Stephen Hawking was one of the most important physicists of the last decade. One of the most important discoveries he made during his career was the named Hawking radiation. This radiation is responsible for black holes emitting energy, and, consequently, their gradual evaporation.
But what is exactly Hawking radiation, and how is it produced?
What is Hawking radiation?
The idea of Hawking radiation is based on the fact that empty space is not actually empty. This can be a concept hard to understand. Even if there aren’t particles in the vacuum, the quantic fields that permit their existence are present.
Because these fields don’t have zero energy, in the smallest scales, pairs of particles, named “virtual particles” are created. These pairs of particles consist of a particle and an anti-particle. In a fraction of a second, they annihilate each other, releasing energy in the process, so that they “return” the energy they used to appear from nowhere.
Stephen Hawking, in an attempt to know if black holes have a temperature, asked himself what would happen if a pair of virtual particles appeared just at the boundary of the event horizon of a black hole (the event horizon is the boundary from which it becomes impossible to escape from the black hole).
In a paper he published in 1974, Hawking proposed that when a pair of virtual particles appear near the event horizon of a black hole, one of them falls into the black hole, while the other gets to escape before they can annihilate each other. The net result is that, to someone observing the black hole from the outside, it would seem that a particle has been emitted. This is what we call Hawking radiation.
The temperature of black holes
Due to Hawking radiation, black holes have a temperature. However, their temperatures are extremely low. Hawking demonstrated that the quantity of energy released by a black hole is inversely proportional to its mass. Therefore, the greater the mass of a black hole, the lower its temperature will be.
A black hole of one solar mass (the mass of the Sun) would have a temperature of 10-8 degrees Kelvin, while a black hole of one million solar masses would have a temperature of about 1014 degrees Kelvin.
These temperatures are just above absolute zero (the lowest temperature possible), and they are extremely low compared to the Cosmic Microwave Background, which is the leftover radiation from the Big Bang, and has a temperature of about 2.725 degrees Kelvin.
Loss of the mass of black holes
Hawking radiation has an energy of positive charge, which implies that the black hole loses the corresponding energy when it emits a particle. This loss of energy (and consequently mass) can also be described as the absorption of negatively charged particles by the black hole.
This constant absorption of negatively charged particles, makes the black hole lose mass over time. As we previously explained, the more mass a black hole has, the less energy it loses, and inversely.
So, black holes don’t live forever. Black holes that stop absorbing matter will finally disappear. However, this process of evaporation takes an enormous quantity of time. It is estimated that a black hole of one solar mass (and remember that there are black holes with tens of millions of solar masses) would take 1064 years to completely disappear. In comparison, the age of the universe is about 1010 years.