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General Theory of Relativity Essay

The universe consists of many fascinating objects. Unfortunately, some of which cannot be known with just the naked eye. This is because it cannot be seen from Earth, and other technological advancements are needed to fully determine its nature. This is also the reason why several people have acquired false notions of what those objects are. One of those objects is the black hole. Science fiction films and and television shows have given the public a different view of black holes (Lochner 2006), poles apart from what they really are. This research paper seeks to discuss the definition and characteristics of black holes.

Before the characteristics of a black hole can be discussed, it is first necessary to define what it is. An introduction is in order. So what is black hole? According to Lochner (2006), it is the “evolutionary end point of massive stars (p. 2). ” There are stars which has sizes much bigger than that of the Sun. In fact, some of them are “at least 10 to 15 times as massive as the Sun (Lochner, 2006, p. 2). ” When a star of this size sustains a supernova explosion, it leaves behind some stellar residue. Since in space there are no other forces to fight gravity, the residue will crumble unto itself.

Lochner (2006) narrates, “The star eventually collapses to the point of zero volume and infinite density, creating what is known as a ‘singularity’ (p. 2). ” Eventually, density will increase; the light rays which is diffused by the star will then be encircling the star. “Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it (Lochner, 2006, p. 2). ” The moment the star reaches the point of no density, all the light is trapped. Hence, it is called the black hole. The phrase ‘black hole’ was coined by John Archibald Wheeler (Bunn, 1995).

Prior to that, the objects were called frozen stars (Bunn, 1995). Even in the time of Isaac Newton, the existence of objects as such has been thought of. Now it is more accurately explained using Einstein’s General Theory of Relativity. This is “a geometric theory of gravitation, which incorporates and extends the theory of special relativity to accelerated frames of reference and introducing the principle that gravitational and inertial forces are equivalent (Lochner, 2006, p. 1). ” This theory then explains the possibility for such occurrences like bent light caused by massive objects and the very nature of black holes.

Such theory enables the event wherein “space and time become so warped that time practically stops in the vicinity of a black hole (Lochner, 2006, p. 1). ” Black holes are identified to have an estimated mass of 4-15 Suns. Since black holes are formed after the death of a star, or supernova explosions, these explosions in turn have after-effects. These effects give way to X-ray binaries which is referred to as black hole candidates (Lochner, 2006, p. 1). Moreover, there exists galaxy-mass black holes. These massive black holes are usually placed in Active Galactic Nuclei, or AGN.

AGN is a “class of galaxies which spew massive amounts of energy from their centers, far more than ordinary galaxies (Lochner, 2006, p. 1). ” The black holes in question are said to have a mass of 10-100 billion Suns (Lochner, 2006, p. 1), a testament to how enormous they really are. The mass of one of these enormous black holes was recently determined by radio astronomy (Lochner, 2006, p. 1). The black hole has been portrayed by pop culture as one which sucks objects in; Lochner (2006) refers to the black hole’s inaccurate depiction as “a cosmic vacuum cleaner (p. 2).

” He notes, “If our Sun was suddenly replaced with a black hole of the same mass, the earth’s orbit around the Sun would be unchanged… Of course the Earth’s temperature would change, and there would be no solar wind or solar magnetic storms affecting us (Lochner, 2006, p. 2). ” This is because a black hole can “exert the same force on something far away from it as any other object of the same mass would (Lochner, 2006, p. 1). ” If in any instance, an object gets “sucked” into the hole, that very same object will pass through what is called “Schwarzschild radius (Lochner, 2006, p. 2).

” “This is the radius r of the event horizon for a Schwarzschild black hole (Lochner, 2006, p. 2). ” In the Schwarzschild radius, the escape speed and light speed is equal. Therefore, in the instance that light passes through, the light would not be able to escape. Say for example, a black hole with the same mass as the Sun, takes its place. Then the radius will still be different. The Sun’s radius is estimated at 700,000 km, while the Schwarzschild radius is only 3km (Lochner, 2006, p. 2). This would entail that the Earth must be of closer proximity to get sucked in a black hole in the center of our solar system.

From where we are it is impossible to see the black holes. How do we then determine their existence? Since black holes are merely massive star remains, it would mean that they are of a small size. Also, because all the light gets trapped into itself, it would be impossible to see. Nonetheless, there are instances wherein a black hole can make itself perceivable. According to Lochner (2006), “if a black hole passes through a cloud of interstellar matter, or is close to another “normal” star, the black hole can accrete matter into itself.

As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms and when the atoms reach a few million degrees Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission (p. 1). ” Thus, black holes are dependent on other stars to make its presence known. The very same X-rays are also determinants of “black hole candidates (Lochner, 2006, p. 2).

” It was said that a “companion star is a perfect source of infalling material for a black hole (Lochner, 2006, p. 2). ” Because the X-ray sources are binary, a binary system is also enforced. This system makes the computation of the black hole candidate’s mass possible. The moment the mass is calculated, it can be deduced whether the said candidate is a black hole or a neuron star. What is a neuron star? It is “the imploded core of a massive star produced by a supernova explosion (Lochner, 2006, p. 2). ” Now these neuron stars are characterized by masses which are estimated to be 1.

5 times more than the sun. Moreover, if there exists random variation of emitted X-rays, this is also a signifier of a black hole’s presence. Lochner (2006) states, “The infalling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity(p. 2). ” In addition, “if the X-ray source is in a binary system, the X-rays will be periodically cut off as the source is eclipsed by the companion star (Lochner, 2006, p. 2). ” All these characteristics are considered in identifying possible black hole candidates.

