If light and black hole seem perfectly opposite at first glance, general relativity teaches us that this is actually not quite the case. Indeed, Einstein's equations demonstrate that just like a collapsing star, light can also "collapse" and form a black hole.
In January 1955, the American physicist John Archibald Wheeler, world renowned expert on general relativity and black holes, published “Geons”, a set of works studying the behavior of electromagnetic waves subjected to gravity (1). More specifically, Wheeler studies how an electromagnetic wave can be confined in a region of spacetime, under the effect of the gravitational attraction generated by its own energy field.
Based on the gravitational field equations of general relativity, Wheeler shows that if a very large quantity of photons is confined within a very small region of spacetime, then this extreme concentration of energy could naturally lead to the formation of an event horizon, and therefore a black hole (1). As such, the physicist calls this phenomenon "kugelblitz", literally "ball lightning" or "ball lightning".
Although general relativity describes the way in which masses bend space-time, this one, before being a theory of mass, is above all a theory of energy. For Einstein, gravity as a geometric deformation of space-time results from the presence of energy. It is only then, through the famous energy-mass equivalence formula E=mc², that the conversion from one to the other is carried out.
In other words, for gravity, there is no difference between energy and matter. This is why photons with zero mass are as sensitive to gravity as a massive body and also generate a gravitational field
However, a kugelblitz is no different from a “classic” black hole. Only its formation mechanism is different. Unlike the stellar black hole – resulting from the gravitational collapse of a star – and the primordial black hole – resulting from the gravitational collapse of an area of space-time under the effect of density fluctuations, the kugelblitz comes from an extremely large concentration of photons in a restricted region of space-time (2). The energy density is so high that an event horizon forms, trapping the light that led to its formation and resulting in a black hole.
Once formed, this black hole can be fully described by the Schwarzschild equations, that is to say the equations used to describe “classic” black holes. Indeed, with regard to the equations of general relativity, once the horizon of events has formed, all types of black holes are confused, the source of their formation no longer has any importance since they are mathematically treated as identical manner.
Following the work of Wheeler, the notion of kugelblitz was taken up and deepened by many physicists. In addition, the question of the potential artificial creation of a kugelblitz naturally arose; however, the researchers demonstrated that, to create an Earth-sized kugelblitz, it would require bringing together the light emitted for ten years from all stars within a sphere 350 light-years in radius around the Earth (2 )…That’s a lot.
