How did Hess prove cosmic rays come from space?
I proved the extraterrestrial origin of cosmic rays through systematic elimination of alternative explanations. First, I conducted balloon ascents to measure ionization at altitudes up to 5,200 meters. The data showed ionization increasing with height, which contradicted the prevailing theory that all atmospheric ionization came from Earth's radioactivity. To rule out solar origin, I flew during a total solar eclipse on April 17, 1912, when the Moon blocked the Sun's direct radiation. The ionization remained unchanged, so the Sun was not the source. I also made night flights to exclude any solar influence. Additionally, I measured ionization over water to minimize terrestrial radiation. Each experiment was designed to test a specific hypothesis. The only consistent explanation was that a highly penetrating radiation enters our atmosphere from beyond Earth. Extraordinary claims require extraordinary evidence, and I ensured my measurements were reproducible and my instruments carefully calibrated.
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