Deep within the universe lurk massive objects which seem to defy the laws of physics. They twist space and time into an endless void that no signal can attempt to escape. These objects are known as black holes, and are objects that are constantly undergoing scientific study. Their existence was first predicted by Einstein’s Theory of General Relativity, and scientists have searched for them ever since. Why are we interested in black holes? The answer to this question is simple; they may very well be the key to unlocking the mystery of how our universe was formed. However, before we can use them as a means of answering one of sciences most daunting questions, we must first understand black holes themselves.

The first step toward the understanding black holes is to gain an understanding of how science happened upon them in the first place. In 1916, Albert Einstein published a theory which he claimed united the ideas presented in special relativity, and classical Newtonian gravitation. This was an important discovery, because until Einstein’s solution these two principles of physics had been at odds with each other. Einstein called his unifying theory “General Relativity.” According to this theory, as an object becomes sufficiently massive and compact it will form a region of space from which no signal can transmit, not even light itself. This means that black holes are points of mass that have the capability to warp space and time in such a way that their gravitational pull forms an event horizon at which no signal from the inside of the hole can reach any point outside the hole. Understandably, scientists wanted to further investigate if the formation of such objects was even possible, and if so, what the implications of their discovery might be.

Before the search for black holes could begin, scientists needed to take a closer look at what general relativity has to say about them. The first thing that relativity has to say relates to the different types of black holes. “According to theory, there might be three types of black holes: stellar, super-massive, and miniature black holes – depending on their size,” . These different black holes hold properties that affect science independently. Stellar black holes are possibly the most observable, being formed as the final step in stellar evolution. It is this type of black hole that has lead to much in the way of understanding these daunting cosmological beasts. Black holes of the super-massive variety taunt scientists with their possibilities, but extremely limited methods of observation. It has been speculated that these black holes may have been responsible for as much as half the radiation emitted during the Big Bang. “Astronomers are finding out that these objects may have been critical to the formation of structure in the early universe, spawning bursts of star formation, planets, and even life itself,” . However, since these scientific marvels lurk at the center of galaxies, and are not inherently visible, scientists must settle for speculation. The final type of black hole as predicted by general relativity is the miniature black hole. These holes are thought to have formed shortly after the Big Bang, and can, in theory, “evaporate.” This means that these rather small black holes have the potential to degrade to a point at which they would explode with a force trillions of times more powerful than any man made explosion. These black holes, however, have never been observed, nor is there any clear evidence that they even exist at all.

Scientists then began to search for any physical signs that black holes might actually exist. In 1963, an astronomer by the name of Cyril Hazard designed a test to isolate a region of intense radioactivity. During the test he was successfully able to locate a star like object near the constellation Virgo as the source of one such region of radioactivity. This source, as well as many others like it, has been dubbed “quasi-stellar radio sources,” or quasars for short. These quasars baffled scientists since they emitted so much radiation for their rather small size (nearly 100,000 times smaller than the radius of out galaxy). It became clear that no star could emit such amounts of radiation. The answer to the quasar question happens to confirm the existence of black holes. “It turns out that super-massive black holes are luminous precisely because the material falling into them is squeezed and cajoled into producing radiation before disappearing forever below the horizon,”. In essence these quasars are the detectable footprint of super-massive black holes. Other smaller stellar black holes have been identified by their “shadow,” which is really the visible event horizon, that they cast on nearby stellar objects.

Once the existence of these strange objects was confirmed, speculation as to their formation began. The current theory describes stellar black holes as the final stage in the death of massive stars in the universe. Scientists know that as large stars age they burn out their supply of hydrogen. This hydrogen acts as the source of the stars energy, and when it is used up the star undergoes its inevitable death. The first stage of this death comes in the form of a supernova. This massive explosion destroys much of the material inside the core of the star. However, the remaining material gathers together to form what is known as a neutron star. At this point the star will either remain the neutral neutron star, or implode upon itself to form a black hole. “If the mass of the stellar remnant is high enough, the neutron degeneracy pressure will be insufficient to prevent collapse below the Schwarzschild radius. The stellar remnant thus becomes a black hole” . This means that black holes form when a neutron star possesses a gravitational field so great that not even the star itself can stand up the pull. It collapses inward endlessly, racing towards infinity.

