A LOOK AT BLACK HOLES AND THEIR COUNTERPARTS

For as long as the Earth has been around, humans have looked up in awe at the night sky and wondered about the intricate pieces that make up what is our universe. For most of Earth’s history, the universe only encompassed what was visible to the naked eye. In 1609, however, everything changed when telescopes were invented and the realm of the universe opened to unveil new and mysterious aspects never before seen. Interestingly enough, though technology has grown to gigantic proportions, there is still a lot we don’t know about the universe, and black holes, white holes, and wormholes are just a few of the aspects we still have much to learn about. At this point in time, astronomists have yet to actually see a black hole. So how do they know that black holes do in fact exist? And how can they deduce that related beings like white holes and wormholes do (or do not) exist? Through careful deduction and intensive examination, scientists like Einstein, Stephen Hawking, and many others have been able to give insight into the deep, enigmatic universe and exactly what these mysterious beings are all about. Einstein's general theory of relativity describes gravity as a curvature of space time caused by the presence of matter. If the curvature is fairly weak, Newton's laws of gravity can explain most of what is observed, for example, the regular motions of the planets. Very massive or dense objects generate much stronger gravity. The most compact objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip. Scientists today call such an object a black hole. Why black? Though the history of the term is interesting, the main reason is that no light can escape from inside a black hole: it has, in effect, disappeared from the visible universe. By definition a black hole is a region where matter collapses to infinite density and where, as a result, the curvature of spacetime 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? How does such a thing come into existence? Black holes are thought to form from stars or other massive objects if and when they collapse from their own gravity to form an object whose density is infinite: in other words, a singularity. The singularity is where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. Here it's no longer meaningful to speak of space and time, much less spacetime. Jumbled up at the singularity, space and time cease to exist as we know them. As a stars lifetime goes on, nuclear fusion in the core generates electromagnetic radiation, including the particles of light known as photons. This radiation exerts an outward pressure that exactly balances the inward pull of gravity caused by the star's mass. As the nuclear fuel is exhausted, the outward forces of radiation diminish, allowing the gravitation to compress the star inward. The contraction of the core causes its temperature to rise and allows remaining nuclear material to be used as fuel. The star is saved from further collapse -- but only for a while. Eventually, all possible nuclear fuel is used up and the core collapses. How far it collapses, into what kind of object, and at what rate, is determined by the star's final mass and the remaining outward pressure that the burnt-up nuclear residue (largely iron) can muster. If the star is sufficiently massive or compressible, it may collapse to a black hole. The first person to discuss black holes was a Cambridge man by the name of John Mitchell. Mitchell argued that it was possible to have a star that was sufficiently massive and sufficiently small in size that its escape velocity would be greater than the velocity of light (Hawking 117). Such a star would obviously not be visible on Earth because its light would never reach us, but Mitchell argued that it might be possible to detect the presence of that star by the effect that its gravitational field would have on nearby matter (Hawking 117). The idea being implied was that light could actually be slowed by a strong enough force of gravity. According to the theory of general relativity, space and time together can be regarded as forming a four-dimensional space called spacetime (Hawking 118). This space is not flat but rather distorted and curved by the matter and energy within it. The curvature of spacetime can be observed in the bending of the light or even radio waves that travel close to a strong gravitational field. This field would be so strong around and within a black hole that not even light could escape. The boundary of a black hole is formed by an area called the event horizon, which is formed by light that fails to escape from the black hole but stays hovering around its edges. To answer the question of whether or not black holes do in fact exist, scientists must rely on indirect evidence, i.e., light that they don’t see. Suppose you have found a region of space where you think there might be a black hole. The first thing scientists must check for when trying to determine whether or not a black hole exists is to measure how much mass there is in that region. If there is a large mass concentrated in a small volume, and if the mass is dark, then it's a good guess that there's a black hole there. There are two kinds of systems in which astronomers have found such compact, massive, dark objects: the centers of galaxies (including perhaps our own Milky Way Galaxy), and X-ray-emitting binary systems in our own Galaxy (internet 1). According to a recent review by two prominent astronomists Kormendy and Richstone, eight galaxies have been observed to contain such massive dark objects in their centers. The masses of the cores of these galaxies range from one million to several billion times the mass of the Sun. The mass is measured by observing the speed with which stars and gas orbit around the center of the galaxy: the faster the orbital speeds, the stronger the gravitational force required to hold the stars and gas in their orbits (internet 