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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