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 he 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 niverse? 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 pace-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 orizon, 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 rom 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 epending 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 pon 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, ravitational 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 ach 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 ext 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 vent 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, ventually 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 et 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 ould 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 f 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 vaporate 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 ain 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 uspected 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 hey 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.

Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a black hole. Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein’s general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, by thinking about gravity under fairly simple circumstances.

Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don’t throw it too hard, it will rise for a while, but eventually the acceleration due to the planet’s gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet’s gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet’s gravity is called the “escape velocity.

As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet’s center: the closer you are, the higher the escape velocity . The Earth’s escape velocity is 11. 2 kilometers per second (about 25,000 M. P. H. ), while the Moon’s is only 2. ilometers per second (about 5300 M. P. H. ).

We cannot see it, but radiation is emitted by any matter that gets swallowed by black hole in the form of X-rays. Matter usually orbits a black hole before being swallowed. The matter spins very fast and with other matter forms an accretion disk of rapidly spinning matter. This accretion disk heats up through friction to such high temperatures that it emits X-rays. And also there is some X-ray sources which have all the properties described above.

Unfortunately it is impossible to distinguish between a black hole and a neutron star unless we can prove that the mass of the unseen component is too great for a neutron star. Strong evidence was found by Royal Greenwich Observatory astronomers that one of these sources called Cyg X-1 (which means the first X-ray source discovered in the constellation of Cygnus) does indeed contain a black hole. It is possible there for a star to be swallowed by the black hole.

The pull of gravity on such a star will be so strong as to break it up into its component atoms, and throw them out at high speed in all directions. Astronomers have found a half-dozen or so binary star systems (two stars orbiting each other) where one of the stars is invisible, yet must be there since it pulls with enough gravitational force on the other visible star to make that star orbit around their common center of gravity and the mass of the invisible star is considerably greater than 3 to 5 solar masses.

Therefore these invisible stars are thought to be good candidate black holes. There is also evidence that super-massive black holes (about 1 billion solar masses) exist at the centers of many galaxies and quasars. In this latter case other explanations of the output of energy by quasars are not as good as the explanation using a super-massive black hole. A black hole is formed when a star of more than 5 solar masses runs out of energy fuel, and the outer layers of gas is thrown out in a supernova explosion.

The core of the star collapses to a super dense neutron star or a Black Hole where even the atomic nuclei are squeezed together. The energy density goes to infinity. For a Black Hole, the radius becomes smaller than the Schwarzschild radius, which defines the horizon of the Black Hole: The death explosion of a massive star, resulting in a sharp increase in brightness followed by a gradual fading. At peak light output, supernova explosions can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud.

This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant. So, a black hole is an object, which is so compact that the escape velocity from its surface is greater than the speed of light. The following table lists escape velocities and Schwarzchild radii for some objects: The black hole masses ranging from 4 to 15 Suns (1 solar mass = 1 Msun = 2 x 1033 grams. ) And are believed to be formed during supernova explosions. The after-effects are observed in some X-ray binaries known as black hole candidates.

The velocity depends on the mass of the planet. The scientists believe if our Sun dies, the sun may turn into a black hole. Black holes were theorized about as early as 1783, when John Michell mistakenly combined Newtonian gravitation with the corpuscular theory of light. The concept of an escape velocity, Vesc, was well known, and even though the speed of light wasn’t, Michell’s idea worked the same. He showed that Vesc was proportional to mass/circumference and reasoned that, for a compact enough star, Vesc might well exceed the speed of light.

His mistakes were twofold: he subscribed to the corpuscular theory of light, and he assumed that Newton’s law of universal gravitation could apply to such a situation. These mistakes happened to cancel each other out, but when the wave theory of light gained favor, the astronomers abandoned these dark stars. In the beginning of the 20th century, Einstein proposed his theory of general relativity.

The formula worked out by Michell and rederived, this time without mistakes in the derivation, by Karl Schwarzschild, gives the Schwarzschild radius for any massive body (that is, a body containing mass): RS= 2GM/c2. Vesc for any body smaller than this radius would exceed that of light, and since general relativity forbids this; any matter within RS would be crushed into the center. Thus RS can effectively be thought of as the boundary of a black hole, called an event horizon because all events within RS are causally disconnected from the rest of the universe. There arent many physical features of a black hole. In an aphorism coined by John Wheeler , “black holes have no hair,” hair meaning surface features from which details of it’s formation might be obtained.

