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ESSAY SAMPLE ON "AN OVERVIEW OF THE CONCEPTS, PROPERTIES AND PROCESSES OF BLACK HOLES" |
Every day we look out upon the night sky, wondering and dreaming of what lies beyond our
planet. The universe that we live in is so diverse and unique, and it interests us to
learn about all the variance that lies beyond our grasp. Within this marvel of wonders,
our universe holds a mystery that is very difficult to understand because of the
complications that arise when trying to examine and explore the principles of space. That
mystery happens to be that of the ever elusive, black hole.
This essay will hopefully give you the knowledge and understanding of the concepts,
properties, and processes involved with the space phenomenon of the black hole. It will
describe how a black hole is generally formed, how it functions, and the effects it has
on the universe.
By definition, a black hole is a region where matter collapses to infinite density, and
where, as a result, the curvature of space-time is extreme. Moreover, the intense
gravitational field of the black hole prevents any light or other electromagnetic
radiation from escaping. But where lies the "point of no return" at which any matter or
energy is doomed to disappear from the visible universe?
The black hole's surface is known as the event horizon. Behind this horizon, the inward
pull of gravity is overwhelming and no information about the black hole's interior can
escape to the outer universe. Applying the Einstein Field Equations to collapsing stars,
Kurt Schwarzschild discovered the critical radius for a given mass at which matter would
collapse into an infinitely dense state known as a singularity.
At the center of the 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. Here it is no longer meaningful to speak of space and time, much less
space-time. Jumbled up at the singularity, space and time as we know them cease to
exist. At the singularity, the laws of physics break down, including Einstein's Theory
of General Relativity. This is known as Quantum Gravity. In this realm, space and time
are broken apart and cause and effect cannot be unraveled. Even today, there is no
satisfactory theory for what happens at and beyond the rim of the singularity.
A rotating black hole has an interesting feature, called a Cauchy horizon, contained in
its interior. The Cauchy horizon is a light-like surface which is the boundary of the
domain of validity of the Cauchy problem. What this means is that it is impossible to
use the laws of physics to predict the structure of the region after the Cauchy horizon.
This breakdown of predictability has led physicists to hypothesize that a singularity
should form at the Cauchy horizon, forcing the evolution of the interior to stop at the
Cauchy horizon, rendering the idea of a region after it meaningless.
Recently this hypothesis was tested in a simple black hole model. A spherically
symmetric black hole with a point electric charge has the same essential features as a
rotating black hole. It was shown in the spherical model that the Cauchy horizon does
develop a scalar curvature singularity. It was also found that the mass of the black
hole measured near the Cauchy horizon diverges exponentially as the Cauchy horizon is
approached. This led to this phenomena being dubbed "mass inflation."
In order to understand what exactly a black hole is, we must first take a look at the
basis for the cause of a black hole. All black holes are formed from the gravitational
collapse of a star, usually having a great, massive, core. A star is created when huge,
gigantic, gas clouds bind together due to attractive forces and form a hot core, combined
from all the energy of the two gas clouds. This energy produced is so great when it first
collides, that a nuclear reaction occurs and the gases within the star start to burn
continuously. The hydrogen gas is usually the first type of gas consumed in a star and
then other gas elements such as carbon,
Oxygen, and helium are consumed.
This chain reaction fuels the star for millions or billions of years depending upon the
amount of gases there are. The star manages to avoid collapsing at this point because of
the equilibrium achieved by itself. The gravitational pull from the core of the star is
equal to the gravitational pull of the gases forming a type of orbit, however when this
equality is broken the star can go into several different stages.
Usually if the star is small in mass, most of the gases will be
consumed while some of it escapes. This occurs because there is not a tremendous
gravitational pull upon those gases and therefore the star weakens and becomes smaller.
It is then referred to as a white dwarf. A teaspoonful of white dwarf material would
weigh five-and-a-half tons on Earth. Yet a white dwarf star can contract no further;
it's electrons resist further compression by exerting an outward pressure that
counteracts gravity. If the star was to have a larger mass, then it might go supernova,
such as SN 1987A, meaning that the nuclear fusion within the star simply goes out of
control, causing the star to explode.
After exploding, a fraction of the star is usually left (if it has not turned into pure
gas) and that fraction of the star is known as a neutron star. Neutron stars are so
dense, a teaspoonful would weigh 100 million tons on Earth. As heavy as neutron stars
are, they too can only contract so far. This is because, as crushed as they are, the
neutrons also resist the inward pull of gravity, just as a white dwarf's electrons do.
A black hole is one of the last options that a star may take. If the core of the star is
so massive (approximately 6-8 times the mass of the sun) then it is most likely that when
the star's gases are almost consumed those gases will collapse inward, forced into the
core by the gravitational force laid upon them. The core continues to collapse to a
critical size or circumference, or "the point of no return."
