Eli5 What is “Hawkings Radiation”?


This was partially inspired by the event horizon post, I’ve heard of Hawkings radiation recently but don’t understand it, any help?

In: Physics

It’s radiation given off by black holes. Over a long period of time (e.g. 10^100 years) the mass of the black hole is converted to radiation and the black hole gets smaller and smaller until it disappears.

Space isn’t actually empty. There are particles that randomly appear, then colide and eliminate each other. Hawking radiation is basically when these particles appear inside the event horizon of a black hole, but it’s partner is outside and escapes. This basically means the black hole is “radiating” particles. Over trillions of years this may even cause the black hole to shrink and disappear.

Space isn’t technically empty, it’s constantly filled with things called Virtual Particles. Virtual Particles spring up into existence from quantum fluctuations (no real simpler way to explain it) in pairs of opposites, one particle and one anti-particle. Conservation of matter says matter and energy cannot be created nor destroyed, but that’s a bit of an over simplification. Net matter-energy must be maintained and virtual particles do not break this because each particle is equal and opposite of each other. Normally they collide together almost immedietly and annihilate into nothing again, but when virtual particles are created at the edge of event horizons, this can’t happen. One particle pops into existence inside the event horizon and is lost, the other outside and escapes, becoming a real particle. Because of conservation of matter-energy, when this happens, the escaping particle steals some mass from the Black Hole, causing it to shrink. The actual process on how it steals mass is super deep into quantum physics but another point is that the escaping mass isn’t previous mass that fell into the Black Hole, it’s new mass that escapes in the same process that adds equal and opposite “Negative Mass” to the Black Hole.

This explanation is a bit of a simplification but not inaccurate description, seeing how we are dealing with the product of Relativity colliding with Quantum Physics, the fact there is an answer that can fit this subreddit is somewhat of a miracle.

At quantum scales, the vacuum of deep space is not actually a vacuum, but a quantum “foam” of virtual particle and antiparticle pairs which continually wink into existence and then immediately annihilate. Ordinarily, this results in no net mass change and no net energy change for the universe at large; however, immediately adjacent to the event horizon of a black hole things behave a bit differently. A photon and its corresponding antiphoton can come into existence in the quantum vacuum, but then instead of immediately annihilating, one happens to cross the event horizon and be lost to the black hole while the other escapes to infinity because it is now unpaired. Technically, the radiation is not being emitted by the black hole (which cannot emit anything by definition), but rather by the space just beyond the horizon, but the effect is the same as if the black hole itself were radiating.

A consequence of this Hawking radiation is that it provides a mechanism for the black hole to lose energy, since the absorbed photon must necessarily carry the same absolute quantity of energy of the emitted photon, only negative as it is the conjugate. This allows for black holes to evaporate, as the negative energy robs it of momentum over time, maintaining the energy balance of the universe.

Every object in the universe emits a ‘blackbody radiation’ equivalent to its temperature. Hawking radiation is a black holes equivalent to that, caused by the propensity for normal empty space’s quantum fluctuations to spawn virtual pairs of identical particles which immediately annihilate thereby leaving nothing but normal background fluctuations. Sometimes half of these particles are just within the boundary of the event horizon and the rest are not, those that are not do not immediately annihilate, and allow information/energy to be extruded from the black hole, denoting their ability to evaporate.

edit: I misspoke, the virtual particles are identical but opposite charged particles, so things like electrons and positrons, a particle and its antimatter counterpart.

Watch [this](https://youtu.be/QqsLTNkzvaY?t=502) It’s from Kurzgezagt so it’s as close to an ELI5 as you get regarding this topic.

Ok you hear a lot about virtual particles its nonsense but sometimes its useful nonsense. When it comes to Hawking Radiation I think its not useful nonsense.

So quantum fluctuations. They are a thing becaue we can’t say a quantum field is on zero energy state, you know there is always a bit of uncertainty. It averages out to zero. Think of it like waves of particles and waves of antiparicles they cancel each other to make the fields zero. (Note that regular fluctuations dont really make particles.)

So Nick Lucid from the Science Asylum made a great video about it with a pretty good analogy.

Imagine quantum fluctuations as a wave, it is a wave so it’s not that hard. Now imagine this wave is a string waving. There are frequencies that the string likes and dislikes. It likes frequencies that fill the string with whole periods. These are the natural frequencies of the string. Now pinch a point on the string. Its now a fixed point so there are only a certain number of frequencies left for the string.

A black hole does something similar to quantum fluctuations. It creates a fixed point so the waves that all came together to cancel in front of the black hole no longer cancel on the other side of the black hole as some of the waves are messed up by the black hole. So particles can pop into existence. This doesn’t necessarily happen on the edge of the event horizon but somewhere near the black hole.

Hawking Radiation can be any particle but its almost always photons. It’s easier to escape from a back hole if you travel at light speed. The light is really not gonna be localised though, its almost always super large radio waves. These carry away a tiny bit of energy from the only available source which is the balck hole itself.

So as HR happens the back hole gets lighter and the event horizon shrinks. HR gets faster as the black hole gets smaller and after an unreasonably large amount of time the black hole explodes.

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