1). Astronomers believe these massive dark objects in galactic centers to be black holes for at least two reasons. First, it is hard to think of anything else they could be: they are too dense and dark to be stars or clusters of stars. Secondly, the only promising theory to explain the enigmatic objects known as quasars and active galaxies postulates that such galaxies have supermassive black holes at their cores. If this theory is correct, then a large fraction of galaxies -- all the ones that are now or used to be active galaxies-- must have supermassive black holes at the center (internet 1). Taken together, these arguments strongly suggest that the cores of these galaxies contain black holes, but these theories are not absolute. However, two very recent discoveries have been made that strongly support the hypothesis that these systems do indeed contain black holes. First, a nearby active galaxy was found to have a water maser system (a very powerful source of microwave radiation) near its nucleus and using the technique of very-long-baseline interferometry, a group of researchers was able to map the velocity distribution of the gas with very fine resolution (internet 1). In fact, researchers were able to measure the velocity within less than half a light-year of the center of the galaxy. From this measurement researchers were able to conclude that the massive object at the center of this galaxy is less than half a light-year in radius. It is almost impossible that anything other than a black hole could have so much mass concentrated into such a small volume (internet 1). The second discovery that sparked researchers to believe these objects were infact black holes was the detection of a spectral line from one galactic nucleus that indicates the presence of atoms near the nucleus that are moving extremely fast (about 1/3 the speed of light). The radiation from these atoms had been redshifted in just the manner one would expect for radiation coming from near the horizon of a black hole (internet 3). These observations would be very difficult to explain in any other way besides a black hole, and if they are verified, then the hypothesis that some galaxies contain supermassive black holes at their centers would be fairly secure (SRDS 3). These discoveries have helped to justify the existence of black holes for astronomists, forging a path for deeper thought about what is really going on within these mysterious beings. While black holes are space objects that pull in all things so that they can never get out, their opposite is a white hole, which allows things to come out of but not fall into. The equations of general relativity have the mathematical property that they’re symmetric in time. In other words, any solution to the equations can be taken reversed so that time flows backwards rather than forwards. If you apply this rule to the solution that describes black holes, you get an object known as a white hole. Since a black hole is a region of space from which nothing can escape, the time-reversed version of a black hole is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only spit things out. White holes are a perfectly valid mathematical solution to the equations of general relativity, but that doesn't mean that they actually exist in nature. In fact, they almost certainly do not exist, since there's no way to produce one. Producing a white hole is just as impossible as destroying a black hole, since the two processes are time-reversals of each other. The basic idea is that white holes cannot exist, since they violate the second law of thermodynamics. The idea of worm holes comes into existence if we consider black holes that rotate and/or have charge. The idea is that it is possible to fall into such a black hole and not hit the singularity. In effect, the interior of a charged or rotating black hole can "join up" with a corresponding white hole in such a way that you can fall into the black hole and pop out of the white hole. This combination of black and white holes is called a wormhole. To fully realize a wormhole, it must be determined the properties of matter required to form the throat of the wormhole (Pickover 128). The tension of matter that would be needed to keep a wormhole open would be extremely large, for example a throat that is four miles across, the quantity of force needed is 10 to the 33rd power pounds per square inch (Pickover 128). This amount of force is highly unreasonable, yet larger wormholes with wider throats would supposedly have more reasonable values. Still, such needed values produced a difficult situation to overcome in the attempt to justify wormholes. Furthermore, the tension needed to keep the wormhole open must be ten to the 17th power greater than the density of the substance used to build the wormhole (Pickover). At this point in time, scientists have yet to find any existence of matter in the universe that has breakings tensions so much larger than its density. Based on such mathematical solutions, it has been deduced that wormholes, like white holes, almost certainly do not exist. As mentioned above in the section on white holes, just because something is a valid mathematical solution to an equation doesn't mean that it actually exists in nature. In particular, black holes that form from the collapse of ordinary matter (which includes all of the black holes that we think exist) do not form wormholes. If anything does in fact fall into a black hole, it doesn’t come out anywhere, but rather hits the black hole’s singularity. Furthermore, even if a wormhole were formed, it is thought that it would not be stable. Even the slightest perturbation (including the perturbation caused by an attempt at anything to travel through it) would cause it to collapse. Finally, even if did wormholes exist and were stable, anything that passed through would be fried by the radiation that pours into the wormhole (from nearby stars, the cosmic microwave