Black holes are one of the most fascinating objects found in the Universe. They are celestial bodies of extremely intense gravity from which nothing can escape – not a planet, not a moon, and not even light.
Anything that crosses the event horizon — the boundary within which escape velocity of a black hole is greater than the speed of light — spirals drastically towards an unknown fate.
In recent years, physicists have uncovered a lot of unknown facts about black holes. Some discoveries laid the foundation for the future, while some are still blowing researchers’ minds. Here are the 15 most intriguing facts and theories of black holes that you should know.
1. Black Hole Was Discovered By Karl Schwarzschild In 1916
Karl Schwarzschild | Image credit: Wikimedia
Although objects with intense gravitational fields (from which light cannot escape) were considered in the 18th century, it was Karl Schwarzschild who gave the first modern solution of general relativity in 1916, characterizing a black hole.
In 1958, David Finkelstein published its interpretation as a region of space from which nothing can escape. An American theoretical physicist John Wheeler then linked the term “black hole” to objects with gravitational collapse predicted early in the 20th century.
He used the term “black hole” during a presentation he gave at the NASA Goddard Institute of Space Studies in 1967.
2. They Cannot Be Directly Observed
The first-ever image of a black hole at the core of elliptical galaxy Messier 87
Since light cannot escape the massive gravitational pull of a black hole, you cannot directly observe it. However, you can see how its gravity affects nearby celestial bodies and gas.
Astronomers study stars to see if they are orbiting or flying around a black hole. When a star and black hole are close together, radiation is emitted, which is usually captured by space-based telescopes and satellites.
In 2019, scientists captured the first-ever image of a black hole, located 500 million trillion kilometers away. It was photographed by a network 8 telescopes across the world. This supermassive black hole measures 40 billion kilometers across and has a mass 6.5 billion times that of the Sun.
3. Types of Black Holes
There are four types of black holes (three real and one hypothetical) –
Stellar black holes: are small black holes with masses ranging from 5 to several tens of times the mass of the Sun. They are formed by the gravitational collapse of a large star.
Supermassive black holes: are the largest black holes with masses ranging from hundreds of thousands to billions of solar masses. Their origin remains an open field of research.
Intermediate black holes are significantly more massive than stellar black holes but less than supermassive black holes. The strongest evidence for such celestial bodies comes from certain low-luminosity active galactic nuclei.
Primordial black holes are hypothetical black holes that could have formed soon after the Big Bang. Their masses can be far less than stellar mass. Stephen Hawking studied these black holes in depth and found that they could weigh as little as 100 micrograms.
4. A Black Hole Has Three Layers
A black hole has three layers: the singularity, the outer and inner event horizon.
The center of the black hole is called a singularity. This is the region where all the mass is compressed down to nearly zero volume. Thus, the singularity has almost infinite density and generates an enormous gravitational force.
The outer event horizon is the very outer layer from where materials can still escape from a black hole’s gravity. The gravitational pull on this layer is not as strong as the center or middle layer.
The inner event horizon is the central layer. This is the region from where materials cannot escape. It pushes the material toward the center of a black hole where gravitational influence is the strongest.
5. Black Hole Can Be As Small As 0.1 Millimeters
A black hole can have mass as small as Earth’s moon and as enormous as ten billion times the mass of the Sun.
Its mass is proportional to the size of the event horizon, which is measured as Schwarzschild radius. It is the radius at which the escape velocity is equal to the speed of light.
The Schwarzschild radius of the Earth is about the size of a marble. This means you have to compress the earth to the size of a marble in order to turn it into a black hole.
Moreover, no black hole is infinitely small. The minimum mass is above or equal to Planck mass which is about 22 microgram.
6. Black Holes Spin Around An Axis
When a star collapses into an extremely small space, it still retains all that mass. To conserve the angular momentum, the black hole’s rotational rate speeds up.
As the black hole spins, its mass causes the nearby space-time to rotate as well. This region is called the ergosphere. This is region (outside the event horizon) where various interesting effects occur.
The smaller its event horizon, the faster it spins. However, there is a speed limit to how fast a black hole can spin [without revealing its singularity to the rest of the Universe].
The heaviest stellar black hole (GRS 1915+105) in the Milky Way is rotating 1,150 times per second. And there is a black hole in galaxy NGC 1365, which is spinning at 84% the speed of light. It has reached the cosmic speed limit and cannot rotate any faster.
7. They Produce Sound
Chandra’s observation of the Perseus galaxy cluster revealed wavelike features that seem to be sound waves | Credit: NASA
In 2003, Astronomers using NASA’s Chandra X-ray Observatory detected sound waves from a supermassive black hole located 250 million light-years from Earth. The ‘note’ is the deepest ever detected from any celestial body.
When black hole pulls something in, its event horizon supercharges the particle close to the speed of light, producing sound. The space-based telescopes capture sound waves that have already traveled millions of light-years from their source (black hole).
But sound can’t travel in a vacuum, then how we are hearing black holes? Actually, outer space is not a complete vacuum. It consists of several hydrogen atoms (plus other gases) per cubic meter, which serve as a medium for very low-frequency sound waves.
8. Black Holes Distort Space and Time
A simulation showing how black hole distorts space-time
Due to extreme gravitational influence, a black hole can distort space-time in the near neighborhood. According to the general theory of relativity, the closer you get to a black hole, the slower time runs.
The event horizon is the boundary around the black hole where every matter, including light, loses its ability to escape. The gravitational force is constant across the event horizon.
