Introduction

Lurking behind the dusty visible portions of the universe lie objects with strange characteristics. Light cannot escape from these objects of mystery , rendering them
virtually invisible. Yet the power that they wield is so colossal that nothing can evade
their intense gravitational grasp. Such objects seem implausible, yet do exist. Black
holes, however comically and incorrectly named, have a place in this vast universe;
almost acting as the fantasized vampires on Earth. Collapsed giant stars with no other
future than to terminate the future of anything that crosses their point of no return, black
holes have been lurking in the shadows of the cosmos waiting to be found. Fascinating
since first discovered, black holes are tantalizingly close yet invisible from Earth and
contain untapped resources for the potential benefit of humans.

History

In 1783, John Michell, a British natural philosopher, was the first to contemplate the
likelihood of such objects as black holes. He was attempting to imagine a very compact
star’s effects on the light particles it emits (corpuscles). He supposed that a particle must
be projected at a high enough velocity to escape the gravity of such a star. The higher the
gravity would become, the more it would slow down these particles of light. Michell
calculated the minimum
speed for a particle to escape using Newton’s laws of gravity, and called this the 'escape
velocity.' This escape velocity was determined by a particle’s square root of its mass
divided by its circumference. This just means that the smaller the circumference
(distance around the center of the star), the stronger the pull of gravity at its surface.
Thus the escape velocity would increase.

Michell also stated that once a star’s circumference reached a certain size the escape
velocity would become the speed of light. Calling this the 'critical circumference', he
predicted that the particles of light would just scarcely be able to escape. Once a star
went beyond the critical circumference, no light would be able to escape. Being unable to
think of any laws to prevent this from happening, he decided that there could exist a great
deal of these throughout the universe. Later on, however, light came to be thought of not
as a particle, but as a wave. This made these 'dark stars' unable to fit into Newton’s
Laws of gravity (light would later become known as both under quantum theory.)

Unfortunately, Michell’s ideas were based on the Newtonian laws, with time as an
absolute factor. When the general theory of relativity was proposed, Michell’s ideas did
not fit into the new sense of time and space. His ideas weren’t forgotten, though. In
1916, Karl Schwartzschild found a way to fit the dark stars into general relativity. Not
many people believed him, and even Einstein didn’t accept dark stars to exist. The reasons behind this lie in peoples' abilities to believe in something of huge mass and zero size. Not many people would just believe that without a large amount of direct evidence! The
importance of the dark stars wouldn’t be found until John Wheeler studied Einstein’s
theory of general relativity, despite it’s seeming unimportance at the time.

Subrahmanyan Chandrasekhar, an Indian graduate student, went to England in
1928 to study under the astronomer Sir Arthur Eddington, an expert on general relativity.
Chandrasekhar determined how a large star could exist and be able to endure its own
gravity even after it ran out of fuel. Once the star would start to shrink, the particles of
matter would be forced closer to each other, and repel one another. Thus the star would
uphold itself using a balance of gravity and repulsion. He also calculated a limit to how
large the star could become for this to occur. If the star was too massive, the force of
repulsion would be overcome by the vehement gravity. This limit is at a star 1.5 times
the mass of the sun, and is known as the 'Chandrasekhar limit.' If such a star was too
large, it would collapse into itself at a single point. Eddington, who didn’t believe this
strange theory, inevitably caused Chandrasekhar to forsake his work.

In 1939, this problem was solved when Robert Oppenheimer came up with results that
made Chandrasekhar’s ideas comprehensible. His work on the atom bomb project in
World War II delayed him, however. Oppenheimer determined that gravity affects light,
therefore at a
certain critical radius, light wouldn’t be able to escape a star. More people became
interested in the 1960s when technology became more modern, as compared to what they at one point had. Technology allowed them to observe immense gravitational effects where there appeared to be nothing, and the radiation let off by black holes at their poles.

Other forms of radiation had to be looked for to first detect black holes since visible
light can’t escape them. In the constellation Cygnus, x-rays were found to be discharged
at one thousand times per second from an invisible source. It was discovered that
whatever was creating this was only 300 kilometers across, and was right next to a giant
blue star. The giant
star’s path was being pulled at by something with an immense but unseen gravitational
force. This was the first time that a black hole was detected in the sky. The x-rays were
thought to be
coming from the friction due to dust and gas being pulled from the star next to it. Many
other stars have since been found to have the same effects.

Black holes weren’t actually called that for quite awhile. The first name proposed
was by Michell, who initially called them 'dark stars,' because they were collapsed stars
that were
unable to emit light. Since Schwartzschild started working with the dark stars, and
calculated a way for them to exist, they were then known as 'Schwartzschild
singularities.'In 1969, John Wheeler annoyed the French by naming these
objects 'black holes.' The translation to French was an inappropriate phrase in France
(the sort of phrase that would cause immature people of which most of the world’s
population are to giggle ferociously), so the French suggested the name be changed to
“astre occlu” which means hidden star. However, this
wasn’t as interesting as black holes, that was the name that the public liked, so it stuck.

General

Unfortunately, this name doesn’t accurately describe these mysterious objects. To
understand a black hole, one must first understand how one is formed. First, a very large
star, no
less than thirty times the size of the sun, runs out of fuel. It will then cool down, and
begin to compress. Then, being too large for the aforementioned repulsion of it’s
particles to
keep itself stable, the star will collapse. The smaller it becomes, the stronger the
gravitational pull at the surface. After it reaches the critical circumference, light, having
the highest possible
velocity, will no longer be escape the surface. The star will then commence to collapse to
a tiny point called singularity, which represents zero size and infinite
density. Therefore, a black hole isn’t a 'hole' in space, but an immensely dense object
pulling in and devouring everything in its path.

