# Black Hole - energy and matter

Can someone explain what is happening with energy (e.g. light) and matter which was collected by a black hole?

I suppose that matter builds a black hole's mass and size, but what about light? Does it just disappear? But to generate light, the star consumed matter (mass).

## 1 Answer

No, the light doesn't just disappear, it adds to the black hole's mass in accordance with $$E=mc^2$$. So if a star produced light from $$m$$ kilograms of mass and all that light fell into a black hole, the black hole's mass would increase by $$m$$ kilograms.

• So does it mean that in this case the light is the carrier of mass? – Luke Aug 9 at 10:37
• @Luke Not really. The energy of light is purely kinetic energy, it has no mass. In contrast, a chunk of moving matter has both kinetic energy and mass. Mass is sometimes called "rest energy". – PM 2Ring Aug 9 at 11:12
• Thanks. So what is the form of mass generated in the Black Hole by light? If the star burned 1kg of Hydrogen, what king of matter will be generated in the Black Hole, so what form of mass will be generated? – Luke Aug 9 at 12:52
• @Luke Firstly, fusion only converts a small amount of rest mass to light & heat. In a typical star core, 1kg of hydrogen fuses into about 993g of helium, producing about $6.4×10^{14}$ joules of energy. If all that energy fell into a BH (black hole), the BH's mass would increase by 7 grams. A BH doesn't generate matter. When matter or energy reaches a BH's EH (event horizon), the spacetime curvature around the BH increases, and that change is "locked in" once the stuff crosses the EH. Nothing inside the EH can ever have an effect on stuff outside the EH. – PM 2Ring Aug 9 at 13:27