There are no perturbations in its event horizon, no magnetic fields. The hole is perfectly spherical and in fact has only three attributes: it’s mass, it’s spin (angular momentum), and it’s electric charge. Of these properties, it is only the mass that concerns astronomers. As a cloud of gas contracts, the interior heats up until the core is so hot and dense that nuclear reactions can occur. This nucleo-synthesis of hydrogen into heavier elements generates a tremendous pressure, according to the ideal gas law P=NkT, and this pressure holds the star up against further gravitational collapse.

This state of equilibrium, during which a star is said to be on the main sequence, lasts until the hydrogen in the core is used up, about 10 billion years for a star like the sun, whereupon gravity will resume shrinking the star. Exactly what occurs next depends on the complicated interactions between different layers of the star, but generally, the star will explode in a supernova. If there is any remnant of this explosion, its further evolution depends almost exclusively on it’s mass. A remnant below 1. 4 M (@) will collapse until it can be supported by electron degeneracy pressure and form a white dwarf.

A remnant between 1. 4 and 3 M(@) is halted by neutron degeneracy pressure and forms a neutron star. Degeneracy pressure is an effect that results from quantum mechanical interactions when the density of subatomic particles increases. As it depends only on this density, it is non-thermal and will remain no matter how much the star cools down. Still for remnants above 3 M(@), not even degeneracy pressure can counter the force of gravity, and a black hole is born. This was the general base that general relativity gave to astronomers, but just because something is allowed to happen doesn’t mean that it does.

Most astronomers resisted such absurd realities. Astronomers are very conservative by nature, and some of the most respected and influential astronomers of the day rejected this idea so soundly that it wasn’t until the 60’s that any actual searches began. At first, the only instruments available were the old familiar optical telescopes. Optical telescopes are just what they sound like, telescopes sensitive to the visible portion of the electromagnetic spectrum . This spectrum can reveal much information regarding the source of the light. The color indicates the temperature of a star.

By combining the type of star, identified by observing lots of other stars with similar characteristics, and our models of stellar processes with a measurement of the star’s luminosity, it is possible to calculate the distance to the star. We can even determine the chemical composition of the star by observing any emission or absorption lines in the spectra. Furthermore, these lines are very distinctive, and if they appear in the correct relation to each other but have been Doppler-shifted towards the red or blue ends of the spectrum, a measurement of the star’s speed relative to the earth can be obtained.

The only distinguishing feature of a black hole is its gravity, however, and searching for a black hole with an optical telescope is next to impossible. A black hole does not give off any light. It’s too small to observe by blocking out stars behind it. It could act as a gravitational lens, but to do so it would have to be directly in line with the Earth and some bright object, and even then there would be no way to distinguish between a black hole or a very dim star. Still, there was on promising method proposed by Russian astronomers Zel’dovich and Guseinov in 1964.

If the black hole was in a binary system with another, normal star, the light curve of the system would give it away. Binary systems comprise about half of all known stars, so it is not unlikely that a black hole might be found next to a normal star. In a spectroscopic binary system, the stars rotate about their center of mass and the light will be Doppler shifted. The light curve of a star is a graph of the intensity or Doppler-shift of light from the star versus time. Here the light curve of the visible companion can yield much information.

The period of rotation about the center of mass can be determined by inspection of the Doppler-shifted light curve itself, and the mass of the visible star is given by the type of star and how luminous it is. All that is then needed is a reasonable estimation of the inclination i of the system, and several important things can be calculated. The mass function f(M) = M2^3 sin i / (M1 +M2)^2 gives a relation between the masses of the two bodies, and the semi-major axis a1=AM2/(M1+M2)^2 sin i (where A is the separation of the centers of mass) gives the size of the orbit, which can also be related to the rotational velocities of the stars.