After a black hole is created, the gravitational force continues to pull in space debris
and other types of matters to help add to the mass of the core, making the hole stronger
and more powerful.
The most defining quality of a black hole is its emission of gravitational waves so
strong they can cause light to bend toward it. Gravitational waves are disturbances in
the curvature of space-time caused by the motions of matter. Propagating at (or near)
the speed of light, gravitational waves do not travel through space-time as such -- the
fabric of space-time itself is oscillating. Though gravitational waves pass straight
through matter, their strength weakens as the distance from the original source
increases.
Although many physicists doubted the existence of gravitational waves, physical evidence
was presented when American researchers observed a binary pulsar system that was thought
to consist of two neutron stars orbiting each other closely and rapidly. Radio pulses
from one of the stars showed that its orbital period was decreasing. In other words, the
stars were spiraling toward each other, and by the exact amount predicted if the system
were losing energy by radiating gravity waves.
Most black holes tend to be in a consistent spinning motion as a result of the
gravitational waves. This motion absorbs various matter and spins it within the ring
(known as the event horizon) that is formed around the black hole. The matter keeps
within the event horizon until it has spun into the center where it is concentrated
within the core adding to the mass. Such spinning black holes are known as Kerr black
holes.
Time runs slower where gravity is stronger. If we look at something next to a black
hole, it appears to be in slow motion, and it is. The further into the hole you get, the
slower time is running. However, if you are inside, you think that you are moving
normally, and everyone outside is moving very fast.
Some scientists think that if you enter a black hole the forces inside will transport
you to another place in space and time. At the other end would be a white hole, which
is theoretically a point in space that just expels matter and energy.
Also as a result of the powerful gravitational waves, most black holes orbit around
stars, partly due to the fact that they were once stars. This may cause some problems for
the neighboring stars, for if a black hole gets powerful enough it may actually pull a
star into it and disrupt the orbit of many other stars. The black hole can then grow
strong enough (from the star's mass) as to possibly absorb another star.
When a black hole absorbs a star, the star is first pulled into the ergosphere, which
sweeps all the matter into the event horizon, named for its flat horizontal appearance
and because this happens to be the place where mostly all the action within the black
hole occurs. When the star is passed on into the event horizon the light that the star
endures is bent within the current and therefore cannot be seen in space. At this exact
point in time, high amounts of radiation are given off, and with the proper equipment,
can be detected and seen as an image of a black hole. Through this technique, astronomers
now believe that they have found a black hole known as Centaurus A. The existence of a
star apparently absorbing nothingness led astronomers to suggest and confirm the
existence of another black hole, Cygnus X1.
By emitting gravitational waves, non-stationary black holes lose energy, eventually
becoming stationary and ceasing to radiate in this manner. In other words, they decay
and become stationary black holes, namely holes that are perfectly spherical or whose
rotation is perfectly uniform. According to Einstein's Theory of General Relativity,
such objects cannot emit gravitational waves.
Black hole electrodynamics is the theory of electrodynamics outside a black hole. This
can be very trivial if you consider just a black hole described by the three usual
parameters: mass, electric charge, and angular momentum. Initially simplifying the case
by disregarding rotation, we simply get the well known solution of a point charge. This
is not very physically interesting, since it seems highly unlikely that any black hole
(or any celestial body) should not be rotating. Adding rotation, it seems that charge is
present. A rotating, charged black hole creates a magnetic field around the hole because
the inertial frame is dragged around the hole. Far from the black hole, at infinity, the
black hole electric field is that of a point charge.
However, black holes do not even have charges. The magnitude of the gravitational pull
repels even charges from the hole, and different charges would neutralize the charge of
the hole.
The domain of a black hole can be separated into three regions, the first being the
rotating black hole and the area near it, the accretion disk (a region of force-free
fields), and an acceleration region outside the plasma.
Disk accretion can occur onto supermassive black holes at the center of galaxies and in
binary systems between a black hole (not necessarily supermassive) and a supermassive
star. The accretion disk of a rotating black hole, is, by the black hole, driven into
the equatorial plane of the rotation. The force on the disk is gravitational.
Black holes are not really black, because they can radiate matter and energy. As they
do this, they slowly lose mass, and thus are said to evaporate.
Black holes, it turns out, follow the basic laws of thermo-dynamics. The gravitational
acceleration at the event horizon corresponds to the temperature term in thermo-dynamical
equations, mass corresponds to energy, and the rotational energy of a spinning black hole
is similar to the work term for ordinary matter, such as gas. Black holes have a finite
temperature; this temperature is inversely proportional to the mass of the hole. Hence
smaller holes are hotter. The surface area of the event horizon also has significance
because it is related to the entropy of the hole.
