Let’s
briefly focus on Einstein’s theory of relativity (time dilation with gravity)-

Einstein’s
theory state that time is slowed when gravity increases.

I know this
sound weird and confusing but let’s understand it gradually.

Let’s
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 straight 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 don’t 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 don’t have a defined surface-we define its event horizon (similar
to surface of black hole).

It is the
region surrounding black hole in which once an object just enters- then- its
escape is impossible! (You can call it as the critical region!)

Black holes
have acceleration 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 don’t have a surface. They are like high mass filled in a point!!!
Hence we can’t 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 black holes 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 behavior with distance from black hole.. And a mere escape is
possible which is not possible in case of small black holes.

So imagine
you standing in free space at a safe distance from black hole!

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
traveled 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 noticeable…

As the earth
has small mass!

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