A spectroscopic binary with no visible companion would be a candidate for a black hole, and if the dim star’s mass is determined to be greater than that of the visible star, it would be a promising candidate. However, this method consists of many uncertainties. Although there were no hard cases for black holes any scientists search, there arose another way a black hole might show itself. If the black hole were in a gaseous nebula, the gas would fall into the black hole.

The inherent magnetic fields of the gas create turbulence, generating heat, which is in turn transformed into electromagnetic radiation. The luminosity of the gas could oscillate rapidly due to the turbulence, and such rapid oscillations would give the black hole away. Another Soviet scientist, Schwarzmann, developed the “Multichannel Analyzer of Nanosecond Pulses of Brightness Variation” in an effort to detect these oscillations, but that method also proved fruitless.

X-ray novas are a special class of X-ray binaries where the system contains a late-type optical companion (a star near the end of its life) and a compact object, which can be either a neutron star or a black hole. Usually the spectrum of the companion in this type of system is very weak compared to that of the gas, but in X-ray novae the fraction of light from X-ray heating is negligible, and we have an excellent opportunity to study the system in detail.

If the accretion disk is due to a black hole, then understanding the companion star in detail will also allow understanding of the processes of X-ray emission. Several X-ray satellites detected Muscae 1991 and calculations began to pinpoint an optical companion. To do this, the exact position of the X-ray source must be known. If there is a star in the visible range at that same position, it is most likely related to the X-ray star, and the light curve can then be studied in detail. In this case, a companion was found.

The similarities of Muscae 1991 with one of the best black hole candidates, V616 Mon, make it seem realistic that it might be a black hole. The evolution of the light curves, the decay rate in magnitude of the novae, and variations in brightness on the order of a day are all similar in the two systems. The spectrum of the nova, its various emission lines and other spectroscopic details, also does not resemble a classical nova in the same stages, but instead resembles that of the black hole candidates Cen X-4 and V616 Mon.

As it is not a classical nova, the distance to Muscae 1991 must be estimated from a known linear relation of the width of the NaD line to distance. This gives a result of 1. 4 kpc (kiloparsecs), which returns some typical values for low mass X-ray binaries and justifies confidence in its validity. Using this distance and the spectral features of the binary, the companion star seems to be a late main sequence star, which is in agreement with current theories of low-mass X-ray binaries.

What this all boils down to is that the binary X-ray nova Muscae 1991 behaves very similarly to other black hole candidates in the galaxy, and gives a picture of the nova as a burst of gravitational potential energy released as matter from the disk accreted onto the compact object. The large amounts of energy released in the nova as X-rays indicates the companion is at least a neutron star and possibly a black hole, but no obvious conclusions can be made as to Muscae 1991’s containing a black hole.

Cygnus X-1 is accepted as a black hole by most astronomers, there is still nothing about it that demands unequivocally to be accepted as such. Cygnus X-1 is the best X-ray astronomy can give us. But X-rays and visible light are not the only ways of probing the sky. Radio astronomy was also discovered accidentally. In the 1930’s, a technician trying to clear up intercontinental phone calls discovered radio waves coming from the Milky Way. Curiously enough, nobody really seemed to care very much; an amateur built the world’s first radio telescope.

A modest 9 meters in size, it had extremely poor resolution, and the larger dishes that were to slowly follow did not fare much better. As in X-ray astronomy, the astronomers couldn’t do anything really useful with cosmic radio waves until they could identify an optical counterpart. Since radio waves are on the order of meters long, diffraction effects would require unreasonably large dishes to acquire any decent resolution. To counter this, astronomers came up with radio inter-ferometry. At first the bodies that shone most brightly in the sky could not be associated with an optical counterpart.

As radio telescopes improved, the error boxes for these sources shrank until, in 1953, a team at Cambridge had a sufficiently accurate estimate that other astronomers at the Palomar 5-meter optical telescope could identify the radio source Cyngus A with an optical source. This source turned out to be a galaxy, and once it’s redshift, and hence distance, were measured, it was found that this galaxy’s radio luminosity was millions of times brighter than that of an ordinary galaxy. The first radio galaxy had been found.

