“Black holes are where God divided by zero.” - Stephen Wright.
The idea of a black hole was first put forward by John Michell in a letter to Henry Cavendish. To me, black holes seem like the most mysterious, strangest and most exciting thing in the entire known Universe. I fail to imagine such a place whose gravity not even light can escape!
Definition of a black hole...
A black hole is a region in space from which absolutely nothing can escape. Even light is sucked in a black hole.
Imagine throwing a ball in the air. It would fall back eventually due to Earth's gravity. What if you threw the ball up at a speed that it barely escapes Earth's gravity? This is known as escape velocity. Escape velocity differs from body to body, and depends on far you are from the body's center. For Earth, it is 11.2 km per second.
Now try to imagine an object that has such a great concentration of space in a very little area that its escape velocity is higher than the speed of light. But nothing can go faster than light. This means everything near it will be sucked in by its gravity. Same is the case with a black hole.
Imagine throwing a ball in the air. It would fall back eventually due to Earth's gravity. What if you threw the ball up at a speed that it barely escapes Earth's gravity? This is known as escape velocity. Escape velocity differs from body to body, and depends on far you are from the body's center. For Earth, it is 11.2 km per second.
Now try to imagine an object that has such a great concentration of space in a very little area that its escape velocity is higher than the speed of light. But nothing can go faster than light. This means everything near it will be sucked in by its gravity. Same is the case with a black hole.
How are black holes formed?
- Black holes are created huge stars, with more than 3 solar masses(3 times the mass of the sun) are about to go into a supernova. Just to remind you, a supernova is a massive luminous explosion when a star completes its life.
- The star may or may not go into a supernova. By then, the stars expend all their fuel. So it cannot produce any more energy and cracks under its own gravity.
- The shrink to a small size, known as Schwarzschild radius.
- Now, it becomes so dense that it attracts everything nearby towards it. This, is a black hole.
Note that not all stars turn into a black hole. For eg: our Sun has too little mass, hence it will die peacefully and turn into a white dwarf.
Is there evidence that black holes exist?
Center of a galaxy
We can't see black holes, because light can't get past them but there is evidence.
The way to find black holes is by knowing if there is a very large amount concentrated in a very small space. These areas might be black holes. There are two places where black holes are rumored to exist, and one of them is centers of galaxies. The masses of the centers of these galaxies may be as high as 7 billion times the mass of the Sun.
Now, these are thought to be black holes for two reasons:
The way to find black holes is by knowing if there is a very large amount concentrated in a very small space. These areas might be black holes. There are two places where black holes are rumored to exist, and one of them is centers of galaxies. The masses of the centers of these galaxies may be as high as 7 billion times the mass of the Sun.
Now, these are thought to be black holes for two reasons:
- There is nothing else they could possibly be, because they are too dark and dense.
- Some promising theories say that very massive galaxies have black holes at their center.
Where does all that matter swallowed go?
Artist's impression : Singularity
There is something called singularity inside a black hole.
Imagine a place with zero volume and infinite density. Difficult, eh? In fact it won't even be a 'place', as the volume is zero. But imagining it is a place, this is where everything swallowed by a black hole goes.
You must be thinking that how can such a place exist. Even I find it impossible to believe. But in singularity, space and time don't exist as we know them. Laws of physics are absolute rubbish out there (Those who don't like physics would be very happy in black hole!). But we can never be sure, as no space probe would ever return from the singularity to tell us the story of a black hole.
Lets trust the scientists on this one!
Imagine a place with zero volume and infinite density. Difficult, eh? In fact it won't even be a 'place', as the volume is zero. But imagining it is a place, this is where everything swallowed by a black hole goes.
You must be thinking that how can such a place exist. Even I find it impossible to believe. But in singularity, space and time don't exist as we know them. Laws of physics are absolute rubbish out there (Those who don't like physics would be very happy in black hole!). But we can never be sure, as no space probe would ever return from the singularity to tell us the story of a black hole.
Lets trust the scientists on this one!
Why don't black holes swallow everything present in the Universe?
Even Earth's gravity pulls things. But there is a certain limit after which we can escape Earth's gravity.
Similarly, a black hole has a limit, called its horizon. If you get inside the horizon, they you will be eaten up by the black hole, no questions asked. But as long as you steer clear of the horizon, you will be safe.
Similarly, a black hole has a limit, called its horizon. If you get inside the horizon, they you will be eaten up by the black hole, no questions asked. But as long as you steer clear of the horizon, you will be safe.
Does Milky Way have a black hole?
Sagittarius A*
Like many other galaxies, scientist think that our galaxy, Milky Way, too has a black hole at its center, though it is not confirmed. Scientists have named it Sagittarius A* (pronounced A star).
This black hole is is 26,000 light years away from Earth and is thought to be 14 million miles across. This means is coulf fit inside Mercury's orbit. It is thought to contain as much mass as 4 billion Suns!
This black hole is is 26,000 light years away from Earth and is thought to be 14 million miles across. This means is coulf fit inside Mercury's orbit. It is thought to contain as much mass as 4 billion Suns!
What if I accidentally fall in a black hole?
Well, I don't want anyone to fall inside a black hole, and we don't have a person who'll be willing to, so let's imagine a person - let's say, Jerry.
After crossing the horizon, at first Jerry will feel absolutely weightless, because the spaceship around him and everything else is being pulled in the same direction as his body. As Jerry gets closer to the center, imagining that his feet are towards the center, he will feel 'stretched'. His legs will experience a greater pull than his upper body. These are called tidal forces. For a large black hole, these can be felt from 600,000 km from the center after crossing the horizon.
Eventually, Jerry's body would rip apart (Sorry Jerry). So you better not fall into a black hole!
After crossing the horizon, at first Jerry will feel absolutely weightless, because the spaceship around him and everything else is being pulled in the same direction as his body. As Jerry gets closer to the center, imagining that his feet are towards the center, he will feel 'stretched'. His legs will experience a greater pull than his upper body. These are called tidal forces. For a large black hole, these can be felt from 600,000 km from the center after crossing the horizon.
Eventually, Jerry's body would rip apart (Sorry Jerry). So you better not fall into a black hole!
The scenery as Jerry falls into a black hole...
Jerry might see images of distant, strange objects. One never knows what a black hole has had for breakfast. These images might be distorted, because light bends inside a black hole. How beautiful that must be! Also, you can see everything outside the black hole as light from outside can reach you, but no one can see you from outside, as light from the black hole cannot escape.