Clearly black holes are the hosts of many strange and mind-boggling properties of physics. What makes them even stranger is the fact that there are two major types of black holes. There are non-rotating, and uncharged black holes, as well as rotating black holes. The properties of these two varieties of black holes have much in common, but they also have many properties that make them uniquely their own. Physicists have spent years attempting to sort out exactly what makes up a black hole, and recent studies have finally begun to produce some answers.

Scientists first began by decoding the mysteries of non-rotating black holes. The first major, and obvious, property of a black hole is its unique event horizon. This is the exact point within the black hole at which no signal can reach the outside. According to Stephen Hawking the event horizon is “the point of which light is just barely unable to escape,” . It is this event horizon that gives black holes their names. It is “the defining feature of a black hole – it is black because no light or other radiation can escape from inside it,” . The second most important feature of a black hole relates to the properties precisely at its center. General relativity predicts that the entire mass of a black hole collapses to a point of zero volume. Since the volume of the hole compresses to zero then the gravitational warping of space-time within the hole is infinite. Physicists give this these combined properties the name “singularity.” This, however, is only true if we are addressing black holes from a general relativistic point of view. If we look at them from the point of view of quantum mechanics we run into a problem. Quantum mechanics “does not allow objects to have zero size – so quantum mechanics says the center of a black hole is not a singularity but just a very large mass compressed into the smallest possible volume”. These two ways of looking at black holes will only be resolved upon the discovery of a theory uniting the two principles.

Another property of non-rotating black holes is the photon sphere. This is a boundary of zero thickness at which photons traveling tangent to the sphere will be trapped in orbit. These spheres are extremely unstable, and photons are very likely to react with nearby objects. This causes them to either escape the black holes gravitational pull, or fall behind the event horizon. Lastly, black holes form accretion disks around themselves. This is a buildup of gasses near the event horizon that has been compressed so much that it emits radiation. This occurs in a process which converts matter to energy in a highly efficient manner. Thus, these accretion disks are detectable due to the massive amounts of energy that they emit, providing scientists with one more bit of confirmation as to the existence of black holes.

The second major variety of black hole is known as a rotating black hole. These black holes are formed as a result of the implosion of a large rotating star. One of the features that make rotating black holes unique is known as the ergo-sphere. This is a region outside of the hole’s event horizon that is dragging space at a speed which exceeds the speed of light. Even though this region is dragging space-time at a rate faster than light itself, it is not within the event horizon, and thus, particles still have the possibility of escape. Do not let the fact that this rate of drag is faster than the speed of light bother you. Although, general relativity does not allow mass to travel faster than the speed of light; it does allow regions of space and time to exceed the “cosmological speed limit.” It is thought that particles escaping this ergo-sphere may be responsible for a phenomenon known as gamma ray bursts. Another interesting property of rotating black holes relates to their event horizon or horizons as it were. When a black hole spins it creates two distinct event horizons; an inner one and an outer one. As the spin of a rotating black hole increases the two horizons approach each other, until they finally are equal to each other. At that point the rotating black hole will cease to be a black hole and will be only a naked singularity. However, there is no evidence that any naked singularities have ever been formed. Finally, there is the idea, which is suggested by general relativity, that rotating black holes posses a ring shaped singularity. This means that any series of outside observers, when asked to identify the center of the black holes mass, will all point to some point on the ring of singularity. Again, this ring of singularity is of zero volume in accordance with general relativity, but not quantum mechanics.

All of this information provides a steady foot hold for the leap towards understanding the physical properties of rotating black holes. This type of black hole has begun to provide scientists with quantitative information about black holes, albeit information that is not easily attained. This is an important step towards total understanding because without any true mathematical description we may never know if our black hole observations hold true. Physicist John Wheeler once said that “black holes have no hair,” that is to mean that they have no indicators as to their internal properties outside of the event horizon. This statement expresses the limits to the understanding of black holes. However, in order to take the first step towards such understanding, one must begin with mass and spin.

The mass of a black hole can rather easily be attained, provided you are investigating a black hole with a mass of stellar proportion. “Astronomers record how fast the companion star circles around the black hole, and, from this, they infer its mass,” . The reason this only works for stellar black holes is that super-massive black holes (which can be billions of suns in size) lie at the center of galaxies, and it is very difficult to see stars moving around the center of a galaxy. Because of this fact it is impossible for any scientist to determine the actual mass of any observable super-massive black holes.