A spinning black hole gives rise to a strange effect called frame-dragging. In this effect, space and time close to the black hole are actually dragged around with it. Space drags so intensely that it is not possible to move in the opposite direction. It’s an infinite regression of distortions where there is no way to move forward.
Overall, the classical laws of physics as we know them cease to operate inside the event horizon, it’s not really possible to conceive anything with infinite density and zero volume.
9. Black Holes Can Kill You In A Horrible Way
If you fell into a black hole, your body would be stretched into a long spaghetti-like strand.
Assuming it’s a small black hole, you will be distorted by the tremendous tidal force of gravity. The tidal force is the difference between the gravity on your head and your feet. The force acting on your head (if you are falling head-first) would be much stronger than the force acting on your feet.
This difference would make you feel like something is tearing you apart, stretching you from head to toe. The closer your head gets to the black hole, the faster it moves. But the lower half of your body is farther away and so isn’t moving toward the center as fast.
As the tidal force exceed the molecular forces that bind your flesh, your body would snap into two pieces, and those two pieces would snap into another two pieces, and so on. You would be extruded through the fabric of space-time like toothpaste through a tube.
10. Black Holes Do Not Suck
Everything inside the event horizon crunches down to one-dimensional singularity
People usually think of a black hole as a cosmic vacuum that sucks matter from all around. It’s a common misconception. Black holes are like any other celestial body but with an enormous gravitational influence on the space in their vicinity. This gravitational pull just causes the matter around them to accelerate rapidly.
Even if you replaced our Sun with a black hole of equal mass, Earth would not fall in. The black hole would have the same gravitational field as the Sun. Earth and other planets would continue orbiting the black hole as it orbits the Sun today.
And since the Sun isn’t big enough, it will never turn into a black hole.
11. Supermassive Black Holes Exist In The Centers Of Most Galaxies
X-ray image of Sagittarius A | Credit: NASA
Researchers believe there is a supermassive black hole at the core of most galaxies, including the Milky Way. These large black holes actually hold galaxies together in space.
Sagittarius A, the black hole located in the center of the Milky Way is 4 million times more massive than the Sun. At just 26,000 light-years from Earth, Sagittarius A is one of the very few black holes in the Universe where astronomers can actually witness the flow of matter nearby.
12. There Are Uncountable Black Hole In The Universe
Our galaxy alone consists of more than 100 million stellar black holes, plus supermassive Sagittarius A at its core. With nearly 100 billion galaxies out there, each having a core supermassive monster and 100 million stellar-mass black holes (while other types are still being studied), it is like trying to count the number of sand grains on Earth.
13. Any Object Can Be Turned Into A Black Hole
Stars aren’t the only things that eventually turn into black holes. Theoretically, you could turn anything into a black hole.
For instance, if you reduced the size of the Sun to 6 kilometers across while retaining all of its mass, it would become a black hole. Its density would reach astronomical levels which would make the gravitational force incredibly strong.
The same theory could be applied to Earth and any other object, such as a mobile, car or even your own body. However, we don’t know any such technique that can shrink down the volume to an infinitely small point while retaining 100 percent of the object’s mass.
14. They Eventually Evaporate Over Time
In 1974, Stephen Hawking theorized that black holes radiate a small number of photon particles, which makes them gradually lose mass and vanish over time. This process of evaporation is named ‘Hawking Radiation’.
The black-body radiation takes place due to quantum effects near the event horizon. Since the process is incredibly slow, only the smallest black holes would have had time to completely evaporate during the 13.8 billion years (the age of the Universe).
15. Supermassive Black Holes Determine the Number of Stars in the Galaxy
There is a balanced relationship between black holes activities and the number of stars. Too many stars would make a galaxy too hot for life to evolve, whereas not enough stars can prevent life from forming.
New research shows how supermassive black holes regulate star formation in massive galaxies. The history of star formation in nearby massive galaxies depends on the mass of the central supermassive black hole.
16. They are Giant Source of Energy
Black holes create energy more efficiently than small stars like Sun.
Since the gravitational influence is very strong near the event horizon, the matter nearest to the fringe of the event horizon orbits much faster than the matter at the outer event horizon (outer layer of a black hole).
The matter moves so quickly that it heats up to millions of degrees Celsius, transforming mass into energy in a form of radiation (known as blackbody radiation).
A black hole can convert 10% of mass into energy. To put that in perspective, nuclear fusion converts only 0.7% mass into energy.
Researchers have even investigated whether it is physically possible to use this kind of energy to build power plants or starships.
17. Black Holes Could Create New Universes
This might sound crazy but some physicists believe that black holes could open new worlds. Our universe may have been born inside a black hole, and black holes in our Universe might be giving birth to new universes of their own.
To understand how this works, picture our current Universe: everything you look at is made possible by a series of events that happened in the past and certain conditions that came together to create life.
If you made changes to these conditions/events by even a tiny amount, things wouldn’t be the same. In theory, the singularity could alter these conditions, creating a new, slightly altered universe.
18. Information Could Escape Black Hole
What happens to the information of particles passing through black holes? Physicists have been trying to answer this question for decades.
The laws of quantum physics state that information cannot be permanently destroyed. However, if information cannot escape a black hole, then to all purposes and intents, it has been destroyed. This seems to violate the rules of quantum mechanics.
According to Stephen Hawking, information never really enter the black hole.
“The information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon” – Stephen Hawking
As an object enters the black hole, its information is captured and stored on the event horizon. Although object may be destroyed inside the black hole, the information will stay smeared on the event horizon.
The information can escape along with Hawking radiation but in a useless and chaotic form. In fact, it could come out in another universe. Hawking suggested that black holes aren’t the eternal prisons they were once considered.
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