background, etc.). This radiation would enter the worm hole and get blue shifted to very high frequencies so that anything tried to pass through the wormhole would get fried by these X-rays and gamma rays. However, since we know that black holes do exist, many have tried to imagine if there are variations of the basic idea of a black hole. The existence of another kind of nonstellar black hole has been proposed by the British astrophysicist Stephen Hawking. According to Hawking's theory, numerous tiny primordial black holes, possibly with a mass equal to that of an asteroid or less, might have been created during the big bang, a state of extremely high temperatures and density in which the universe is thought to have originated roughly 10 billion years ago (Hawking 161). These so-called mini black holes, unlike the more massive variety, lose mass over time and disappear. The idea is that subatomic particles such as protons and their antiparticles (i.e., antiprotons) may be created very near a mini black hole. If a proton and an antiproton escape its gravitational attraction, they annihilate each other and in so doing generate energy that they in effect drain from the black hole. The way in which it is thought that such radiation can escape from the field of a black hole is that according to the uncertainty principle, particles can travel faster than light for a short distance, enabling both particles and radiation to get out through the event horizon and escape from a black hole (Hawking 121). However, the particles that have emerged from the black hole will be quite different from what fell in. Only the energy would be the same (Hawking 121). If this process is repeated again and again, the black hole evaporates, having lost all of its energy and thereby its mass, since these are equivalent. Hawking goes on to propose that if such disappearing black holes did exist, everything that had fallen into the black hole before it evaporated would go off into a little baby universe of its own (Hawking 121). The idea is that a small, self-contained universe would branch off from our region of the universe. This baby universe may join on again to our region of space-time and if it did, it would appear to us to be another black hole that formed and evaporated (Hawking 121). However, the baby universes proposed by Hawking that take the particles that fell into them occur only in what is called imaginary time (Hawking 121). Particles can only continue on through the black holes without hitting their singularities in imaginary time. Although the idea of baby universes is not feasible at this point, they do have important implications for our attempt to find a complete unified theory that will describe everything in the universe (Hawking 123). Another very important aspect of black holes that coincides with the idea of space and light being bent is the idea of time dilation. By definition, time dilation is an effect caused by gravity or the curvature of spacetime, or by acceleration, in which the rate at which time passes in one location will be measured differently by a clock right there and another clock somewhere else (Ferguson 132). As a black hole is approached, the extreme gravitational pull slows time down until the event horizon is reached at which point time, in theory, completely stops. Therefore, an observer near a black hole ages more slowly than one farther from the black hole. Also, the proper time of a clock on the surface of a collapsing star is different from the apparent time of the collapse, measured by a distant observer (Pickover 43). While an object approaching a black hole would appear permanently frozen at the event horizon, as its image gradually faded, in actuality the object would pierce the event horizon and plunge toward the singularity (Pickover 43). At this point in time, astronomers using NASA's Rossi X-ray Timing Explorer (RXTE) spacecraft reported today that they have observed a black hole that is literally dragging space and time around itself as it rotates. This bizarre effect, called "frame dragging," is the first evidence to support a prediction made in 1918 using Einstein's theory of relativity (internet 5). So in effect, the dilation of time has in fact been observed and gravitational time dilation is thought to be quite real. Though there is still much to learn about black holes, scientists and astronomers have made great advances in helping to decipher the secrets of black holes. Though a black hole has never actually been seen, its presence can be deduced from the extreme impact it has on the area of space around it. And while astronomers would like to believe that one day white holes and worm holes will in fact be proven to exist, at this point in time they are merely the opposing mathematical answers to the world of black holes, looking feasible on paper but in reality only existing within the minds of those who chose to contemplate them. While the scientific, or rather mathematical, answers to black holes and how they exist have been found, it is interesting to imagine what exactly it would be like to travel inside a black hole. Would it be a doorway to a parallel universe? Or would it crush whatever entered it into nothingness, accepting the remnants of its explorer as a piece of its dark, foreboding existence? While black holes may sound frightening and unrelenting at first, they are in fact a source of infinite mystery and even beauty. Black holes are essentially places that have disappeared from the realm of our universe, and within their hidden boundaries, time and space become magically intertwined.

BIBLIOGRAPHY

Ferguson, Kitty. Black holes in Spacetime. New York: Venture Books Inc. 1991

Hawking, Stephen. Black holes and baby universes and other essays. New York: Bantam Books. 1993

Pickover, Clifford A. Black Holes: A Travelers Guide. New York: John Wiley & Sons, Inc. 1996

Filkin, David. Stephen Hawking’s Universe. New York. Basic Books. 1997