Hawking radiation and how it causes black holes to evaporate
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This is more like eli15, but here goes.
Empty space isn’t really empty. Much like the air around you isn’t empty, there are lots of particles bouncing around, empty space has lots of stuff too. That stuff is weird, though. Pairs of particles literally appearing out of nothing, but only temporarily, before they delete themselves again. A little debt of energy that is cancelled before it can even be noticed.
Near black holes, weird things happen. On the edge of a black hole, near the event horizon, these pairs of particles can pop into existence. But sometimes, one of the particles ends up falling into the black hole before it gets to disappear with its counterpart. That counterpart can escape away from the back hole. Because stuff can’t actually come from nowhere (conservation of energy), the black hole must lose a little mass in order for this particle to leave the black hole. This is a teeny tiny amount of loss, and since it happens only at the event horizon, the amount of loss is related to the surface area of the black hole, proportional to its radius. The total energy in the black hole is proportional to the cube of its radius, and black holes are very very big. So it takes a long, long time for hawking radiation to do anything.
As the black hole shrinks, the surface area becomes bigger relative to the volume, and the rate of loss speeds up. By the end of the life of a black hole, it’s shrinking very fast, and all of the remaining energy ends up getting released quite quickly; assuming that hawking radiation works the way we think it does, the final evaporation of a black hole is quite a big burst of gamma rays, which you don’t want to be anywhere near.
Black holes are so heavy that the gravity they create prevents anything close to them from ever escaping. The border between escape/no escape is called the “event horizon” by scientists. Anything inside the event horizon will be sucked into the black hole, while things outside the event horizon can potentially escape.
New particles are often popping into existence in pairs, a particle and its anti-particle. Usually these particles don’t last very long, they are created then collide with their anti-particle and disappear again. Hawking theorized that if a particle pair appeared exactly on the event horizon, one particle might go into the event horizon, and the other leave it. This prevents the particle pair from self-annihilating, and leaves the one particle outside of the event horizon. This spare particle is Hawking radiation.
Hawking radiation causes black holes to evaporate because energy/matter must be conserved. When these particle pairs normally pop into existence, they are created from zero mass, then self-annihilate to have zero mass again. Since Hawking radiation can’t do this and allows some of the particles (which have energy/mass) to escape, this mass must be taken away from the black hole in order to balance the books on conservation of mass/energy.
Basically the entire surface area of a black whole is emitting hawking radiation faster than it can suck it back in, so if the black whole doesn’t eat anything for a very, very long time then it will get too small and go boom.
The standard pop science explanation for Hawking Radiation involves virtual particles popping in and out of existence, where one falls into the event horizon and the other escapes. You will see a lot of people in this thread using that explanation. Unfortunately, it simply isn’t true. For example, if the particles are truly random, then you would expect an equal amount of particles and their anti-particles to fall in randomly. So that explanation can’t quite be the solution.
Unfortunately, there is absolutely no ELI5 level explanation available around what’s really going on. The real explanation is very complex and requires basically graduate level quantum physics. So the virtual particle ELI5 is literally the best we have. It’s not wrong, but it’s also clearly not right. It also doesn’t really work to simply call it a “simplification” of what’s going on, either.
I am usually pretty confident that I can ELI5 just about any subject that I’m familiar with. Hawking Radiation, I have concluded, simply can’t be done.
I’ll start with the evaporation part. Each black hole has a “mass”, sort of like how heavy it is. If you throw an extra star (or a rock, or an unfortunate astronaut) into a black hole, the black hole gets a little heavier. But the black hole also sucks in things that don’t have any weight, such as light. It turns out that that also makes the black hole a little heavier.
If the black hole is emitting light due to Hawking radiation, then it is like the reverse of sucking in light. As a result, the black hole becomes lighter and lighter (heh!). We call this decrease in mass “black hole evaporation”. Eventually, the black hole loses all of its mass, and it simply disappears in a great final flash of light.
Now onto the origin of the Hawking radiation itself. Others have mentioned the common pop sci picture of a pair of short-lived particles popping into existence, one of them falling into the black hole and the other one emitted as Hawking radiation. I’ll take a crack at the “real” explanation that theoretical physicists actually use. It may be ELI16 at best.
First of all, the black hole doesn’t actually “emit” anything, since it is impossible for anything inside the surface of the black hole (called the event horizon) to escape its strong gravitational pull. As I’ll hopefully explain below, it is actually the black hole’s strong gravitational effect on its surrounding space that causes the empty space itself to literally glow on its own.
To explain this glow, we need to know two things—the Unruh effect and the Equivalence Principle. The Equivalence Principle says that the gravitational effect on someone standing still on the surface of, say, the Earth is equivalent to that person being constantly accelerated “upward” in empty space. In other words, to understand what you would observe right outside of a black hole, we only have to understand what you would observe if you were constantly accelerating (at a very fast rate!).