The next step towards rotational black hole understanding is determining its spin. This area has, until recently, seemed unattainable. There are now currently three techniques being studied, one of which may provide the answer to the question of a black holes spin. The first approach is known as “continuum fitting.” This approach begins with the determination of a black holes mass as described above. Immediately it is evident that this method will not work for super-massive black holes. The next step in this method is to calculate the actual heat given off by the black hole’s accretion disk. This is done by examining the continuum spectrum of X-ray emissions from the black hole. By determining the heat, it becomes possible to determine the holes innermost stable circular orbit. This is possible “because the closer a clump of gas gets to the hole, the greater the hole’s gravity affects it, which makes the gas orbit faster and heat up due to friction,”. It then logically follows, by the principles of general relativity, that the spin can be determined since the radius of the innermost stable circular orbit fluctuates with the rate of spin of the hole. The faster a black hole is rotating; the closer to the event horizon the innermost stable circular orbit will lie.

The second method for determining the spin of a black hole relies on the use of the iron emission line. The process is very much the same as in continuum fitting, but the iron emission spectrum has undergone much more thorough study. This method uses one of relativity’s basic principles to its advantage, that principle is known as gravitational redshift. The iron emission line “blurs near a black hole due to its gravitational redshift. As gas closes on the black hole, time runs slower,”. The idea that time is running slower near the event horizon of a black hole seems to defy all logical thinking, but it is true. As you approach the event horizon of the hole it takes light longer to break free of the gravitational pull and reach an outside observer. “As you get closer and closer to the horizon, the light that you’re emitting takes longer and longer to climb back out to reach [an observer]. In fact, the radiation you emit right as you cross the horizon will hover right there at the horizon forever and never reach [the observer]. You’ve long since passed through the horizon, but the light signal telling [the observer] that won’t reach [the observer] for an infinitely long time,” . This is the phenomenon of gravitational redshift (called redshift because the light emission spectra of any object at this point will appear red in color to an outside observer). The extent of gravitational red-shifting allows astronomers to determine how close the innermost stable circular orbit is to the event horizon. From this value, as with continuum fitting, it becomes possible to determine the rate of the hole’s spin. Unfortunately, this method suffers the same inherent flaw as continuum fitting in that it can only be applied to stellar black holes with a determinable mass. Any black hole for which the mass value is unknown cannot have a calculated spin by this method.

The third scientific approach to spin determination relies on the study of stellar black holes that are known to “sing.” The so called singing is actually the black hole emitting high-frequency notes that are given the title of “quasi-periodic oscillations.” This can be used to determine whether or not a black hole is spinning. “The oscillation frequency tells researchers how hot the X-ray-emitting gas orbiting the black hole is. This, in turn, reveals the orbital radius of the gas. If that radius turns out to be smaller than the innermost stable circular orbit of a non-rotating black hole, they conclude the object is spinning,”. The major problem, other than this method once again requires knowledge of the mass of the hole, is that the information seemingly has no way of being converted into actual numerical spin data. However, researchers believe that once the means of making this conversion are attained this method will prove the most accurate for of spin measurement.

Scientists are anxiously awaiting a method which will provide them with accurate spin data. This is because the spin numbers may very well hold the key to understanding some of the most basic properties of black holes. Information on how black holes are truly born, or how they grow is most likely contained within the mathematical relations of the spin. Scientists may soon find it possible to identify distinct relations between spin and radiation jets, or gamma ray bursts from black holes. Both of these have baffled scientists and will continue to do so until truly accurate spin relations are found. Perhaps most important, though, is the implications that spin data has for general relativity. It is commonly accepted that this theory is what drives all the forces of black holes. “If there is a big surprise out there, if Einstein is wrong, one way of finding out is by pushing these spin measurements,”. It is clear that the expanding knowledge of rotating black holes holds much in the way of how we view physics as a whole.

Nearly a century ago an ex-patent office clerk discovered a theory that changed how the world viewed gravity. He called this theory general relativity, and that theory predicted the existence of one of sciences most baffling findings, the black hole. This object has the ability to warp space and time to a point approaching infinity, and boasts an event horizon beyond which light itself cannot escape. These cosmological giants have been around since the dawn of our universe, possibly having been key players in its formation. It is no wonder that scientists have searched for a complete understanding of these objects. That search has lead to the advancement of much of modern physics, confirming that what Einstein thought is true. It has also lead to a greater understanding of how stars evolve, how radiation is emitted, and how gravity is affected by mass. Even with all of these inspiring discoveries black holes continue to remain one of sciences greatest mysteries. Their puzzle is slowly being assembled with promise that the final picture just may change how science will forever view the world in which we live.