Almost a year ago, when the first-ever image of a black hole in our neighboring galaxy was released, the whole world went crazy for it. I know, you too! So now, after studying Stellar Astrophysics in detail in the last few articles, today, I thought of taking a step forward to meet the pop science’s favorite topic: Black Holes. So, in today’s article of our “Basics of Astrophysics” series, let’s explore these beasts!
Well, we all are well acquainted with one basic definition of a black hole, “an object so massive and so dense that nothing, not even light, can escape from its gravitational attraction” Right? But, how did the concept of these bizarre objects landed at our doors? Well, It’s a long story…
Hinted at as early as the 1780s by John Michell, Black holes have been sucking up scientific attention from the very beginning. However, the most important and modern proposal about these beasts came up with Einstein’s theory of general relativity. In 1687, Isaac Newton established that a force of gravity acts in between each and every object in this universe, however, he was himself never very clear about the origin of this entity. But, Einstein gave a completely new definition to gravity in his General Theory of Relativity.
According to him, all the massive objects cause a distortion in space-time, which is felt as gravity. This means that gravity is nothing but a distortion of space-time. Just like the trampoline cloth gets twisted when you jump on it, massive objects also warp the space-time in a similar manner. Heavier the object, more pronounced is the bending!
Still considered one of the most beautiful and elegant scientific theories of all time, this theory of Einstein showed that whenever a massive star dies, it leaves behind a small, dense remnant core. From the possible solutions of Einstein’s field equations, Karl Schwarzschild concluded that if the core’s mass is more than about three times the mass of the Sun, then the force of gravity overwhelms all other forces, producing a black hole. If you want to know how a star dies and how these dead cores form, you can read the previous articles of this series given below.
Basic Terminologies Related To Black Holes:
Let’s have a look at some of the basic terminologies related to Black holes.
Event Horizon: The event horizon is the critical boundary beyond which nothing, even light cannot escape. In the reference frame of the in-falling matter, everything is fine. But, to an outside observer, things appear differently because of gravitational time dilation. As the gravitational pull increases, light from the in-falling material starts becoming redshifted and as the material reaches the event horizon, due to tremendous redshift, it fades away. So, an outside observer can never witness the formation of the event horizon of the black hole. We discussed the concept of the gravitational redshift in the 6th article of the series.
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Singularity: According to General Relativity, a gravitational singularity exists at the center of a black hole. Singularity is the region where the spacetime curvature becomes infinite. The singular region has zero volume and is thought to have infinite density. The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.
Photon sphere: A photon sphere or photon circle is an area or region of space where gravity is so strong that photons are forced to travel in orbits. It is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole. For the non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The rotating black holes, on the other hand, possess 2 photon spheres: one rotating in the same direction as the black hole, and the other one rotating in the opposite direction.
Ergosphere : Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still. This region of spacetime is called the ergosphere. Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing the latter to slow.
Innermost Stable Circular Orbit: According to Newtonian gravity, the test particles can orbit stably at some arbitrary distances from a central object. However, in general relativity, there exists an innermost stable circular orbit (often called the ISCO), inside of which, any infinitesimal perturbations to a circular orbit will lead to inspiral into the black hole.
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Classification of Black Holes:
Broadly, black holes are classified into three main categories. They are:
- Micro black holes
- Stellar mass black holes
- Supermassive black holes
Now let us learn about each of these one by one.
Micro Black Holes
Also known as quantum mechanical black holes, the micro black holes are hypothetical. The fact that black holes smaller than stellar mass can form was first theorized by Stephen Hawking in 1971. These micro black holes have a certain mass limit. According to the concept of Schwarzchild radius and Compton wavelength, the minimum mass of a micro black hole is 22 micro-grams, also known as the Planck mass.
The quantum mechanical black holes must have played an important role in the early universe in tremendous energy and density. Such black holes, however, were unstable and must have evaporated through the Hawking radiation. In 1975, Hawking showed that due to quantum mechanical effects, smaller the black hole, faster it will evaporate. So it results in a sudden burst of particles as the micro black hole suddenly explodes. Mathematics tells us that the energy required to create these micro black holes is of the order of 10^19 GeV. This is far more than the maximum energy we can achieve with current technology.
Stellar Mass Black Holes
The second in the classification of black holes is a stellar-mass black hole. These are one of the most studied black holes and unlike the micro ones, they do exist in nature. Their formation mechanism is also known to scientists. As the name suggests, a stellar-mass black hole forms when a massive star collapses. The massive stars have the potential to host full-scale fusion of heavy elements in their core. They progressively fuse elements such as carbon, neon, oxygen, silicon, sulphur and so on. In the previous article of the series, we saw how a stellar-mass black hole forms.