The Unruh effect is trickier. It says that even in empty space with no matter or light, an accelerating person would observe a constant light around them (albeit very faintly), simply due to their acceleration relative to the surrounding space. This light they observe surrounding them is not only real, but also looks exactly like the light naturally emitted by an object at a particular temperature, called the blackbody radiation. The faster the acceleration, the higher the “temperature” of the observed light. (I’m not aware of a simple intuitive explanation of this effect. The derivation of this effect, although short, requires knowledge of statistical mechanics, general relativity, quantum field theory, as well as the use of time as an imaginary number.)
If a person constantly accelerating in empty space should see themselves surrounded by light of a particular temperature, by the Equivalence Principle, so should a (very courageous) person standing on the surface of a black hole. (Remember, gravitational pull downward is equivalent to being accelerated upward.) This means that the person near the black hole would observe light surrounding them. This is the Hawking radiation. The stronger the gravitational pull of the black hole, the higher the temperature of the Hawking radiation.
I’m happy to go into either more details or more basics.
The virtual antiparticle explanation is a… problematic analogy. Hawking seems to have grabbed on to a popular explanation that was easier to explain than the actual idea and carried the primary point across – that black holes can die.
My understanding is that we don’t fully know how it works. Hawking’s work shows *that* it happens, gives predictions about how quickly it happens, and we can follow the chain of understanding he used to get there.
But we don’t know exactly how it happens. And we probably won’t until we get a theory of everything.
This is admittedly unsatisfying so I will point you towards [The Science Asylum’s video on Hawking Radiation ](https://youtu.be/rrUvLlrvgxQ) as a starter and PBS Space Time’s series on Hawking Radiation for a more thorough explanation.
The radiation part has been adequately explained (or at least a simplification has). Now, for the evaporation bit:
You may know E=m c^2. This equation means that energy and mass are kinda the same. Now, energy is conserved in any closed system. This means that, if a black hole is sometimes radiating out particles with mass, and thus energy, the black hole must be losing that mass/ energy. So, as long as the black hole is not sucking up more energy than it is losing, it will shrink. Currently most black holes are probably not shrinking, since they have many chances to gobble up massive things, and because even empty space has some energy to be eaten- the cosmic microwave background radiation means space is not absolutely cold, but is at about 3K, so there is an energy source for even the loneliest black holes- for now! Space is getting colder, and stuff is getting further apart, so one day the background heat won‘t be enough, and black holes won‘t find any big things to gobble up.
Now, Hawking radiation is a property of the surface of the black hole*. More surface means more hawking radiation. Since small 3D objects have more surface by comparison to their volume (a 100 by 100 by 100 cube has volume 1 million, but surface 60 thousand, while a 1 by 1 by 1 cube has volume 1 and surface 6), smaller black holes actually give of their energy faster in proportion to the energy the hole has stored. This means that this creates a runaway effect, and the black hole shrinks faster and faster**. Now, this doesn‘t mean the process is fast by any means- solar mass black holes probably take ~10^67 years to evaporate.
*Here I mean the surface of the sphere with radius equal to the schwarzschild-radius. The black hole itself maybe kinda possibly doesn‘t have a defined area.
**It can be shown that the emitted power is proportional to mass^-2. So, halve the mass, four times the radiation.
I want to add something here that no one seems to have mentioned…
So, in quantum mechanics, there is a law known as conservation of quantum information. This law states that information can not be created or destroyed, that it must be conserved. The observation of this law has consistently held true. However, black holes challenge this. If black holes suck in everything and doesn’t allow anything to escape, it would violate conservation of information and “break” the theory of quantum mechanics.
Stephen Hawking thought a lot about this, and made math work that would account for conservation of quantum information in regards to black holes. In his theory, it basically says that quantum information is conserved by being emitted as Hawking Radiation from black holes. The consequence of this, in regards to the math, is that black holes will eventually emit everything as Hawking Radiation and evaporate.
So, Hawking Radiation is essentially a mathematical “compromise” that allows black holes to exist in a way that doesn’t break our current understanding of quantum mechanics. Unfortunately, due to the nature of black holes, it may be impossible to prove that Hawking Radiation is a real thing that happens. We may find a better theory of Quantum Mechanics in the future that accounts for black holes in a way that doesn’t require Hawking Radiation. However, for now, it works on paper.
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.
Everything in the universe has a temperature. Black holes have a temperature. Empty space itself has a temperature. Smaller black holes are hotter than larger black holes.
Temperature always flows from hot to cold. Therefore, if a black hole is hotter than the space around it, it must transfer heat by radiating something. That “something” is Hawking radiation. Because radiation is energy and energy is mass, the black hole emitting radiation must lose mass.
If black holes didn’t do this, it would be possible to break some of the universe’s physical laws by throwing specific things into black holes.
This is a grossly oversimplified explanation of black hole thermodynamics, but that’s my best ELI5.
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