested that supernova explosions are the sources of the particles. In the 1960s, Arno Penzias, while working on antennas for studying cosmic rays, accidentally discovered the cosmic microwave background.

How it works

Particles are accelerated in shock waves of supernova remnants (first-order Fermi mechanism). Magnetic clouds scatter particles, causing them to cross the shock front repeatedly and gain energy. Upon collision with the atmosphere, a cascade is initiated: a proton produces pions, which produce muons, electrons, and photons. Detectors such as the Pierre Auger Observatory register either particles at ground level or the fluorescent glow of the air. Ultra-high-energy rays lose energy through photopion production when interacting with the cosmic microwave background—this creates the Greisen–Zatsepin–Kuzmin limit.

Open problems & fun facts

Source of particles with energy above 10^20 eV (the 'Oh-My-God particle' and similar).; Detailed composition of ultra-high-energy rays: protons? iron nuclei?; Exact acceleration mechanism in astrophysical sources: diffusive shock acceleration, magnetic reconnection?

💡 Cosmic rays helped discover the muon—the heavy cousin of the electron. Initially they were considered unwanted noise for detectors, but they gave us a new particle.