Stars are among the most fascinating things in the universe, but their birth and death are not so fascinating, rather mysterious and puzzling. There are humongous numbers of stars in the cosmos and at the end of their lives some die quietly and some go out in spectacular explosions giving birth to black holes. A black hole is a region in space where the force of gravity is so strong, not even light, the fastest known entity in the universe, can escape. The boundary of the black hole is called the event horizon, a point of no return. When something crosses the event horizon, it collapses into a black hole’s singularity, an infinitely small, infinitely dense point where space, time, and the laws of physics no longer apply. In the initial days of space research, it was believed that black holes are objects into which things could enter, but never leave. However, this all changed when Stephen Hawking put forward a theory regarding a special kind of radiation, which later became known as Hawking radiation.




According to the classical mechanic, a black hole is an object in space that engulfs everything and doesn’t allow anything to escape when it is inside the gravitational field, even the light itself. This postulates of the classical view of black holes suggest that the mass of a black hole cannot decrease, it can either increase or remains the same. If mass and energy are added to a black hole, then its radius should get bigger. If the radius gets bigger, then its surface area will get larger as well according to the equation:  4PiR*2 (formula for the surface area of a sphere).  To Stephen Hawking this idea of the surface area staying the same or increasing seemed very similar to the second law of thermodynamic; In any natural process, the entropy of any closed system increases or remains constant, it never decreases. As a result, Hawking proposed a theorem for the black hole, and it is called the second theory of black hole mechanics. And it says; in any natural process the surface area of the event horizon of the black hole increases or remains constant, it never decreases. Similar to the second law, there are also ways to state the other three laws of thermodynamics in a way that are true for black holes as well.

In thermodynamics, there is something called a black body. A black body is something that only absorbs radiation and it cannot reflect or transmit any radiation. Analogously, a black hole is something that also does not transmit are reflect any radiation, it only absorbs radiation. Therefore, thermodynamic suggests that perhaps black holes are physically a thermal body. If a black hole can be thought of as a black body, then it must have a temperature associated with it, because a black body in thermodynamics always has a temperature. But if it has a temperature, it must shine in some way. Now we have a riddle because according to classical physics, a black hole is not supposed to release anything. Objects go in, no objects are supposed to come out. These two thoughts stunned Hawking and he set out to prove that why black holes should not shine. But when he applied the laws of quantum mechanics to the laws of general relativity, he found the opposite to be true. He realized that stuff can come out near the event horizon of the black hole. In 1974 he published a paper where he outlined a mechanism for this shine.

So, how does it all happen? The theory of very tiny- quantum theory- says that space is never empty at the atomic level, there is always a tiny amount of activity. But the theory of very big- classical theory- says that nothing exists at the edge of a black hole, because it is just an empty space. Hawking realized this is where the two theories clashed. According to quantum theory, there are pairs of particles in empty space that emerge out of a void, exist for a tiny moment, and then destroy each other. These are called virtual particles. This phenomenon of formation and annihilation of virtual particles happens throughout space. The pair of particles are like two poles of a magnet, cannot exist without each other, one has a positive mass and the other has a negative mass. Hawking asked, what would happen if this pair of particles pumped up against a black hole. He realized that the positive particle would have just enough energy to escape the black hole, but the particle with negative mass would fall in, and it would do something extraordinary to the black hole. A particle that goes inside the black hole eventually decreases the mass of the black hole, because it effectively has a negative mass, but the particle that goes off to the distant observer is observed as part of radiation. The particle we get as radiation possesses positive energy and this energy comes from the black hole, as a sequel the black hole gets negative energy and that is the reason why the energy of the black hole decreases. And it is the same as the decrease in the mass of the black hole as it is given by the mass-energy equivalence equation: E=mc*2. So the virtual particles are created in space by borrowing energy so that nothing violates the law of conservation of energy, the energy of the shine is really coming from the mass of the black hole. And this is what we call hawking radiation.

He was the first to set out a detailed theory of cosmology explained by emerging quantum theory and the general theory of relativity. Scientists still don’t completely understand how quantum mechanics explains gravity, but Hawking radiation continues to inspire research and provides clues into the nature of gravity and how it relates to the other forces of nature.