We know that the speed of ball thrown upward
in the air go on decreasing because of Earth’s
gravity.
Speed of ball becomes zero at certain height
and then ball start coming downward and speed
of ball go on increasing until it hits ground.
We also know that gravitational force of Earth
goes on decreasing with the distance.
Suppose we throw the ball with velocity that
it reaches such distance where Earth’s gravity
doesn’t act on it.
In this case ball continues to travel away
from earth in space.
For throwing a ball outside Earth’s gravitational
field we require minimum velocity.
This velocity is called as escape velocity.
The escape velocity for Earth is 11.2 km/s.
Neglecting air atmosphere on earth if we throw
any object with 11.2 km/s then that object
escapes Earth’s gravitational field and
travels in space.
If we do same thing on moon, ball must be
thrown by 2.4 km/s velocity to make escape
from moon’s gravitational field.
It means escape velocity of moon is 2.4 km/s.
For Sun, the escape velocity is 618 km/s.
More the mass of the object in the space more
is the escape velocity.
For the objects with same mass escape velocity
is greater for lower size object.
It means escape velocity of white dwarf of
equal mass of Sun is greater than Sun because
radius of white dwarf is much lesser than
the Sun.
The escape velocity of white dwarf having
mass equal to sun is 6451 km/s, the escape
velocity of neutron star is 150000 km/s which
is half the speed of light.
The escape velocity of black hole is greater
than 300000 km/s that is greater than speed
of light.
It means light also cannot escape black hole.
This is what the black hole.
This is the object from which light doesn’t
escape that’s why we cannot see that object
hence named black hole.
The value of radius of the object at which
its escape velocity becomes equal to speed
of light is called Schwarzschild radius.
The Schwarzschild radius of Sun is 3 km, it
means Sun will become black hole if it compressed
to sphere of 3 km radius.
Schwarzschild radius of Earth is 9 mm, not
even 1 cm. if the Earth having radius 6400
km compressed to small marble size, Earth
will become black hole.
In 1915 Karl Schwarzschild first time solved
Einstein’s equations of special relativity
which equations were extremely difficult to
solve.
In upcoming theory of General Relativity,
the idea of black hole came on the surface.
Schwarzschild found the formula for gravity
around extremely dense singular objects.
It has been proved that no light can escape
from an object of specific radius.
This radius is named as Schwarzschild radius.
This equation has obtained from accurate mathematical
procedure.
In fact, this happens because of infinite
bending of the space-time curve by extremely
dense objects.
Time literally stops at the black hole.
According to Einstein’s special theory of
relativity no object can travel with speed
faster than the light.
Means no object can escape the black hole.
In the sense of thermodynamics, black hole
is perfectly black body which absorbs everything
fall on it.
Around the black hole there is boundary of
no return, called as event horizon.
Black holes are absolutely very hot and emits
radiation named as Hawking radiation which
tends to reduction of the mass of the black
hole according to mass-energy equivalence
principle E = MC2.
In 1784 John Michell and in 1796 Simon Laplace
first time proposed idea of black hole.
They briefly proposed the presence of an extremely
massive and dense body that even light could
not escape it.
Scientists observed that supermassive black
holes exist in the centre of galaxies.
Black holes are also in various sizes.
Major are classified as stellar mass black
holes and supermassive blackholes.
Stellar size black holes are having mass 25
to few hundred solar mass but supermassive
black holes have mass equal to mass of millions
of Suns.
There are only two properties of black holes.
One is rotation and other is mass. in 1963
Scientist Kerr used Einstein’s equations
to prove that black hole are rotating objects.
The electrical charge property cannot be possessed
by a black hole.
Therefore, we can say black hole is electrically
chargeless, rotating, massive body.
Black hole is basically extremely simple body
but physically and mathematically plays vital
role in modern astronomy.
Black holes gave us proof of gravitational
waves in 2016.
GW150914 is the name of first observed gravitational
waves.
These waves were emerged from binary black
hole system when they were fusing together.
These objects were 13.4 billion light years
away from us.
Means they were fused together before 13.4
billion years and the gravitational waves
emerged from that incident reached to Earth
in 2015.
There is one black hole at the centre of our
galaxy Milky Way named Sagittarius A which
is having mass 4.3 million times the Sun.
This is nearest supermassive black hole from
us but still we couldn’t capture it.
On 10 April 2019 a world-spanning network
of telescopes called Event Horizon Telescope,
observed and captured picture of first ever
black hole.
The captured black hole is situated at the
centre of Messier 87 galaxy in Virgo supercluster.
M87 is around 55 million light years away
from us.
The M87 black hole is 6.5 billion times massive
than Sun.
Hot and shining disc of materials that encircles
the black hole can be seen clearly.
This disc is looking bright from one side
and goes fainter to another side.
This is because material is coming toward
us to look shining and material going away
from us to look faint.
The Event Horizon Telescope is collection
of more than 200 telescopes using an array
of observatories scattered around the world.
These combined arrays of telescopes make Earth
size telescope.
There are a lot of misconceptions about the
black holes.
Let’s try to clear this out.
First is that Sun will never become a black
hole.
If sun is having mass 25 times greater than
it is having right now, definitely it would
become a black hole.
Second is that a lot of people thinks that
black holes sucking everything near them,
or the black holes are cosmic vacuum cleaners.
This is not true at all.
They have powerful gravity, but it will be
affecting any other body when that body come
close enough to black hole.
Mass of black hole is its power.
Imagine that if we replace the Sun by a black
hole of equal mass of sun then the Earth along
with all other planets and objects would orbit
that black hole exactly like as it does now.
From 150 million km distance Earth doesn’t
need to know Sun is tiny or big object, it
only deals with mass and distance.
