This article on Hawking radiation is a guest article by Karnika Bhardwaj who is pursuing Bachelor's in Physics from Delhi University.
The Pillars of Physics
“A Theory of Everything” is what many physicists are concerned about since the era of Archimedes. This ultimate theory will provide us with the key of our universe (or multiverse). The close resemblance is the two theoretical frameworks known as “General Relativity” and “Quantum field theory”. The General Theory of Relativity provides with the description of gravity in the region of large scales and high masses. The Quantum field theory provides with the description of the non-gravitational forces. Together, these are the two main pillars of Physics.
General Relativity And Black Holes
General Relativity was given by Einstein in 1915. It provided us with a set of beautiful non-linear partial differential equations and its solutions were even more beautiful. The first solution that came out was after a year from a German physicist Karl Schwarzschild. It gave us a perfectly spherical non-rotating black hole. The another solution was provided by Roy Kerr. It was a rotating black hole and thus began the era of black hole research.
In the mid 60’s, Roger Penrose devoted himself to the study of black holes and proved an important theorem that the gravitational collapse of a large dying star must result in a singularity, where space-time cannot be continued and General Relativity breaks down. Penrose in collaboration with Hawking further investigated this matter with the help of a more complex mathematical model derived from the General Relativity. This matter turned up the heat of the black hole research by stating that the singularities were not a problem but an aspect of the general relativity.
Black Holes And Entropy
The entire idea a black hole we had till then was that nothing can come out of it. It can’t emit light. It can’t reflect. What goes in never comes out. When we talk about the area of a black hole, what we are really describing is the boundary region after which no information can ever escape. Then Hawking realized that if it absorbs stuff, it must be taking entropy out of one side of the boundary and according to second law of thermodynamics, entropy always increases. So, each times it gets something in, its entropy increases. Hawking then suggested that the black holes must actually be evaporating. They must actually be radiating. According to the quantum theory, the fluctuations in the field allow the formation of virtual particles in space. This phenomenon keeps going.
The Hawking Radiation
These pairs are created and annihilated very quickly and thus, they cannot be observed directly. Hawking showed that when the formation of the pair of particle and anti-particle (formally known as, ”Virtual Particles”) takes place near the event horizon, then the positive particle may escape, leaving its anti-particle inside the event horizon. The escaped particle is then observed as the thermal radiation radiated by the black hole. This is known as “The Hawking Radiation”. The anti-particle of negative mass falls into the black hole, and reduces the mass of the black hole. The consequence is known as “the Information Paradox” or “Hawking Paradox”.
This paradox states that the physical information of the particle which fell into the black hole is completely lost which contradicts our known physical laws which states that the physical information is always conserved. Hawking defeated this for long run of 30 years until he accepted his defeat in what is now known as the “black hole war”. Leonard Susskind solved this paradox by bringing in the notion of holographic universe.
Black Holes or Something Else?
Even 100 years after the theoretical discovery of black holes, there are still many unsolved problems. While the entire black hole paradigm suggests the formation of gravitational singularity, the works of many other scientists show that this is not possible. There are many alternate black hole models which can explain the experimental observations which the current model cannot. One of them is the observed magnetic field around a black hole which is way stronger than the theory predicts. This is solved by the ECO (Eternally Collapsing Object) model which says that the black holes are actually collapsing object that hit the singularity only at infinite time.
Hawking radiation has no experimental proof. This is the reason why Stephen Hawking never won the Nobel Prize for this work. Who knows, maybe one day we will find the evidence.