Black Holes Astronomy Encompasses Vast

Black Holes

Astronomy encompasses vast topics and includes many subjects. Among these subjects is the area of study involving black holes. Although there has been a great deal of research concerning this subject, there are still many factors concerning black holes that are still unknown. The purpose of this disccusion is to explain the phenomenon of black holes and how the theory of the black hole evolved. Let us begin this discussion by providing a definition for black holes.

What are black holes?

Hawkins (1998) explains that Black holes are theoretical articles that symbolize the eventual state of collapsed matter. The author explains that In principle they may be of any mass, but there are certain mass ranges where we believe conditions can easily arise for them to form. For example, dead stars with more than four times the mass of the Sun no longer have the energy to resist the power of their own gravity to crush them almost out of existence, into the secluded annihilation of a singularity. This is the infinitely dense and infinitely small state of being where atoms are decimated into their primordial parts and cannot interact with any form of radiation (Hawkins, 1998)."

The theory also asserts that not even light can escape the gravitational pull created by black holes. Black holes can also be thought of as compact bodies that are not atomic. In any case the mass of a black hole is one aspect of the totality of what was created as a result of atomic material that was a part of the early universe. However the idea that this material reverted back to a state that is nonatomic is not relevant as it relates to discovering dark matter (Hawkins, 1998).

In any case, the theory of black holes is usually expressed as it relates to Einstein's theory of general relativity (Hawkins, 1998). This theory asserts that gravity is almost identical to the shape of space. In addition this theory purports that gravity is not only an attractive force but also relates to the slopes and curves of space. In other words, gravity is responsible for the manner in which massive bodies are able to distort space (Hawkins, 1998). For example, a heavy object such as a ball would stretch and curve the surface of a rubber sheet and as a result cause less weighty balls to move toward the heavier ball (Hawkins, 1998). A black hole is similar to this in that it is the place where "space is so curved in upon itself that even light and other carriers of information cannot escape, but keep going round and round within the black hole's "event horizon" thinking that they are traveling in a straight line. Like a bottomless whirlpool, a black hole can draw in material and information from the rest of the Universe, but it gives back nothing (Hawkins, 1998)."

This means that the entire Universe is finite and similar to a black hole because it is restricted by its own event horizon, within which light can travel in a straight line. This is true whether or not the light curves back upon itself (Hawkins, 1998). A straight line is defined as the course that light travels. In this kind of universe, straight lines that are parallel will meet in the way that lines of longitude meet at the poles (Hawkins, 1998). From this detached perspective the overall form of the Universe is analogous to the surface of a sphere. As such, the value of ? would be more than 1.

With these things being understood, the universe would inevitably collapse under its own weight into a spectacle because eventually it would no longer possess the energy to resist the strength of its own gravity (Hawkins, 1998). The author points out that a closed universe has often been presented as a snake swallowing its own tail. The geometry of such a universe is also identical to that of the surface of a sphere, as the sum of the angles of a triangle is more than 180 degrees (Hawkins, 1998). In addition within an open universe, the typical density is not more than the critical density. In fact it is believed to be saddle shaped and as such the sum of the angles of a triangle will be less than 180 degrees (Hawkins, 1998). At this point parallel straight lines associated with the path light travels will become progressively more remote as it relates to their capacity to communicate with each other in perpetuity (Hawkins, 1998).

As it relates to how large a black hole is, Bunn (1995) asserts there is not a restriction in principle as it relates to how massive a black hole can be. Therefore, any amount of mass can develop into a black hole if it is packed together at a high enough density. Many researchers believe that the majority of black holes were produced be the collapse of the massive stars, and as a result those black holes may weigh the same amount as a massive star. The author points out that "A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. (Here I'm using scientific notation: 10^{31} means a 1 with 31 zeroes after it, or 10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million times as much as the Sun, or 10^{36} kilograms (Bunn 1995)."

According to the research the idea or concept of the black hole is a rather simple one. The black hole is simply occurs when matter collapses and not longer has the energy to fight against the gravitational pull. In addition the pull created by the black hole is so strong that not even light can escape it. Now that we have garnered a greater understanding of the definition of a black hole let us concentrate upon the history of this theory.

History of Black holes

According to Hawley and Holcomb (1998) from a historical standpoint the black hole is viewed as an extreme outcome of the Einstein's theory concerning general relativity. However, the theory of the black hole can even be explained if one considers Newton's theory of gravity. This gravitational theory asserts that any planet or star has at its surface gravitational acceleration (Hawley and Holcomb 1998). In order to escape from the planet or the star must have a velocity that is significant enough to rise above the gravitational pull. This type of velocity does indeed exist and it is referred to as escape velocity (Hawley and Holcomb 1998).

On the planet earth, escape velocity is 11 kilometers per second. If this escape velocity was the same as the speed of light and there was a star with that significant an escape velocity light, would not be able to depart from the surface of that star; the star would be dark (Hawley and Holcomb 1998). In addition any light that exist on the surface of this star could ascend, but similar to a ball tossed into the air, the light would ultimately turn around and fall back down (Hawley and Holcomb 1998). When this theory was first asserted astronomers did not yet know that the speed of light in vacuo is the final speed limit, but with that additional knowledge, it can be surmised that nothing could escape from this type of star (Hawley and Holcomb 1998).