Once this alpha ladder reaches nickel-56, the reaction chain stops. The further fusion of nickel into zinc isn’t thermodynamically favorable. This causes the core to shut down. In such a scenario, the star collapses under its own gravity. If the star is quite massive, nothing can halt this collapse and the star is crushed into a black hole.
Supermassive Black Holes
As the name indicates, the supermassive black holes are the largest black holes that are found at the centers of the galaxies. They can be a billion times as massive as the Sun. But these black holes can have a density less than that of water. The reason is simple: the Schwarzchild radius of the black hole is directly proportional to its mass and the volume is proportional to the cube of the radius. This makes density to be inversely proportional to the square of the mass. Hence, more the mass, less is the density of the black hole.
Also, the tidal force of such a black hole is very less. The tidal force on a body at the event horizon is likewise inversely proportional to the square of the mass. Thus a person on the surface of the Earth and one at the event horizon of a 10 million M? (10 million solar masses) black hole experience about the same tidal force between their head and feet.
The mechanism through which such bizarre objects take birth is still a mystery and an open field of research in Astrophysics. There are several hypotheses. One hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter. Some scientists also speculate that the stellar-mass black holes formed from the death of the first stars in the universe could have given way to such supermassive black holes. The correct explanation, however, is yet to come.
Detection of Black holes:
As the black holes do not themselves emit any electromagnetic radiation other than the hypothetical Hawking radiation, so it is definitely not possible to detect them directly. Due to this, astrophysicists searching for black holes have to rely on indirect observations. These indirect observations are made by detecting the Gravitational waves using interferometers. Apart from this, gravitational lensing, analysis of the mysterious motion of objects around some invisible force, the accretion of matter, etc also helps in detecting black holes.
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The first strong candidate for a black hole, Cygnus X-1, was discovered in 1972 using these indirect observations. However, one of the major breakthroughs in the study of black holes came last year. On April 10, 2019, the first simulated image of a black hole in our neighboring galaxy was released after rigorous hard work of over 2-years of around 200 astronomers armed with supercomputers and the Event Horizon Telescope.
Although, we have our first image of a black hole with us today. Still, a lot of differences of opinions exits among different scientific communities regarding the authenticity of black holes. Some believe in these giant beasts, while others completely discard them. Despite all the controversies, Black holes still occupy one of the major sections of the study of our universe as a whole.
Do Black Holes Really Exist?
Ever since the beginning, astronomers have divided opinions on the existence of true mathematical black holes as predicted by General Relativity. Scientists like Einstein, Eddington, McCrea, Mitra, among others, did not believe in the existence of true mathematical black holes. On the other hand, scientists like Hawking, Chandrasekhar, Thorne, among others, had a different opinion. Over the course of time, many flaws were found in the currently accepted model of black holes and many alternative models were proposed. Some of them include gravastars, naked singularities, and MECO.
Of all these models, MECO is quite promising. MECO stands for Magnetospheric eternally collapsing object and was first proposed by Indian astrophysicist Abhas Mitra and later generalized by Darryl J. Leiter and Stanley L. Robertson. One of the major differences between the true black holes and MECOs is that the latter can produce their own magnetic fields, a fact backed by observation. Also, as stated in the currently accepted model, MECOs do not collapse to a singularity. MECOs are eternally collapsing objects. They will only hit singularity at infinity.
If you want to know more about the MECOs, you can follow this link. In this blog, Prof Mirta has written about the flaws in the current model and MECOs overcome those flaws and prove to be a better theoretical model. The blog also contains links to all the peer-reviewed publications for reference. You can also contact Prof. Mitra here and watch his TEDx talk on flaws on the current black hole model below.
Also watch: The flaws in the current model of black holes and the concept of ECO
Author’s Message :
This article is completely dedicated to the most loved astrophysical objects, the black holes. While interacting with people and especially students, we have realized how influential pop science is. From the children in school to the renowned science fiction writers, everyone loves to explore these pop-science topics. It is good to watch science fiction movies and books that project Astrophysics as an extremely glamorous field. These definitely play a major role in pushing people towards the study of the cosmos.
But those who really wish to pursue a career in astrophysics should be careful. Reading about these cool topics such as white holes, time travel, black holes, wormholes, etc. pump the students and they go for Astrophysics. But here, they come across some serious Physics and Mathematics, which they hadn’t expected. Subjects such as Statistical Mechanics, Quantum Mechanics, Electrodynamics, Mathematical Physics, Relativity, Optics, Spectroscopy are the very basics of astrophysics and many students struggle to get hold of them.
So, we advise our budding Astrophysicists that it is good to read as many books as you can. However, make sure that you also start with Physics and Mathematics. Keep in mind that the road to astrophysics is through physics. If you master the above-mentioned subjects, Astrophysics will really be a cakewalk for you.