Gravity

Black Hole

Black Hole

The general relativity view.

• Gravity is not a force. It is an effect caused by the curvature of space-time fabric. (Theory of 
• Space-time fabric is a 4D sheet, which gets bend or curved due to any mass or object. (Like a trampoline sheet gets bent when a heavy object placed over it) .
• Mass and energy are the same things.
• Heavier the object more the curvature in space-time hence more gravity. 

I mentioned before that heavy object causes more curvature in space-time hence more gravity. Space-time is like a single elastic fabric made from the three spatial and one time dimensions. (You can read my previous blog gravity). The object bends both space and time, and the bending of time component makes the clocks run slower in more gravity. Or you can say time is slower near heavy objects or planets or stars. 

The flow of time changes due to the curvature of space-time.

Now, a black hole is way heavier than our Sun making the time run even slower around it, and when any planets enter the black hole event horizon, the time almost freezes for a distant observer (Bob), due to which the Bob will never be able to see it crossing the event horizon. But because the distance between the Bob spaceship and black hole will increase, it will eventually disappear due to the redshift of the visible spectrum. Alice fell into the black hole horizon after the planet would observe it passing the event horizon and ultimately going into the singularity.

So, how Black Hole formed astronomically?

Star is just a gas cloud with a dense iron core. It is common sense that a bomb explosion causes outward pressure. The same way the nuclear reaction caused by the sun creates an outward pressure to balance the gravity, hence maintaining its shape.

When the star runs out of fuel (hydrogen), there isn't enough pressure maintained to cancel the gravitational collapse, and it eventually dies. It either becomes a neutron star or black hole depending upon the mass of the star. Our sun doesn't have enough mass to become a black hole.

Space-time curvature and mass comparison

Depending upon the angular momentum of the star, it becomes either rotating or non-rotating black holes. 

"The black hole after the formation usually lost their information of what type of object they were before, the only information they have of mass, angular momentum and charge. Hence, as said by John Wheeler that black hole has no hair."

Black Hole has horizon called 'event horizon' 
and the centre of Black Hole is called singularity where everything even time breaks.

But what exactly is a singularity.

Roger Penrose and Stephan Hawking proposed the singularity theorem.

In physics, singularity majorly means gravitational singularity, explained as a discontinuity in the fabric of space-time. And this discontinuity is also responsible for time travelling through a black hole. 

So, by discontinuity, I don't mean that it has some stars in-between or some asteroids, but what I mean is somewhat absurd. Let's do a thought experiment:

Take paper and a pen, make a line connecting two corners, now make a hole in between the paper so now you won't be able to join both corners unless you draw the line around that hole. So, that hole is acting as a singularity (or discontinuity) for that paper. Now try to imagine them in the space-time fabric; a hole in space-time fabric is a singularity.

Singularity: disconnected space-time shown through the accretion disk

"There is space singularity, which is the discontinuity of space dimensions that are theoretically present in non-rotating black holes like the Schwarzschild black hole. Time singularity, theoretically present in rotating black holes (Kerr black hole). Space-time singularity, proposed by Hawking in the big-bang model."

Rotating Black Hole

How does a hole or singularity important for time travel?

Well, instead of going around the singularity, you can jump into one, and you will get out on another side of space-time fabric (singularity acts like a shortcut). 

 But the singularity has to be of perfect size (of a large black hole). Otherwise, you will stretch like spaghetti from toe to head, and then you will die in it. Suppose there is a planet 80 light-years away, and there is a singularity in between which connect our solar system and that Planet's solar system making the shortcut 25 light-years. So, this is how singularity works. This path connecting both space-time fabric is called 'WORM-HOLE'. But the idea of the wormhole is purely hypothetical. So, as we know that the time is slower there let's say 1 year of Earth is 1 hour around the black hole, so when you took a ship in 2020  travel black and return to the Earth then you will see  Earth has in 2021 (so you will be in future) hence time-travel. 

Wormhole

The wormhole is well fit in rotating black hole as shown in interstellar movie by Kip Throne there they used the Kerr black hole.

Wormhole connecting two different regions