A Complete Introduction to Black Holes:
One of the most mysterious objects in space are Black Holes, dark voids formed by the collapse of massive stars. What secrets do these dense beasts hold? In this article we will unravel the mystery surrounding these fascinating structures.
Origins:
The idea of the existence of Black Holes was first proposed by John Michell in 1783. In it, he pointed out that a start that was sufficiently massive would have such a strong gravitational pull, that even light could not escape it. A similar suggestion was made by the Marquis de Laplace, apparently independent of Michell. The exact mass of the star required for it to form a black hole was calculated by Indian-American Astrophysicist Subrahmanyan Chandrasekhar. On his way to England, to study in Cambridge, he calculated the maximum mass a star could have, and still separate it from it’s own gravity when it ran out of fuel.
When a star becomes very small, the matter particles get very near each other. But the Pauli exclusion principle says that two particles can’t have both the same position and same velocities. Therefore, they must have very different velocities. This makes them move away from each other and tends to make the star expand.
However, as Chandra realized, there is a limit to the repulsion that the exclusion principle can provide. The theory of relativity limits the maximum difference in the velocities of particles in the star to the speed of light. So, if a star was sufficiently dense, the exclusion principle would not be enough to prevent it from collapsing.
Scientists like Eddington and Einstein refused to believe Chandra’s conclusions. Chandra abandoned this line of study after being ridiculed for proposing something so unnatural. Eventually, scientists realized he had been right. When he was awarded the 1983 Nobel Prize, it was is part of his early research in “limiting the masses of stars”. The maximum limit came to be known as the “Chandrasekhar Limit” which’s value is 1.4 msun or 1.4 times the mass of our sun, or roughly 2.7846 x 10³⁰ kg.
Formation:
Stars are balls of gas, which pulled together by gravity in gigantic gas clouds. The gas is Hydrogen, and a star has enormous amounts of it. As gravity tries to pull the gas to the center, the temperature starts to rise and the Hydrogen molecules fuse to form Helium, the next element, by the process of nuclear fusion. This releases a tremendous amount of energy, which causes the star to expand countering the force of gravity.
This balance between pressure and gravity keeps the star alive and kicking for millions of years. But nothing lasts forever.
After millions of years, the star starts to run out of fuel. The pressure against gravity starts to die out and gravity starts to compress the star. The helium starts fusing into lithium, lithium into beryllium. But when all the stars fuel turns into iron, whose fusion releases no energy, the star’s death has arrived.
The gravity of the star compresses it even more and the star dies with a final burst of light, releasing more energy than it ever did in its whole life, a supernova.
Decision of Fate:
Remember from the introduction, the Chandrasekhar Limit? Well, the time has come to put it to use. We will call it M.
After the supernovae, if the star weighs less than M(1.4 msun), it turns into a white dwarf, with a radius of a few thousand miles and a density of hundreds of tons per cubic inch. A white dwarf is supported by the exclusion principle between the electrons in its matter.
Stars weighing equal to M are compressed by their gravity into a neutron star with a diameter of about 12 miles while weighing 1.4 msun. Just a tablespoon of a neutron star can weigh more than a billion tons. These stars are supported by the exclusion principle between the protons and the neutrons.
On the other hand, stars with masses more than M have a problem. Their gravity is so powerful that it compresses it till it cannot be compressed any more, till the smallest possible unit of length, the Planck length (1.616255(18) × -10³⁵ m). The star’s entire mass is compressed in that single point in space. Thus, a Black Hole is formed.
Misconceptions about Black Holes:
Let's discuss some of the most common misconceptions about black holes and what really the truth is:
- Black Holes are very big : One of the most common misconceptions about Black Holes is that they are very large. As discussed above, Black Holes are one of the smallest things in the universe, measuring a diameter of 1 Planck length. However, their sphere of influence is very large (pun intended).
- Black Holes have infinite gravity : I have heard many people say that Black holes have infinite gravity. This is completely wrong. Black Holes have the same gravity as a star of the same mass outside the surface. If our sun was replaced by a black hole of the same mass, our orbits would stay the same. The difference is felt only inside the surface. The star’s gravity starts decreasing and we go closer to the center, while the black hole’s gravity keeps increasing. When we go inside a star, the amout of matter pulling us decreases, while in a black hole, the singularity always lies in our future, and never in the past.