Now that the technology was in place, more and more of these galaxies were discovered and they began to be studied in great detail. The results troubled astronomers; radio galaxies had two lobes of radio emissions with the dim optical galaxy in the center. These lobes stretched out millions of light-years, indicating a stable source of emission, and conservative estimates of the energy involved in their production was on the order of 10^61 ergs, as much energy as would be released in ten billion supernovas.

Radio galaxies were among the first in what are today classified as AGN – active galactic nuclei. Other types of AGN include Seyfert galaxies, N galaxies, BL Lacertae objects, and quasars. They all demonstrate violent behavior that can’t be associated with the ordinary behavior of stars and interstellar dust, whether it be matter and energy ejected from the nucleus to luminosities of truly astronomical proportions. While all these objects were regarded as puzzles, it was really the quasars that could not be explained by any astronomical processes at all.

Of course they do exist, and astronomers rushed to find explanations for them. It was in this storm of hypotheses that the idea of a super-massive black hole lost it’s exotic nature and became the most reasonable explanation. In fact, many of the other realistic explanations also support this idea, for they could evolve into a super-massive black hole . If there are a lot of star-star collisions occurring, the stars will lose enough energy such that they become bound in a binary which fairly rapidly decays, if they do not coalesce directly with each other.

Such models of AGN could have two natural results without invoking black holes: supernova explosions, or clusters of pulsars. The supernova explosions are only as efficient as regular nuclear burning in stars, and must occur at a rate of about 5 to 10 a year. Furthermore, these supernovas cannot be ordinary stellar supernovas but rather a sort of ‘hypernova’, wherein neutron stars must pass through the cores of super-massive stars, due to calculations of the energies released.

If the cluster evolves into a cluster of pulsars, it is the rotational energy of the pulsars that powers the quasars. Through horrendously complicated interactions of particles and strong electromagnetic fields, this energy could be released into the universe, but both this and the supernova model have another serious flaw; there is no directionality of the radiation that could result in the observed jets of quasars and other AGN.

To correct this would require a flattened cloud of gas that would either hasten the death of the cluster and it would collapse into a black hole, or the luminosity would be so great that the resulting wind of radiation would drive the gas into space, thereby destroying the model entirely. Other models involve the rotational energies of massive uncollapsed bodies. Known as super-massive stars, magnetoids, or spinars, they are all basically the same; a massive, spinning flattened disk (a super-massive rotating star will evolve into a disk).

One way these spinars could liberate energy is by gravitational contraction, releasing up to a few percent of their rest mass as energy. However, to remain stable against collapse, a very large ultraviolet radiation pressure must be present, and such radiation is not found in radio galaxies, though they might be in high-redshift quasars. A pulsar is a rotating neutron star with skewed magnetic poles . Radiation is emitted in the direction of the magnetic poles, and if this beam passes earth, it has the same effect as a lighthouse.

The incredible angular momentum of a pulsar makes its pulses extremely regular, to a degree of accuracy elsewhere found only in atomic clocks. As such, the orbit of a binary pulsar can be scrutinized in extreme detail, and has been. The results are amazing; the period of the stars is declining and their orbit is slowly decaying to exactly the degree predicted by general relativity. A better proof of gravitational radiation could hardly be imagined. The first person to attempt to detect this radiation was Joseph Weber.

He eventually came up with the first bar gravity-wave detector. This was a long aluminum cylinder, 2m by m, that should be compressed with an incoming gravity wave. To detect this compression he wired piezoelectric crystals, which respond to pressure by generating an electric current, to the outside surface of the bar. Although it didn’t work, other bar detectors were built that used a device called a stroboscopic sensor to filter out random vibrations. This was an ingenious device, but it too proved to be a non-contributor in the advancement of learning more of the galaxy.

Just as X-ray astronomy went from simple detectors in the noses of rockets to full fledged X-ray telescopes housed in orbiting satellites, and radio astronomy went from crude dishes to continent spanning arrays, gravity wave detectors may show a completely new spectrum. And, just as X-rays brought a completely new universe into focus, one can hardly imagine what a gravitational view of the universe will reveal. At the very least, we will have definitive proof or denial of black holes, but we may find that black holes are some of the more subtle features of the universe.