Surely you have heard that black holes don't emit anything, even light can't escape from them! But what if there is a peculiar sort of radiation, one whose origin lies in quantum mechanics, that defies even this logic and is emitted from black holes?
The core of Hawking radiation is that it describes the process of hypothetical particle formation near a black hole’s boundary. The term ‘hypothetical’ is of key importance here and we will look more into it as we go further. The radiation also has another important consequence: A black hole’s temperature is inversely proportional to its mass. In simple words, this means that the larger a black hole is the cooler it is and vice versa.
Though the peculiar thing about this radiation is that it relies on the principles of both general relativity and quantum mechanics. As context, these two fields are long since thought to be irreconcilable. Our calculations breakdown whenever we try to find a way to apply both of them at a single phenomenon. Surprisingly, black hole radiation is one of the rare processes that are based on application of both giant fields.
The radiation was first proposed by Stephen Hawking in 1974 and although it has been widely discussed since then, its experimental confirmation is still debatably due. If in the future we do confirm it, we can we sure that black holes can emit energy and shrink in size with time.
Why should black holes glow?
Our key to understanding this question is entropy. Put simply, it is merely the measure of disorder in our universe. The second law of thermodynamics states that entropy and thus disorder should only increase with time. Imagine it like a cup proceeding to fall and break into pieces. It goes from a ordered state to a disordered one, thus following the law. Indeed the reverse would mean the violation of this fall and this doesn’t happen, broken cups going back to their original states is hardly a common occurrence.
Looking more closely at black holes we can see that since they ‘absorb’ matter and thereby remove it, they should decrease the entropy of universe making it less disordered. If this were true then it would be a direct violation of thermodynamics. Also the boundary of a black hole, called the event horizon should be most affected by absorption of matter. Naturally a person would expect it to increase in area with every mass that falls in. Jacob Bekenstein proposed that a possible way to conserve thermodynamics would be by imagining that the increased area should represent entropy that would’ve been lost. Hawking further expanded on this by saying that if the event horizon is imagined to have an entropy then it should also glow, since entropy is another way of describing heat energy.
Hawking initially thought to disregard Bekenstein’s proposal because a glowing black hole wouldn’t really be a black hole in strict sense. Instead, he later went on to discover that black holes actually do shine with cold light!
How do black holes produce Hawking radiation?
We will not exaggerate, the actual mechanism behind the emission of radiation from event horizon is a complex one that requires immaculate understanding of advanced sciences. But that does not mean we can’t explain the intuitive idea behind it.
We all know the basic and ever-constant rule of cosmos: Energy is always conserved. We start by expanding along this. In quantum mechanics, vacuum isn’t actually empty. Instead it is full of positive and negative particles forming and annihilating each other at instantaneous speed. It is much like how you would describe zero as sum of negative and positive one. Rather than calling zero ‘nothing’, we imagine it to be the result of countless negative and positive equal numbers colliding.
Vacuum is much similar to this, like and unlike pairs annihilating each other. But these particles are hypothetical, unreal. They do not actually exist and their only purpose is to form and destroy the opposite particle. On a rare occasion when such a ‘twin particle pair’ is formed near the event horizon, the black hole absorbs one of the particles. This split leaves half of every pair to escape the black hole as actual, real radiation.
In order to map this interaction properly, we would need a comprehensive understanding of gravity's role in quantum mechanics, but Hawking's results suggest that black holes 'scatter' some features while leaving others intact by altering the mix of quantum properties near their event horizons caused by curved space. A black hole can shrink as a result of these intact properties, which resemble specific radiation temperatures.