Black holes as a Light/Radiation source

The word black hole is of a very recent origin. It is an assumption of an idea that whenever a matter particle comes near the vicinity of a black hole, that particle falls into the black hole due to its strong gravitational field. This idea was based upon the fact that a sufficiently massive star can have such a huge gravitational field that not even light can escape through its surface. Most of us whenever hears the word ‘BLACK HOLE’ we often think of an object in the cosmos that is black in color and that pulls everything towards it, even light (moreover like an impending doom!). But in reality black holes are objects that emit large amounts of radiation and glow red hot (only this light fails to reach us) and rather than a depiction of a place of end of time, they actually hold keys for unlocking the mysteries of the universe.

To understand how this happens that black holes are glowing bodies and emit radiation, one needs to know how they are formed. A star mostly consists of hydrogen. As this hydrogen is packed into a tightly packed space, the hydrogen particles begin to collide and start a nuclear reaction with a big explosion (this gives the star its usual shine) and converts hydrogen to helium. This is a process with which most of us are familiar. What happens next is that when a star runs out of hydrogen or its fuel, it starts collapsing due to the total gravitational effect of the particles between each other (note that earlier when a star had its fuel tank full, it didn’t collapse because energy from the nuclear reactions was keeping it at a fixed size and when the fuel ran out, there was no energy from the reactions to keep the size of the star stable). So when a star starts to collapse after it finishes up all of its hydrogen, one may think that the star may go on shrinking till every matter is at a single point just like in case of a black hole. But this doesn’t happens because when matter particles are tightly packed, their velocities are increased and they vibrate and greater speed which produces and anti-gravitational effect. This is known as the Pauli’s Exclusion Principle. If you take a star full of neutrons and start squeezing them, Pauli Exclusion Principle requires you to excite the particles to higher and higher energies, so there are enough distinct quantum states(denoted by principal quantum number, the azimuthal quantum number, the magnetic quantum number, and the spin quantum number) for them to occupy. This causes a force that pushes back against the collapse. So according to the exclusion principle when star is collapsing, this principle generates a force or an anti-gravitational effect that repels the matter particles away from each other and at a certain point the star becomes stable. This stage of a star is known as a White Dwarf. If the exclusion principle is used when the neutrons are producing an anti-gravitational effect, the resulting stage of the star is called a Neutron Star. So one may ask that when collapsing star results in a black hole? That depends upon the size of the star before it starts to collapse, that is if the star’s mass is below a certain critical limit then it will form a white or a neutron star. This limit is called The Chandershekhar Limit. On the other hand if the mass of the star is above the Chandershekhar limit then the huge mass and its huge gravitational field of the star overpowers the anti-gravitational force of the exclusion principle and the star goes on contracting and due to this the star may explode(supernova) or it may form a black hole. In case of star collapsing into a black hole, all of the mass of the star is at a single point known as a Singularity. When light passes near this point, note that light does not get absorbed by the singularity but rather is trapped. It begins to travel around the singularity in a spherical manner that gives the black hole its circular shape (due to the strong gravitation of the singularity or the black hole, path of lights is bended). We cannot see this light because it fails to reach us. Light rays bend their path around a black hole because a wave can also behave as of particle nature. This boundary or an envelope of light around the singularity is called the Event Horizon. At the event horizon even lights fails to escape and as general relativity says that nothing can travel faster than light, nothing else can escape from the black hole. One thing more general relativity showed that time is not absolute, that time behaves differently to different observer. For example if passing of time is denoted by periodic pulses of lights then, time near a black hole will pass slowly ( light path is bended or trapped in the black hole) as compared to passing of time elsewhere. This effect shows us that time flows slowly near an object of large gravitational field. This is not just a theory, but this effect has its practical applications. It was discovered that time clock for the satellites orbiting the earth was running slowly as compared to the clocks on earth. Had this error not been detected, GPRS data would have varied by kilometers! One may say that these satellites are travelling in time.

As now we know that how black holes come into existence, one can understand that what does ‘Black holes aren’t so Black’ means.

Just near the event horizon, there is a lot of difference in energy level between outside of the black hole and inside. This area is empty of matter but not of energy. This fluctuation in energy is known as quantum fluctuation, as it arises due to uneven distribution of matter and energy in an area where black hole is present. What this means that the ratio of mass inside a black hole and outside is so great that it creates disturbance in the area near it. This disturbance breaks the classical Newtonian principle that ‘matter can neither be created nor destroyed’, as it results in the formation of an antiparticle and a particle. When these two collide they annihilate each other and nothing is left behind. To make this easy, imagine a positive charge and a negative charge (analogous of particle and antiparticle pair) and when they come close they cancel each other’s charge so that the net charge is zero. Similar happens when a particle and an antiparticle come close they annihilate each other so that net mass is equal to zero.

Antiparticles is said to have a negative mass and a particle is said to have and positive mass. Saying which is particle and which one is anti-particle is just a matter of perspective. Had we and all the matter around us was made up if antiparticles we would have simply named them particles! What happens is that when these pairs of positive and negative matter are formed they quickly vanish or get annihilated, but sometimes due to strong gravitation the black hole (which is made up of matter or particles or precisely positive mass) the antiparticle with negative mass is attracted towards the black hole and falls into it. This results in decrease in the mass of the black hole as antiparticles destroys normal particle. When the antiparticle is absorbed in the black hole, its leftover companion the particle escapes as radiation. The light given off by this radiation merges to the event horizon. As the mass of the black holes decreases, the radiation emitted becomes more and more intense.

One may now think that what is the importance of showing that black holes give of radiation? Well one of its major advantage is that it showed that theories of general relativity and quantum mechanics can be combined. Scientists for decades are trying to find a theory that can be applicable to the whole of the universe. This theory as proposed will be called the Grand Unification Theory (GUT). General relativity deals with massive objects like black holes, gravitation whereas quantum mechanics deals with small objects like matter particles. While making the GUT, scientists always had difficulty to combine these two theories which both are very important. With the discovery of black holes giving of radiation or to say that matter particles interacting with gravity and black holes gave insight to the scientists that theories of relativity and quantum mechanics do interact with each other in real life.

Originally published at on October 25, 2015.

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