Entropy, for a black hole, can be said to be the logarithm of the number of ways it
could have been made. The logarithm of the number of microscopic arrangements that could
give rise to the observed macroscopic state is just the standard definition of entropy.
The enormous entropy of a black hole results from the lost information concerning the
structural and chemical properties before it collapsed. Only three properties can remain
to be observed in the black hole: mass, spin, and charge.
Physicist Stephen Hawking realized that because a black hole has a finite entropy and
temperature, in can be in thermal equilibrium with its surroundings, and therefore must
be able to radiate. Hawking radiation, as it is known, is allowed by a quantum mechanism
called virtual particles. As a consequence of the uncertainty principle, and the
equivalence of matter and energy, a particle and its antiparticle can appear
spontaneously, exist for a very short time, and then turn back into energy. This is
happening all the time, all over the universe. It has been observed in the "Lamb shift"
of the spectrum of the hydrogen atom. The spectrum of light is altered slightly because
the tiny electric fields of these virtual pairs cause the atom's electron to shake in its
orbit.
Now, if a virtual pair appears near a black hole, one particle might become caught up in
a the hole's gravity and dragged in, leaving the other without its partner. Unable to
annihilate and turn back into energy, the lone particle must become real, and can now
escape the black hole. Therefore, mass and energy are lost; they must come from
someplace, and the only source is the black hole itself. So the hole loses mass.
If the hole has a small mass, it will have a small radius. This makes it easier for the
virtual particles to split up and one to escape from the gravitational pull, since they
can only separate by about a wavelength. Therefore, hotter black holes (which are less
massive) evaporate much more quickly than larger ones. The evaporation timescale can be
derived by using the expression for temperature, which is inversely proportional to mass,
the expression for area, which is proportional to mass squared, and the blackbody power
law. The result is that the time required for the black hole to totally evaporate is
proportional to the original mass cubed. As expected, smaller black holes evaporate more
quickly than more massive ones.
The lifetime for a black hole with twice the mass of the sun should be about 10^67
years, but if it were possible for black holes to exist with masses on the order of a
mountain, these would be furiously evaporating today. Although only stars around the
mass of two suns or greater can form black holes in the present universe, it is
conceivable that in the extremely hot and dense very early universe, small lumps of
overdense matter collapsed to form tiny primordial black holes. These would have shrunk
to an even smaller size today and would be radiating intensely. Evaporating black holes
will finally be reduced to a mass where they explode, converting the rest of the matter
to energy instantly. Although there is no real evidence for the existence of primordial
black holes, there may still be some of them, evaporating at this very moment.
The first scientists to really take an in depth look at black holes and the collapsing
of stars, were professor Robert Oppenheimer and his student, Hartland Snyder, in the
early nineteen hundreds. They concluded on the basis of Einstein's theory of relativity
that if the speed of light was the utmost speed of any object, then nothing could escape
a black hole once in its gravitational orbit.
The name "black hole" was given due to the fact that light could not escape from the
gravitational pull from the core, thus making the "black hole" impossible for humans to
see without using technological advancements for measuring such things as radiation. The
second part of the word was given the name "hole" due to the fact that the actual hole is
where everything is absorbed and where the central core, known as the singularity,
presides. This core is the main part of the black hole where the mass is concentrated and
appears purely black on all readings, even through the use of radiation detection
devices.
Just recently a major discovery was found with the help of a device known as The Hubble
Telescope. This telescope has just recently found what many astronomers believe to be a
black hole, after focusing on a star orbiting empty space. Several pictures were sent
back to Earth from the telescope showing many computer enhanced pictures of various
radiation fluctuations and other diverse types of readings that could be read from the
area in which the black hole is suspected to be in.
Several diagrams were made showing how astronomers believe that if somehow you were to
survive through the center of the black hole that there would be enough gravitational
force to possible warp you to another end in the universe or possibly to another
universe. The creative ideas that can be hypothesized from this discovery are endless.
Although our universe is filled with many unexplained, glorious phenomena, it is our
duty to continue exploring them and to continue learning, but in the process we must not
take any of it for granted.
As you have read, black holes are a major topic within our universe and they contain so
much curiosity that they could possibly hold unlimited uses. Black holes are a sensation
that astronomers are still very puzzled with. It seems that as we get closer to solving
their existence and functions, we only end up with more and more questions.
Although these questions just lead us into more and more unanswered problems we seek and
find refuge into them, dreaming that maybe one far off distant day, we will understand
all the conceptions and we will be able to use the universe to our advantage and go where
only our dreams could take us.
Bibliography
1.) Parker, Barry. Colliding Galaxies.
2.) Hawking, Stephen. Black Holes and Baby Universes.
3.) Encyclopedia Brittanica. Volume II, Black Holes. ? 1996
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