Hawking Radiation : Evaporation of Black holes

What is Hawking Radiation?

How do Black Holes radiate energy when nothing is known to escape from them? 

A black hole is known to radiate particles. It is through these radiation waves we determine the properties of that black hole and of the matter that falls into it. But how does this radiation occur if nothing can escape its gravitational pull?

There are mainly two reasons for this radiation:

1. Acceleration and tearing down of particles when matter falls into it.
2. Hawking Radiation.

Now, in the first case, the acceleration of particles at high speed in the accretion disk of a black hole causes the matter to be stretched along. By doing so, a large amount of heat is produced which results in radiation. But in the second case, the resultant radiation isn't caused b the matter that falls in. It occurs as a random process in the quantum theory called Hawking radiation. 

What is Hawking Radiation?

The late physicist Stephen Hawking predicted that black holes do radiate energy based on the principles of quantum mechanics. According to the Casimir Effect in quantum mechanics, it is known for a fact that space, even when it's empty, is always filled with particles and antiparticles that appear and disappear in and out throughout the fabric of space-time. These virtual particles (which comes in pairs) can't be seen through our detectors because they don't exist in space for that long to be detected. 

Another reason for this is that the pairs borrow energy from the existing system to exist but since the energy is 'borrowed' they won't wait too long to be involved in any energy transfer or reactions (because energy is conserved throughout our space). But if any of these particle pairs loses its partner, it becomes a real particle. Once it's a real particle, it can be detected as well as its energy effects.

In black holes, a particle-antiparticle pair loses its partner near the event horizon (boundary of a black hole) due to the gravitational pull of black holes. So when one of them falls into the event horizon, the pair separates and the escaped particle turns into a real particle. This real particle is then identified as a particle with positive energy or an ordinary particle. Thus the spare particle attains a real state and escapes off as energy radiation. 

But one question still remains. If the escaped particle was transitioned to a real particle, how will the borrowed energy get paid back or get redistributed uniformly? Or where does the escaped particle get enough energy to become a real particle? 

The answer is from the black hole. How? The antiparticle that fell into the black hole (which carries negative energy and mass) actually reduces the black hole's mass and thus its energy, as it falls into it. So technically, the black hole loses its energy due to the negative particle, but the lost energy of the black hole is regained in the same amount by the escaped particle to be a real particle. Thus, energy is back to constant and these transitioned real particles get radiated off from the event horizon as Hawking Radiation.

 It just appears as if the black hole is radiating energy particles from its event horizon when it's an actual transition of virtual particles into real particles by ''sacrificing'' their partner.

This also leads to the evaporation of black holes. After each radiation, the mass of the black hole gets reduced even more. But because black holes gain mass at the same time by gobbling up the matter around them, this evaporation process can be incredibly slow. So, it's clear that black holes gain mass at a larger scale than losing mass.