Once not even believed to exist, black holes are now known to be found all
throughout the universe. Very large black holes are found at the centers of galaxies,
whereas smaller ones are scattered just as unevenly as the stars all around. The giant
black hole at the center of the Milky Way, Sagittarius A, was recently observed for two
weeks straight. It was found to have high-energy eruptions, lobes of very hot gas at
twenty-million degrees Celsius, and faint x-ray streams up to one light year in length. All
of these amazing findings are thought to be related to activity near the event horizon (a
sort of point of no return) of the black hole.

The event horizon is equivalent to a point of no return. Once anything passes it, it
can’t escape, no matter what. If a person fell into a black hole, he or she would see the
history of the
universe right there in front of him or her. However, the difference in the gravitational
pull from the person’s head to his or her feet would be so great that he or she would be
stretched out and
most likely either resemble a thin piece of yarn, be very quickly dead, or even more likely
both. The gravity of a black hole is so great that time and space are distorted and at the
point of
singularity, anything pulled in is torn apart atom by atom. Along with the event horizon
and point of singularity, most black holes have large discs of hot gas encircling them,
which is attracted by their extreme gravity.

Black holes that are very massive, supermassive black holes, are the type located at the
centers of galaxies. They are now thought to actually form before the galaxy surrounding
them.
When viewing early galaxies, a study showed that there was already a fully formed
supermassive black hole at the cores, despite the still forming galaxy surrounding them.
These black holes are
thought to formed by gathering materials from the dense cores of other, older galaxies.
Ohio State University researcher Marianne Vetergaard has said in light of this, 'Looking
at this evidence, I have to think that black holes start forming before galaxies do, or form
at a much faster rate, or both.'

As for the other end of a black hole’s life, nothing will be able to be viewed for many
many years. However, Stephen Hawking predicted what would happen when a black
hole stopped spinning. Known as Hawking Radiation, the black hole would still give off radiation, thus still losing energy. He also
found that the temperature of the black hole is related to its surface gravity. Along with
that idea was that as the black hole loses energy and shrinks, its temperature and surface
gravity will increase, causing it to evaporate. This will happen over a very long period of
time, which is why no one will be able to witness one for many many years and possibly never, but after the black hole shrinks beyond the size of the nucleus of an atom, it will
become so intensely hot that it will
brutally explode in a short portion of a second.

Uses

Some more interesting things about black holes are their uses. One use is a potential
energy source. Roger Penrose of Oxford University calculated how exactly this would be
accomplished. First, in order to harness this energy, some sort of structure would have to
be built to orbit the black hole, which could supply a fuel to deposit into it. Each particle
of the fuel would be broken in to two pieces at the event horizon, with one falling past it.
The other particle would give off a boost of energy as it’s comrade was demolished.
These bits of energy could be collected and used to sustain a ship or civilization.

However, this wouldn’t last forever. Cemetrios Christdoulou, a researcher for
Princeton University, revealed that black holes would eventually stop spinning after all of
this, and would not be able to produce energy anymore. Despite the prospect of great
energy resources, the nearest black hole is well over 15 light years away, which is about
eighty-eight trillion miles away. This is a short distance relative to the size of the universe, but seeing as this would most likely never be accessible
from Earth, this resource will have to wait for technology to improve a bit.

Black holes could also be used for time travel. To make it simple, the closer to
the speed of light something travels, the slower time is for it. Einstein abandoned
absolute time and one of
his ideas was that 'Each person traveling in his or her own way must experience a
different time flow than others, traveling differently.' He couldn’t actually do an
experiment to show this since the effects on time aren’t noticeable until one reaches
speeds closer to the speed of light than anyone ever could, so he inferred about how
things would function in such cases. Later on, experiments using very accurate atomic clocks and extremely fast jets were used to help prove this idea.

This relates to black holes in that orbiting around a black hole would bring one to very
high speeds close to the speed of light, causing the time in that person’s view to seem
normal, but to everyone else very slow. The person moving that fast would feel as if he
or she were traveling ordinarily, but would actually be moving forward in time faster than
everyone else, thus creating time travel. This can only go one way, though. Time travel
backwards would be very difficult if not impossible. Once again the nearest black hole
would be too far away to even try this. Certain experiments involving atomic clocks on
high-speed jets have proven this to be true.

Of course, going forward in time faster than usual would be difficult due to the
immense amounts of energy that would be required. The amount of energy that would
most likely be needed to move forward in time would be similar to that of what a
medium-sized star would use.

Conclusion

This is just a small sampling of the vast information on black holes. These
enticing objects have made heads turn in the world of physics as well as the world in
general. Their theorization was looked on as a joke, but today as a great idea of long ago.
Black holes have intrigued humans since first discovered, and that has helped some to
discover all of the fascinating facts about them, including their potential uses. In
conclusion, black holes may seem
so very distant and invisible, but in actuality are closer than ever. Many scientists and
philosophers have been calculating and thinking about these faceless wonders, and thanks
to their
dedication and brilliance, humans today can have the knowledge of something invisible in
the abyss of the universe.

The resources used in the writing of this entry are as follows: Isaac Asimov’s The
Collapsing Universe, The Secrets of the Universe binder set, an article by Pamela Gay
from Astronomy magazine titled “Lobed monster in Sagittarius,” Steven Hawking’s two
books The Illustrated A Brief History of Time and The Universe in a Nutshell, Carl
Sagan’s Cosmos, an article from Astronomy magazine by Vanessa Thomas entitled “The
black hole, then the stars.”, Kip Thorne’s Black Holes and Time Warps, and an aritcle
from Discover magazine by Jeffery Winters titled “Weird Black-Hole Topography
Proposed.”