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March 29, 2008
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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.
Word Count: 3074
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