Black Hole

“Black holes are where God divided by zero” – Albert Einstein. 

Black holes are giant black bodies in the outer space, which have baffled the minds of many! Many believe their existence as an enormous body that pulls everything in and destroys it and many other believe their existence as a place where time stops and freezes everything. But apart from these beliefs, there are real questions like: how black hole is formed? How it was discovered and what are the theories that explain the nature of the black hole.

Black holes are formed when a star collapses upon itself. In short, a star causes a supernova and explodes to form a black hole. Simple enough, isn’t it? Now let’s get into the discovery of black hole and many theories surrounding it.

BEGINNING OF BLACK HOLE THEORY:

 The idea of Black hole was first put forth in the 18^th century by John Michell. He put forth this idea in a letter published in 1783-84. The calculations in the letter assumed that such body might exist, which will absorb anything and everything. The gravitational pull of such body will be very strong and its density will be equal to Sun’s density. He also mentioned that such non-radiating body will be formed when a star collapses (or dies).  Not everything Michell mentioned in his paper was true and it was based on assumptions. But this idea of Michell interested many scientists and thus study of black holes began, with proper equations and calculations. 

In the later years, Einstein came up with general theory of relativity. In which, he did mention that motion of light can be influenced by gravity. Few months after Einstein, Karl Schwarzschild came up with solutions to Einstein’s equations. He concluded that there is area around black holes with very high gravitational pull and every mass becomes singular in this area. It was identified as Schwarzschild’s radius. Later many theories came up which suggested that certain assumptions made by Michell were not true. Like, a body like black hole cannot have density as equal to sun; as with so much density its mass will be very high. This will lead to curvature in the space-time in such a way that the space would close up around the body, leaving us outside it (nowhere).  In 1931,  Subrahmanyan Chandrasekhar proposed that a non-rotating body above a certain limiting mass have no stable solutions. Meaning that body will either collapse in the black hole or it may not. The limiting mass is 1.4 M and is called Chandrasekhar limit

Hence it was concluded that no laws of physics could stop a star from collapsing to Black hole. All these were the old theories in which it was suggested that black holes are non-rotating, non-radiating bodies which  can only pull matter inside it. 

 THE NEW ERA:

Further studies in the field of black holes led to new understandings of the black holes. In 1958, the Schwarzschild’s radius was identified as ‘event horizon’.  Event horizon is the boundary in space-time beyond which events cannot affect the outside observer. In simple terms, it means that it is a “point of no returns”. That is, the gravitational pull at this boundary is so high that it becomes impossible, even for light, to escape it. The area of event horizon increases when a matter or radiation falls in black hole. If two black holes collide, the area of event horizon will be greater than or equal to the sum of area of event horizons of the two black holes.

Stephen Hawking suggested that black holes follow the 2^nd law of thermodynamics. If a matter of high entropy is thrown in a black hole, the area of event horizon increases. However, this theory has one flaw. If a black hole has entropy then it should also have temperature. And if it has temperature, it should emit radiation. Consider two vessels; one is being heated at high temperature and the other one is kept at room temperature. The heated vessel with emit more radiation than the cold one but both of them emit radiation. So by this law, black holes should emit radiation. But by their old definition, black holes shouldn’t emit anything.

This leaves the theory in contradiction. However, in the year 1973, by quantum mechanics it was proved that black holes do emit radiation. Stephen Hawking himself solved the equations to find out that not only rotating but non-rotating black holes too emit radiation at a steady rate.

Now the question is, where is this radiation coming from? The answer lies in the quantum mechanics. The particles are coming from the “empty” space just outside the black hole. This “empty” space consists of particle/antiparticle, which collide with each other to become neutral. Normally the energy around the black hole is positive but inside it, due to strong gravitational field, the positive particle too becomes negative.  Now since the particle becomes anti particle in the gravitational field, it does not need its partner to become neutral. Thus, two cases arise; either the particle falls in the black hole to become anti particle or escape from the vicinity as a positive particle. These escaped particles will appear as emitted particles to an observer at a distance.

DEATH OF BLACK HOLE:

Lower the mass of black hole, higher the temperature. Higher the temperature, higher is the rate of emission, thus loosing mass at higher rate. What happens when the mass finally becomes zero is still unknown. But it is guessed that it will explode and emit energy equivalent to millions of hydrogen bombs.

PRIMORDIAL BLACK HOLES:

Some black holes have a lifetime of millions of years. On the other hand, there is an assumption that there are primordial black holes, which were formed in the beginning of the universe. They have low mass and some of them must have evaporated by now. But some of them may have the age of the universe. These primordial black holes emit X-rays and gamma rays. They are considered to be very hot and bright and emit energy at a rate of ten thousand megawatts. To prove their existence, we have to detect gamma rays emitted from them. But considering their distance, it is impossible to detect them. But if there is a primordial black hole at a distance of Pluto from Earth, it might be possible at some level. We have to develop a huge gamma ray detector, which seems difficult. We can use Earth’s atmosphere as a detector. The gamma rays create electrons and protons as they enter earth’s atmosphere. When this pair collides with other elements, it gives rise to more elections and protons. This gives rise to Cerenkov radiation (flashes of light in the night sky). But these flashes can also be caused due to lightning, debris in atmosphere etc. So this method is also not feasible. So even if we can’t prove the existence of primordial black hole, it is evident that black hole would and should emit radiation like a primordial black hole.

To conclude with, there are many more mysteries to be solved about black holes. There is more to the nature of black hole, which we have to study. Recently, Scientists have started a new project, the Event horizon telescope (EHT), to study the event horizon of Milky Way’s black hole, Sagittarius A* and another black hole in Messier 87. This project will take us a step forward in the study of black holes!