In theoretical physics, a kugelblitz (German: "ball lightning", not to be confused with ball lightning) is a concentration of light so intense that it forms an event horizon and becomes self-trapped: according to general relativity, if enough radiation is aimed into a region, the concentration of energy can warp spacetime enough for the region to become a black hole (although this would be a black hole whose original mass-energy had been in the form of radiant energy rather than matter). In simpler terms, a kugelblitz is a black hole formed from energy as opposed to mass. According to Einstein's general theory of relativity, once an event horizon has formed, the type of mass-energy that created it no longer matters. A kugelblitz is so hot it surpasses the Planck temperature, the temperature of the universe 6956540000000099999♠5.4×10−44 seconds after The Big Bang.
The best-known reference to the kugelblitz idea in English is probably John Archibald Wheeler's 1955 paper "Geons", which explored the idea of creating particles (or toy models of particles) from spacetime curvature. Wheeler's paper on geons also introduced the idea that lines of electric charge trapped in a wormhole throat might be used to model the properties of a charged particle-pair.