Lightning has been observed for millennia. How it starts has not been explained. The electric fields measured inside thunderstorms are too weak to ionize air directly — by about an order of magnitude. Something bridges the gap between the field that exists and the field that would be needed.
The bridge is cosmic rays from deep space. Relativistic particles from outside the solar system enter the atmosphere and seed electron avalanches in the thunderstorm's electric field. These accelerated electrons collide with nitrogen and oxygen molecules, producing bursts of X-rays — terrestrial gamma-ray flashes, a phenomenon observed by satellites since the 1990s but never satisfactorily connected to lightning initiation.
The X-rays then eject new seed electrons from nearby air molecules via the photoelectric effect — Einstein's 1905 mechanism for metal surfaces, occurring here in open air. This creates a self-amplifying feedback loop: cosmic rays produce fast electrons, fast electrons produce X-rays, X-rays produce more seed electrons, and the loop concentrates charge until a conductive channel forms and the lightning bolt fires.
Every lightning bolt on Earth is initiated by particles from outside the solar system. The most violent local atmospheric event is triggered by the most distant cause. The photoelectric effect in air — the same physics that won Einstein a Nobel Prize for explaining electron emission from metals — operates inside every thundercloud, on every molecule that intercepts a passing X-ray.
The mechanism unifies two previously separate atmospheric phenomena: terrestrial gamma-ray flashes and lightning initiation are not coincident events but successive stages of the same causal chain.