Hawley and Holcomb (1998) also explain that the theory of the general relativistic black hole formed after the manifestation of the finished version of Einstein's general theory of relativity in the year 1916 (Hawley and Holcomb 1998). The authors further explain that Despite the great complexity of the Einstein equations, the German astronomer Karl Schwarzschild found one of the first solutions, almost immediately after Einstein published his results. Schwarzschild assumed a perfectly spherical, stationary ball of mass M, surrounded by a vacuum, that is, empty space. This is not a bad approximation to a star; the Sun, at least, rotates slowly and is very close to spherical, and, as far as we know, the Sun is a typical star. Schwarzschild then solved Einstein's equations to compute the spacetime curvature in the exterior of the star. Such a solution consists of a specification of the geometry of spacetime; this description can be encapsulated in the metric coefficients (Hawley and Holcomb 1998)."

The conjecture of Schwarzschild simplified the mathematics required to explain the theory of the black hole. First, he solved for the gravity in a vacuum outside the mass. This allowed him to set the stress-energy term T?

in Einstein's equation equivalent to zero and focus only on the geometry term (Hawley and Holcomb 1998). Because Schwarzschild was investigating the space surrounding a spherical mass, he used spherical spatial coordinates, composed of a distance R. from the midpoint of the mass, in addition to the inclination from the starting point expressed in terms of two angles, including altitude ? And azimuth ? (Hawley and Holcomb 1998).

The author further explains that the gravity coming from this type of star has to be spherically symmetric. This means that the star should only depend on the distance from the star (Hawley and Holcomb 1998). As a result of this fact, it was not possible to disregard the angular terms (Hawley and Holcomb 1998). Lastly, the star along with its gravitational field do not change with time, this means that the metric terms are independent of time, but only when the time coordinate is chosen correctly (Hawley and Holcomb 1998). The time coordinate Schwarzschild used was a rational one because it can be correlated to the time calculated by an observer who was a significant distance away from the central mass, where gravity's effects ebb down to zero (Hawley and Holcomb 1998).

The authors point out that Schwarzschild radius is the identical to the radius used for the Newtonian dark star (Hawley and Holcomb 1998). However because Newtonian gravitation is suitable as it relates to a good approximation, it should be anticipated that the radius would not be too far from its prediction (Hawley and Holcomb 1998). However the black hole is a much more interesting and foreign theory than is the dark star, and as such reflecting upon the black hole as if it were a Newtonian dark star may cause a misunderstanding of the important aspects of the black hole (Hawley and Holcomb 1998).

With these things being understood, why is every round object not considered a black hole? The answer to this has to do with the fact that the Schwarzschild radius can be found in the outer surface of any "normal" object, including a neutron star. For instance, the Schwarzschild radius of the Sun is 3 kilometers; on the other hand the solar radius is more than 1 million kilometers. In addition the Schwarzschild radius of the Earth is below 1 centimeter. Again, the solution presented by Schwarzschild relates only to the empty space contained in the exterior of the sphere (Hawley and Holcomb 1998). This means that if the Schwarzschild radius is less than the radius of the body, it is immaterial inside the body (Hawley and Holcomb 1998). The author also asserts that the metric contained inside of a star is not consistent with a Schwarzschild metric, but is instead a different metric that incorporates the existence of the matter which produces the gravitational field (Hawley and Holcomb 1998). A black hole can only be formed if the object has totally collapsed and vanished beneath its Schwarzschild radius (Hawley and Holcomb 1998).

The authors further explain that at the Schwarzschild radius, the coefficient of the time interval in the Schwarzschild metric is zero (Hawley and Holcomb 1998). As a result, the time interval itself becomes infinite (Hawley and Holcomb 1998). Likewise, radial intervals decrease to zero, which is the definitive length contraction. These effects occur as a result of the choice of coordinates, and these coordinates are not ever absolute even as it relates to Newtonian physics. Nonetheless, the length contraction, time dilation, and other relativistic effects that are dependent upon the metric coefficients, are actual physical occurrences and can be calculated with the right type of instruments (Hawley and Holcomb 1998). In addition the gravitational field in the vicinity of the black hole is more significant at small radius than it is when it is at some distance away, and as a result light moving from near the object endures a gravitational redshift (Hawley and Holcomb 1998).

As it relates to the black hole, any light sent from the Schwarzschild radius is perpetually redshifted (Hawley and Holcomb 1998). As a result the sphere that is derivative of the Schwarzschild radius is reflective of a surface from which light is not able travel to an outside observer. In addition an observer from outside this horizon can not see within the horizon because the inside of the black hole is infinitely unable to communicate with the rest of the universe (Hawley and Holcomb 1998). Likewise the Events that take place inside the black hole can have no contributory contact with events outside the black hole (Hawley and Holcomb 1998). This limit that exists between the inside and the outside of a black hole is referred to as an event horizon (Hawley and Holcomb 1998). The event horizon can be described as the point of no return, at this point no can ever escape the black hole. The Schwarzschild radius is responsible for identifying the event horizon of the black hole (Hawley and Holcomb 1998). The authors further insist,

From outside a black hole, the event horizon seems to be a special location. What would happen if an advanced civilization were to launch a probe toward a black hole? To the observers watching from a safe, far distance, the infalling probe's clock slows down; radio signals from the probe come at increasingly longer wavelengths due to the gravitational redshift. The probe approaches closer and closer to the horizon, but the distant observers never see it cross over into the hole. Time seems to come to a halt for the probe, and the redshift of its radio beacon goes to infinity, as measured by the faraway astronomers. At some point the last, highly redshifted signal from the probe is heard, and then nothing more. The probe disappears forever (Hawley and Holcomb 1998)."