The Official String Theory Web Site:--> Black Holes :--> Hawking radiation questions (basic / advanced)

In the previous section the two main classical properties of black holes -- the total area of event horizons can only increase, and the surface gravity is constant over each event horizon. We call these classical properties because they were discovered by solving the Einstein equations, which are equations that do not use quantum mechanics.
What happens when we add quantum mechanics to the analysis of classical black holes?
The easiest way to combine quantum mechanics with classical general relativity is to look at particle scattering in curved spacetime, where the spacetime curvature can't react to the scattering quantum particles. This is a bit of a fake, because we're keeping the gravity part classical and only using quantum physics for the particles. But even so, stuff happens that is very astounding and important to the theory of black holes.

#### Black holes decay!

Theoretical physicists who studied quantum particle scattering in a curved spacetime discovered that the definition of particle and antiparticle depends on the observer, which is against the usual rules of the theory of general relativity. The implication of this is that the number of particles being counted depends also on the observer
The picture that emerged from all of these studies is that if a physicist were tossed into a black hole, he wouldn't see anything special happen at the event horizon. He would just be crushed by the huge gravitational forces at the center. However if he were held just outside the event horizon by a rope attached to his thumbs, his toes would be burning from a hot soup of particles being emitted from the black hole.
But how can particles get out of the black hole? Even light can't get out of a black hole. (Light is made of massless particles, and if massless particles can't escape, then neither can the particles with nonzero mass.)
In classical black hole physics, the event horizon is an absolute barrier to everything trying to get back outside. However, quantum mechanics brings with it quantum uncertainty, and quantum vacuum fluctuations where particle-antiparticle pairs are always being created, then destroying one another, virtually, in the vacuum.
In the animation above, P stands for particle and A stands for antiparticle. A particle-antiparticle pair is created for a brief instance just outside the black hole event horizon. Before the pair can destroy one another as usual, the antiparticle is sucked behind the event horizon, while the particle is ejected in the opposite direction. (Or vice versa.)
According to the physicist observing the event horizon by hanging from a rope by his thumbs, the black hole has emitted a particle through the event horizon. To a distant observer, the black hole's mass has now decreased by the mass of the emitted particle, and the area of the event horizon has gotten smaller!
But how can this happen? This means that the total area of black holes can and will decrease in time, and black holes can decay, contrary to the classical prediction using the Einstein equations and neglecting quantum physics.

#### Where are the quantum microstates?

Not only does the black hole decay, but the particles it spits out when it decays have a thermal distribution. These decaying black holes start to look like thermal objects that classical physicists have studied in thermodynamics since the 19th century.
But in the 20th century, in the quantum revolution, it was discovered that all 19th century classical thermodynamics could be described as the bulk limit of sums of quantum microstates. The thermodynamics of steam power plants reduced to understanding the quantum microstates of water and air molecules, for example.
So then, what are the quantum microstates that give rise to black hole thermodynamics?
String theorists believe they have the answer.

Hawking radiation from a black hole

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