Black Holes : The Recycle Bins of Universe

"A Black Hole is a region in spacetime where the Gravity is so strong that not even light can escape." This is the most standard definition of a Black Hole. The idea of a Black Hole was first proposed by an English clergyman - John Michell in 1784. However upon the discovery of wave nature of light, his idea was quite overlooked as it wasn't clear how a wave would be influenced by gravity. Some years ahead in time, Karl Schwarzschild discovered the first solution of the mathematical equations of Einstein's General Relativity which characterized a Black Hole. Physicist John Wheeler coined the term "Black Hole" - since they attract everything and almost nothing can escape from them. In this blog article, the formation of Black Hole, its anatomy and some strange consequences will be presented.

Image of Powehi captured by the Event Horizon Telescope.

              How is a Black Hole formed? It is definitely not easy to create an object that would basically attract everything that comes close enough to it. The density of such an object should be nearly infinite. A tremendously high amount of energy and work needs to be done to compress something into such a small space. The quantitative relation between any object's mass and the radius it needs to be compressed into to become a Black Hole was given by Schwarzschild. The radius is called as - "Schwarzschild Radius". This relation in itself is quite simple :

                                                         

                                                       R_g = 2GM/c².

              

                                                         

 Rg here is the Schwarzschild Radius, M is mass of the object, G is the Gravitational Constant and c is the speed of light. In the equation, 2, G, and c are just constant values. Hence, the radius is proportional to (depends on) mass of the object. Greater the mass, greater is the radius. If you were to convert Earth into a Black Hole then you must compress its entire mass into the size of a small marble. This really gives us an idea of how dense a Black Hole is. It is impossible at the present time and perhaps also in the future to artificially synthesize a Black Hole. But, this doesn't mean they don't exist at all. Even though the genius astrophysicist - Stephen Hawking disagreed with the notion of Black Holes at some point of time. We now have solid evidence of its existence. Black Holes do form naturally. However, as discussed before, the process is violent. They are typically formed when huge stars having a mass of almost 8-20 times greater than the mass of our Sun, die in an explosion called as - "Supernova". Doing a quick walkthrough of Stellar Physics - A star produces energy by means of nuclear fusion occurring at its core i.e. it fuses hydrogen and helium to produce new elements along with release of energy. The outer layers of a star tend to collapse onto its relatively denser core due to gravity. But, the radiation released from the nuclear fusion occurring at the core just balances this weight of the outer layer.

 With time, this fuel reservoir of hydrogen and helium in a star depletes and as a result it fuses heavier and heavier elements releasing lesser energy every time. Until a point is reached when no energy is released from this fusion and the weight of outer layers of stars is no longer balanced. They collapse onto the dense core of the star, which is now even denser because of the heavier elements fused and then rebound out causing a huge explosion - "The Supernova". When the mass of this star is great enough, the impact on the core due to this explosion is tremendous causing it to compress into a small volume. Thus, forming either a neutron star or a Stellar Black Hole. There also exists another class of Black Holes known as Supermassive Black Hole. It is believed that these black holes reside at the center (nucleus) of almost every galaxy in the Universe and that the stars in these galaxies revolve around this Black Hole in simple or complicated orbits. Our own Milky Way galaxy holds one at the center. and its named as Sagittarius A*. Supermassive black holes are far larger than stellar black holes and are believed to be formed either by the merging of smaller stellar black holes or by collision of multiple stars in compact star clusters. Astrophysicists have also hypothesized the existence of another class of Black Holes called Intermediate Black Holes, which are sort of midway between stellar black holes and supermassive black holes. Oddly enough, there are no firm observations which prove they exist. Another hypothetical class of black holes called Primordial Black Holes are believed to exist. These black holes might have formed in the early universe just after the Big Bang when the Universe was non-homogeneous and density irregularities caused matter in some regions to collapse into a Black Hole.

              Let us discuss the structure of a black hole now. Note, that by structure I mean those parts which could be observed by us using different means. What exactly happens inside a Black Hole is something of great mystery and something about which we know very little. Broadly speaking a black hole comprises of an accretion disk, the photon sphere, an event horizon and finally the singularity. An accretion disk consists of heated matter like gas, dust and plasma which orbits the Black Hole. Due to their high energy, they radiate X-rays and ultraviolet rays. The photon sphere as the name suggests is a sphere of photons (light rays) surrounding the Black Hole at a fixed distance. This sphere is made of photons which just manage to escape the Black Hole. However, due to its immense gravity their paths are bent forming a sphere around it. The next part of a Black Hole is crucial, because it is what marks the point of no return  - "The Event Horizon". Any object or creature unlucky enough to cross this point is doomed to fall into the black hole and stay there eternally. The event horizon  is the point beyond which one must travel faster than light to escape. Followed by event horizon is finally the "Singularity". A point in spacetime having infinite matter density. The density is so high that the spacetime around this point is highly warped giving rise to all sorts of strange effects.

Warped space and time around a Black Hole.

              

              But what exactly happens if one falls into a Black Hole? You must have heard numerous times that due to the strong gravitational effects the unlucky human would experience something called - "Spaghettification" meaning that since, the lower part of the human would be closer to black hole than the upper part. The gravitational pull on it would be immensely stronger than the upper part of the body. This will make the body stretched like a spaghetti. However, it is believed that this shouldn't be the case with supermassive black holes, where the tidal forces wont be that strong. What would happen in such circumstances? Due to the strong warping of spacetime around the singularity, any observer falling into it will experience "gravitational time dilation" i.e. the observer's time will slow down as compared to someone away from it. A second for the observer might be hundreds of years on Earth. Due to this, the observer would see the entire future unravel in front of him/her in just a matter of time. Since, the space around the Black Hole is warped too, the observer would notice strong distortions in the geometry of space. This is all we can speculate until the observer crosses the "event horizon". What happens after that? - Nobody knows at all. What happens at the singularity? Again, we don't have a firm theory for that. To answer those questions a Quantum Theory of Gravity needs to be summoned. In previous blog articles, I discussed about Quantum Mechanics - the theory of very small. Einstein's General Relativity which predicted the existence of Black Holes is apparently a large scale theory and fails to apply in the realm of Quantum Physics and vice versa. In normal situations this discrepancy can be ignored. But in bizarre scenarios like at the heart of Black Hole. there is so much matter condensed in such a tiny space that both relativistic and quantum effects ought to be considered. Thus, a marriage of the two theories will allow us to peek into the singularity and also not cross the event horizon.

             Numerous efforts are being made to obtain such a theory and many potential candidates had emerged with time (the famous String Theory being one of them). But each theory is incomplete in one aspect or the other. Inspite of that, it never stopped some great physicists from presenting elegant theories about the quantum effects which could possibly be accompanied with a black hole. Stephen Hawking's famous theory of "Hawking Radiation" is one of them. The work of physicists like Gerard 't Hooft, Jacob Bekenstein and Roger Penrose gave rise to the interesting subject of Black Hole Thermodynamics. I shall discuss their beautiful work in the next blog article. Until then, remember not to fall into a Black Hole unless, you want to be a spaghetti.

- Thank You.