Lets briefly focus on Einsteins theory of relativity (time dilation with gravity )-
Einsteins theory states that time is slowed when gravity increases.
I know this sounds wierd and confusing but lets understand it gradually.
Lets understand why time slows with Gravity.
You just need to remember that the universe has 4 dimensions. The 3D space and the 4th dimension is time!
Now consider the Earth. Imagine two points at the ends of its Diameter! The light will have to travel a distance around the circumference of Earth to reach from point A to Point B.
Now suppose the Earth was absent! Then the light would travel in a straigth line from A to B. This means that the presence of Gravity has caused a distraction in the 4 dimensions i.e; it is bending the space-time(or the 4 dimensions). Since the space is getting bent due to gravity(i.e; The path from point A to point B is bent due to presence of Earth) The fourth dimension time( which is definitely not an independent dimension according to theory of relativity) is also being elongated or slowed. Hence time slows near gravity.
Please read this again if you dont understand…
Close to bodies such as Sun or Earth which have relatively smaller mass ,such effects (time dilation effects) are generally too small to be noticed and can be ignored!
However when we speak about black holes that are of billions times mass of sun.. time dilation effect cannot be ignored!
Black holes have an event horizon. We know nothing can escape black holes.
However since black holes dont have a defined surface-we define its event horizon(similar to surace of balck hole).
It is the region surrounding black hole in which once an object just enters- then- its escape is impossble! ( you can call it as the critical region!)
Black holes have accelration due to gravity that shows different variation than planets! Like we know-for the Earth-how it varies with depth and altitude -this is because earth has a surface.
However black holes dont have a surface. They are like high mass filled in a point!!! Hence we cant define its surface but we define its area of influence and event horizon of black hole is that region where its influence just begins!
For supermassive blackholes once you are on the event horizon of black hole there is a possibility you will escape because the acceleration due to gravity shows exponential behaviour with distance from black hole.. and a mere escape is possible which is not possible incase of small black holes.
So imagine you standing in free space at a safe distance from blackhole!
your friend on the other hand is with you.
Now he starts moving towards the black hole.
And reaches close to its event horizon!
As soon as he reaches the event horizon. Time has slowed tremendously for him.
The time dilation is too strong near black hole.
Suppose he spends few seconds in the event horizon of black hole before escaping it (escape is difficult ). We know time for him is slow there .
that is if for his friend it shows 1 hr have passed the guys(at the event horizon ) clock show’s that less than 1 hr has passed(due to high mass of black hole the clock is ticking very slow and 1hr for his friend may correspond to even 1 second for guy at the event horizon ). That is after few seconds the guy at the black hole may become younger than he would have if he was with his friend far from it.
the guy has relativisticly travelled in past by getting younger !!
You may think then why astronauts on space stations are not affected this as they spend 1 year around Earth.
they are indeed affected!
The effect is hardly noticable…
As the earth has small mass!