Hawking radiation and how it causes black holes to evaporate
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ELI5 for this is inherently a bit untrue, but it’s *sort of* true, and probably the best most of us can understand.
Anyway, you can’t create matter or energy, but you can change one into the other. For instance, radioactive decay changes small amounts of mass into energy, and when a particle of matter hits a particle of antimatter, they’re both transformed into energy.
Now there’s a thing where sometimes (really, all the time) particles seem to appear out of nowhere in pairs, like a bit of matter and a bit of antimatter. Then those two hit each other and transform into the exact amount of energy that was needed to create them in the first place, so everything balances out to zero. That’s really ELI5-ed, and a physicist would hate the way I said it, but a physicist could write 15 books on the subject without using any words a 5 year old would even recognize as words. This is not ELI doctorate in physics.
Anyway, Stephen Hawking, who was smarter about these things than pretty much everybody else put together, came up with the idea that sometimes these particles would appear near the event horizon of a black hole, and one would fall in. That means that they couldn’t hit each other to make the energy needed to create them, and one of them would have just appeared out of nowhere.
A lot of those particles will hit other particles and antiparticles and turn into energy, and that energy is called Hawking Radiation, but the matter and energy have to come from somewhere, because you can’t create matter or energy from nothing. So what happens is that the black hole would have to lose that much mass. Black holes have a LOT of mass, so the loss isn’t much, but over really long periods of time, that means that every black hole in the universe would eventually lose all of its mass, and evaporate.
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