What is a black hole?
A black hole is a region in spacetime where the gravity is so dense, that nothing, even particles and electromagnetic radiations such as light, can escape from it. The gravity is so strong because the matter has been clasped into a tiny space.
A German physicist and astronomer named Karl Schwarzschild suggested the phenomenon of a black hole. Black holes can be massive, intermediate, and stellar-mass black holes.
A black hole has a boundary, which is known as the event horizon. It is a point in space beyond which it is impossible to escape the black hole’s gravity. Once anything falling into the black hole passes the event horizon, it can never leave the black hole again and is drawn inescapably and certainly towards the black hole’s center.
There are some interesting scientific facts about black holes.
1. You can’t directly see a black hole.
We can’t see a black hole through our naked eyes or through any telescope. This is because, a black hole does not illuminate any light i.e. light cannot escape the black hole’s gravity.
The event horizon marks the limit of a gravitational field of a black hole so strong that any known object in the universe, including a photon of light, would need to attain escape velocity greater than the speed of light to overcome the pull of the gravitational field. As the speed of light marks the upper-speed limit in this universe according to General Relativity, light cannot achieve the required escape velocity.
2. There are different types of black holes.
There are different categories of black holes on the basis of their presence of a charge, spin, and mass. The major four types of black holes are:
Schwarzchild Black holes
The simplest kind of black hole is a Schwarzschild black hole, which is a black hole with mass, but with no electric charge, and no spin.
Kerr-Newman Black Holes
These are the types with mass, electric charge as well as spin. The Kerr–Newman metric explains the geometry of spacetime for a rotating charged black hole with mass M, charge Q, and angular momentum J.
Kerr Black Holes
Kerr black holes are the ones with mass and spin but no electric charge. The Kerr metric or Kerr geometry describes the geometry of empty spacetime around a rotating uncharged axially-symmetric black hole with a quasi-spherical event horizon.
Reissner-Nordstrom Black Holes
Reissner-Nordstrom Black Holes are the ones with mass, electric charge but no spin. the Reissner–Nordstrom metric is a static solution to the Einstein-Maxwell field equations. It corresponds to the gravitational field of a charged, non-rotating, spherically symmetric body of mass M.
3. We would have to shrink the earth to the size approximately with that of a coin to turn it into a black hole.
Any object, when compressed to its Schwarzschild radius, becomes a black hole as long as the Schwarzschild radius is greater than Plank length. It turns out, you would have to compress the Earth to 8.87 millimetres, in order to make it a black hole.
4. There is a massive black hole at the center of our milky way galaxy.
Observational evince indicates that nearly all large galaxies contain a supermassive black hole, located at the galaxy’s center. In the case of the Milky Way, the supermassive black hole is supposed to be at the location of Sagittarius A*, at the Galactic Core.
5. The opposite side of black holes is estimated to be white holes.
A white-hole is the exact opposite of a black hole. Where nothing can escape the black hole, a white hole could spray out the matter and light like fountains. So, a black hole won’t let you escape it and a white hole won’t let you in. While general relativity describes white holes in the theory, no one knows how one might actually form. But as a black hole gradually evaporates, the question is where does all the matter that the black hole swallowed would go.
As, General relativity won’t let the information out and quantum mechanics forbids its deletion. Hence, it is said to be possible that the other side of a black hole could be a white hole.
6. If a star passes too close to a black hole the star can be torn apart.
The star is distorted by the black hole’s tidal force, but otherwise normal, and only loses gas slowly (over many millions of years) onto the black hole. A direct, head-on collision is rare, but if it happens, it would be very violent and the star would be torn apart very quickly. So quickly, that the core of the star probably won’t have time to respond to the changing conditions before it gets torn apart. A fully torn-apart remnant of the star may form a disk around the black hole.
7. A spinning black hole has a speed limit.
Every spinning black hole has a speed limit. A black hole does not emit light and energy due to singularity. Hence, we cannot have a naked singularity, a singularity which can be seen by the universe, because that would mean that the singularity is itself emitting light and energy which is against the law. Hence there is a speed limit which restricts the black hole from spinning, more than the limit, and revealing its singularity.
Astronomers have detected supermassive black holes spinning at the limits predicted by these theories. At the heart of the galaxy, one black hole NGC 1365, is turning at 84% the speed of light. It has reached the cosmic speed limit; it can’t spin any faster without revealing its singularity.
8. If you were to fall into a black hole, the theory suggests that gravity would stretch you out like spaghetti.
Once you reach the event horizon of a black hole there is no going back, it’s a point of no return. From there, the black hole will start to stretch you long, like a spaghetti. This process of matter stretched like spaghetti in dense gravitational fields is called as spaghettification.
If you were to fall into a black hole, your feet will be attracted more towards the black hole then your head and hence you will be stretched as the gravity will affect each part of your body differently.
9. Time slows down near the event horizon of the black hole.
The theory of relativity says, speed of light is constant and everything else is relative. Hence gravity and time are relative too. You may think, at the event horizon, with the gravitational pull, time would go faster, but in reality, time passes by slowly.
As you reach the event horizon, you are moving at such high speeds due to the strong gravitational force from the black hole, that time will slow down. Also, gravity wraps time. Hence, for an outside observer, a body falling into a black hole, may take an infinite amount of time to cross the event horizon, but for the body falling into the black hole, time passes by finitely. This is how time measurements change near the black hole.
10. The Black Hole Era
In the book, The Five Ages of Universe, authors Fred Adams and Gregory Laughlin, have discussed five eras through which the universe is expected to go through, one of these is the black hole era. The five eras are the primordial Era, the Stelliferous Era, the Degenerate Era, the Black Hole Era, and the Dark Era.
The Black Hole Era is defined as 10^40. In this era, according to the book, the organized matter will remain only in the form of black holes, as all the matter in the universe will start forming black holes.
After 10^100 years, these black holes will evaporate themselves and the universe will enter the Dark Era.