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Black Holes
James Chambers
What is a Black hole?
• The theory of general relativity predicts that a
sufficiently compact mass will deform space-time
to form a black hole.
• A black hole is an object that has collapsed under
its own weight to a point, creating an object that is
fantastically small, yet enormously dense. It sucks
in everything it can absorb, and once formed,
nothing, not even light, can escape its gravitational
pull.
There are two main types of black
holes.
• 1. Stellar Black Holes are born from the death of
stars, which are roughly a few times the mass of
our sun. These stars end their lives when the
hydrogen fuel that makes up the star's interior
burns off, causing the star to collapse.
• 2. And supermassive black holes, which range in
mass from a few hundred thousand times the
mass of our sun to a few billion times that mass,
and exist at the center of galaxies.
• The black hole that lives at the center of the
Milky Way is 4 million times the mass of our sun.
There are two schools of thought on how
supermassive black holes form.
• It's possible that they are seeded from the death
of the earliest stars of the universe, which were
massively large.
• Another, recent theory involves discs of gas that
swirl and funnel like a tornado. The early
universe was filled with gas and radiation. In
some spots, gravity caused gas to fall into halos of
dark matter and form into gas discs. Instabilities
in these discs caused the swirling effect, in which
the gas begins to funnel from the outside in.
Evidence of Black Holes have been theorized
about around for hundreds of years.
• Newton showed there was
evidence of black holes. In his
theory, an energy argument
tells us that there is an escape
velocity from the surface of
any spherical object of mass. If
this velocity is greater than the
speed of light then light from
this object cannot escape to
infinity. Thus the condition for
such an "unseeable" object is
“a black hole”
The difference between Einstein and Newton’s
theory’s on gravity and black holes
• in Newton's theory a particle could
overcome a black holes gravity with
strong enough engines to provide the
energy needed for escape.
• This is not so in general relativity, Einstein's
theory of gravitation. In that theory, escaping
the black hole is equivalent to moving faster
than light, an impossibility in relativity.
•
•
•
•
1. Quiet region
2. Ergosphere
3. Event Horizon
4. Singularity
• 1. The quiet zone is the
area just inside the black
holes influence but still far
enough away that its pull
still negligible.
2. The Ergosphere
• a region of
space-time in
which it is
Impossible to
stand still.
• “Frame-dragging” General relativity predicts
that any rotating mass will tend to slightly
"drag" along the space-time immediately
surrounding it. Any object near the rotating
mass will tend to start moving in the direction
of rotation.
For a rotating black hole, this effect become
so strong near the event horizon that an
object would have to move faster than the
speed of light in the opposite direction to
just stand still.
3. The Point of No Return
“The Event Horizon”
• Where reality starts to come apart
• Around a black hole there is a mathematically
defined surface called an event horizon that
marks the point of no return.
• It is called "black" because it absorbs all the
light that hits the horizon, reflecting nothing,
just like a perfect black body in
thermodynamics.
• Quantum mechanics predicts that event
horizons emit radiation like a black body with
a finite temperature.
• The event horizon is referred to as such because if an
event occurs within the boundary, information from
that event cannot reach an outside observer, making it
impossible to determine if such an event occurred.
• To a distant observer, clocks near a black hole appear
to tick more slowly than those further away from the
black hole. Due to this effect, known as gravitational
time dilation, an object falling into a black hole appears
to slow down as it approaches the event horizon,
taking an infinite time to reach it.
• At the same time, all processes on this object slow
down causing emitted light to appear redder and
dimmer, an effect known as gravitational redshift.
Eventually, at a point just before it reaches the event
horizon, the falling object becomes so dim that it can
no longer be seen.
• Although someone entering a black hole
would not notice any of these effects as they
crossed the event horizon. According to his
own clock, he crossed the event horizon after
a finite time.
How matter
is
pulled
into
a
Black
hole
• Matter will orbit a black hole slowly in
a decaying orbit.
If a craft were to enter the event horizon it
would be subject to Spaghettification
• It would be disassembled.
• The process of spaghettification is as follows.
• First, the object that is falling into the black hole splits
in two. Then the two pieces each split themselves,
rendering a total of four pieces. Then the four pieces
split to form eight. This process of bifurcation
continues up to and past the point in which the split-up
pieces of the original object are at the order of
magnitude of the constituents of atoms. At the end of
the spaghettification process, the object is a string of
elementary particles.
4. The Singularity
• At the center of a black hole lies the singularity,
where matter is crushed to infinite density, the
pull of gravity is infinitely strong, and space-time
has infinite curvature.
• Space-time curvature becomes so strong that the
general relativistic laws break down and laws of
quantum gravity take over.
• Here it's no longer meaningful to speak of space
and time, much less space-time. Space and time
cease to exist as we know them.
• Enter the strange world of quantum gravity.
Birth of a Black Hole
• Gravitational Collapse
When a gigantic star reaches the final stage of
its life and is about to go supernova as it spent
all of it’s nuclear fuel. So it stops burning and
heating up and cannot create the nuclear energy
required to feed the star and let it to continue
the balance to support its own gravitational
draw against the intense pressures brewing
inside.
• The radius of the star shrinks to a critical size,
called the Schwarzschild radius and it starts to
devour anything and everything that comes a
bit too close, including light. Gravity does its
job and the core of the star caves in and
implodes. The outer shells of the star explode
into the space. They may even fall into the
already dense black hole making it even
heavier and denser.
• Basically what happens is that nuclear fusion reactions take place in
the core of the star which causes an acute outward pressure but
that pressure is optimally balanced by the intense inward pull of
gravity by the star’s mass. But when a star is in its death throes, the
fusion reactions combining hydrogen into helium (like in the Sun)
stop and a new kind of nuclear reaction take place that convert
helium into carbon. This is followed by carbon turning into oxygen
and oxygen to silicon and then to iron.
• That’s the point where nuclear fusion stops and the outer layers of
all the elements produced (hydrogen, helium, carbon & silicon)
keep burning around the central core of iron.
• The mammoth iron core builds up and finally explodes which is
called a supernova explosion.
• A star with a mass greater than 2 times that
of our sun, after going supernova its shrinks
and compresses into a new black hole.
A Supernova
Supermassive black holes
• Are present in the centers of most galaxies, including our very own
the Milky Way.
• If you put billions of suns together, you will know the mass of one
supermassive black hole. And that’s why the name as they are
super-colossal in size.
• Astronomers aren’t very sure as to how black holes are formed of
the supermassive variety. Maybe a couple of smaller black holes
coagulate together, or humongous gas clouds cave in to form them.
• The British Scientist Stephen Hawking proposed that trillions of
nonstellar black holes or mini or primordial black holes were
created along with the universe in accordance with the ‘big bang’
theory
How a Black Hole dies!
• This discovery came about when
Stephen Hawking discovered that black
holes should radiate energy due to
quantum mechanical processes. This
radiation is called Hawking radiation. As
a black hole radiates energy, it shrinks
and the more it shrinks, the more it
radiates and so finally it will completely
evaporate.
• However, the timescale for this is
extremely long: a black hole of the mass
of the Sun will take more than a billion
times a billion times etc. of the age of
the universe to evaporate completely!
The information used was from the text
Understanding Our Universe by Palen, Kay, Smith, Bluemthal,
Nov. 2011