- Black Holes devour everything close to them : As discussed above, black holes don’t have infinite gravity. So they don’t devour everything close to them. Only after you cross the Event Horizon(will be discussed), can you never come back.
Structure of a Black Hole:
As we have discussed the origin of and the common misconceptions about Black Holes, we can go into discussing its structure.
A black hole has mainly two parts, an event horizon, and a singularity. Among these the event horizon is imaginary. There is also an ergosphere, but it is related to a complex topic of spinning black holes, so I am not mentioning it here. You can see my article about them if you are interested.
The Event Horizon:
When you get close to a black hole, its gravity start’s pulling you. Slowly, your speed starts increasing. This speed is called escape velocity, or the velocity required to escape from the black hole’s gravity. The event horizon is the imaginary circle around the black hole where escape velocity v equals the speed of light, c. Since nothing can travel faster than light, nothing can escape once they reach the event horizon, not even light. The diameter of the event horizon is directly proportional to the mass of the black hole. It is the limit of what we can see from the outside, as no light escapes from there. This is what earns a black hole it’s name.
The Singularity:
The singularity is the Planck length sized point which contains all the mass of the black hole. It is at the center of the event horizon and it the actual body of the black hole.
Journey to a Black Hole:
Let's discuss what you’ll see if you travel to a black hole. The circumstances are:
The mother ship is in a safe orbit around the Black hole outside the event horizon. You have a radio transmitter attached to your watch which sends a signal every second. You jump out of the ship towards the Black hole. You’ll reach the Event Horizon at 09:00:00 AM.
As you get closer and closer to the event horizon, your crew-mates at the mother ship will notice that the difference between intervals between individual signals from your transmitter is increasing. This is because the radio signals have to fight extra hard to escape due to the black hole’s gravity. The difference will be very slight till 08:59:59, but they will have to wait forever for the 09:00:00 signal as you have crossed the Event Horizon, and the radio signal is forever stuck inside, along with you. From your jump, they will see you travel slower and slower till you come to a stop at 09:00:00. Your image will slowly turn redder and redder(being red-shifted) till it becomes too dim to see and you are trapped in the black hole forever with your 09:00:00 signal and everything else that ever ‘fell in’. The slowing down and the red-shift are due to the light rays from your body bending due to the immense gravity of the black hole.
On the other hand, for you, the mother ship appears to travel slower and slower while you accelerate very fast. You won't even notice when you travel past the event horizon. Soon, you start feeling the tidal forces of the black hole. Centimetres of difference will cause an enormous difference in gravity and you will be pulled till you become very long and very thin. This process is called spaghettification. Soon the gravity will be so powerful that every atom of your body will be ripped apart and you’ll die a quick and painful death.
But let’s go beyond death. Soon you’ll reach the singularity and the black hole’s gravity will compress you into the singularity.
Hawking Radiation:
From the above-stated characteristics of Black Holes, you may conclude that Black Holes never die. But late American Physicist Stephen W. Hawking said that black holes can evaporate by a process called Hawking Radiation.
The theory states that a pair of virtual particles(resultants of quantum fluctuations), one positive and one negative may form just outside the event horizon of a Black Hole. Virtual particles are another of the little quirks quantum mechanics offers to us. While one escapes to safety and becomes a real particle the other enters the black hole, annihilating a real particle inside and thus lowering its mass by a tiny bit. For those who are new to this concept, you can think of the space as a mathematical equation, where we add a +1–1 to it. While we move our +1 away, it becomes a part of the equation (in other words, becomes a real particle), while we can move the -1 to the “black hole” part of the equation and removing a +1 from there. This is equivalent to removing a +1 from the black hole itself.
But Black holes can take enormous amounts of time to evaporate completely. It takes a Black Hole of 1 msun 10⁶⁷ years to evaporate completely. If the universe lasted 10¹⁰ more years for each of its 10⁸⁰ electrons, that would still be 10 million times less than the lifespan of the most massive black hole.
Conclusion:
Black Holes are mysterious things with a strange birth, a long life and an even stranger death. I have just scraped the surface of what is possible in the world of black holes. But hope you got the basic idea of them and will continue to venture further into this wild world.
-Rak Laptudirm
