What is Hawking Radiation?

Black holes are full of mysteries. They have such tremendous gravitational force that even light doesn't escape from it. Black holes are usually known for their size. They are huge and can become bigger by consuming surrounding objects. But, we have never heard of black holes the size of Earth, or black holes the size of a coin. Do such black holes even exist? If yes, why don't we get to observe them? You will find the answer to these questions in this post.

Hawking Radiation:

The major reason that we don't see much of small black holes is the phenomenon called the Hawking Radiation. This phenomenon occurs at the event horizon of the black hole. Event horizon is the imaginary boundary associated with a black hole. If anything goes beyond the event horizon, it can never escape the black hole. Black holes take in astronomical objects like stars and planets. Due to the tremendous gravity, these objects are literally shredded to atoms and converted to energy. So, just at the event horizon, sometimes this energy creates a particle-antiparticle pair. From this pair, one entity is consumed by the black hole and the other one, which is outside the event horizon, escapes from the gravitational force of the black hole. This radiation of particles from the event horizon of a black hole is called as Hawking Radiation. As this phenomenon was discovered by Stephen Hawking, it is named Hawking Radiation.

Hawking's insight was based on a phenomenon of quantum physics known as virtual particles and their behaviour near the event horizon. Even in empty space, subatomic "virtual" particles and antiparticles come briefly into existence, then mutually annihilate and vanish again. Close to a black hole, this manifests as pairs of photons. One of these photons might be pulled beyond the event horizon, leaving the other to escape into the wider universe. Careful analysis showed that if this happened, quantum effects would cause a "partner wave" carrying negative energy to be created and also pass into the black hole, reducing the black hole's total mass, or energy. In effect, to an observer it would appear as if the gravitational force had somehow allowed the black hole's energy to be reduced and the energy of the wider universe to be increased. Hence black holes must gradually lose energy and evaporate over time. Considering the thermal properties of black holes, and conservation laws affecting this process, Hawking calculated that the visible outcome would be a very low level of exact black body electromagnetic radiation produced as if emitted by a black body with a temperature inversely proportional to the mass of the black hole.

An alternative view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole while the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). This causes the black hole to lose mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunneling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon

Back to the question

Do small black holes exist? Yes. They do exist but for a very short time. Hawking radiation is inversely proportional to the size of the black hole. So, the smaller the black hole is, the greater is the rate of radiation. And Hawking Radiation is the process of ejecting an actual particle, so it reduces the mass of the black hole. A coin-sized black hole will barely exist for a few microseconds before decomposing. This is the reason why tiny black holes are so rare.

Thank you for reading.