For further identification, there are X-ray satellites which examines the skies for X-ray sources that may point out black hole candidates. For the longest time, there has been an identified black hole candidate in the name of Cygnus X-1 (Lochner, 2006, p. 2). “It is a highly variable and irregular source with X-ray emission that flickers in hundredths of a second (Lochner, 2006, p. 2). ” When one exhibits such an irregularity, it becomes a black hole candidate. How? It is because it is impossible for an object “to flicker faster than the time required for light to travel across the object (Lochner, 2006, p.

2). ” Lochner (2006) highlights this fact: “In a hundredth of a second, light travels 3000 kilometers. This is one fourth of Earth’s diameter (p. 2)! ” From this, it can be concluded that the region from which the x-rays surrounding Cygnus X-1 are derived, is relatively small. Now Cygnus X-1 has a companion star with the name HDE 226868. This companion star is “a B0 supergiant with a surface temperature of about 31,000 K (Lochner, 2006, p. 2). ” Now observations found that the spectral lines of HDE 226868, which is the “light given off at a specific frequency by an atom or molecule (Lochner, 2006, p.

2),” had been changing within 5. 6 days. It was also said that the mass of HDE 226868 is estimated to be 30 times greater than the Sun’s mass. This would mean that Cygnus X-1may possess at least a mass of 7 solar masses. Why 7 solar masses? This is what is required to create the tremendous gravitational pull that would result in the fluctuation in the spectral lines of HDE 226868. Astronomers thought that since 7 masses does not characterize a neuron star or a white dwarf, which is a star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size, it must then be a black hole.

However, this issue about Cygnus X-1being a black hole has also been surrounded by much skepticism. There is some speculation that the HDE 226868 may be too small for its spectral category, which in turn implies that Cygnus X-1is smaller than was previously declared. Moreover, uncertainty also shrouds the the mass calculations. It is because “uncertainties in the distance to the binary system would also influence mass calculations (Lochner, 2006, p. 2). ” If the computations are inaccurate, the Cygnus X-1may end up only having 3 solar masses.

If Cygnus X-1 has only 3 solar masses, it could be classified as merely being a neuron star, and not a black hole. The good news is that there are more binaries which reveal the possibility of a black hole, that which is much stronger than in Cygnus X-1 (Lochner, 2006, p. 2). In the year 1975, an X-ray transient known as A0620-00 was discovered. In the mid-80s, it was found that the mass of this object was more than 3. 5 solar masses. This fact alone eradicates the possibility of a neuron star, since neuron stars usually possess solar masses of 1. 5.

In fact, the discovery of A0620-00 may have put into question the feasibility of other theories. Nonetheless, the best finding regarding black holes is V404 Cygni. This star was found to have an estimated 10 solar masses. Several journals have also written about the existence of black holes. In the 1995 edition of “Annual Reviews of Astronomy and Astrophysics,”there was a review conducted by Kormendy and Richstone, which implied the eight galaxies were thought to have “massive dark objects in their centers (Bunn, 1995, p. 1). ” These cores were found to have masses which range from 1 million to several billion times that of the sun.

Their massiveness was determined by noting how “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 (Bunn, 1995, p. 1). ” In fact, this is how astronomy usually measures masses. There are two reasons why these massive galactic centers were deemed as black holes. To begin with, the centers are “too dense and dark (Bunn, 1995, p. 1)” to even be considered as a group of stars, or just merely stars.

“Second, 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 (Bunn, 1995, p. 1). ” Even though these reasons point out that the galactic centers are really black holes, there is no sufficient evidence to prove it. Nonetheless, there is a continuous discovery for proofs that systems do include black holes. According to Bunn (1995), “a nearby active galaxy was found to have a “water maser” system (a very powerful source of microwave radiation) near its nucleus.

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 (p. 1). ” They also determined that the velocity was “less than half a light-year of the center of the galaxy (Bunn, 1995, p. 1). ” It is from this fact that they deemed the object as a black hole, simply because only a black hole can have that much mass concentrated in such a small volume (Bunn, 1995, p. 1). All these results are included in January 12, 1995 issue of Nature, vol. 373. , as was reported by Miyoshi et al (Bunn, 1995, p.

1). Is there a possibility that the Sun can be a black hole? No. According to Bunn (1995), “only stars that weigh considerably more than the Sun end their lives as black holes (p. 1). ” For about five billion years, the Sun will remain in its present state. After that, the Sun will undergo a phase wherein it will be a red giant star. The Sun will then end its life as a white dwarf star. If there are black holes, are there white ones? According to Bunn (1995), “the equations of general relativity have an interesting mathematical property: they are symmetric in time.

That means that you can take any solution to the equations and imagine that time flows backwards rather than forwards, and you’ll get another valid solution to the equations. If you apply this rule to the solution that describes black holes, you get an object known as a white hole (p. 1). ” If a black hole then pulls objects in, a white hole would then push things out. The former sucks in, the latter spits out. However, there is no proof that white holes exist, and there are no studies to point out if their existence is a possibility. There has also been speculations about the existence of what is called worm holes.

What is a worm hole? It is “a theoretical opening in space-time that one could use to travel to far away places very quickly (Lochner, 2006, p. 2). ” It is characterized by “two copies of the black hole geometry connected by a throat – the throat, or passageway, is called an Einstein-Rosen bridge (Lochner, 2006, p. 2). ” As is indicated in the definition, it is merely theoretical. There is no scientific basis nor experimental evidence for such existence. However, it is indeed amazing to think that such existence is possible. Do black holes disappear or evaporate? Even astronomers are not sure as to how black holes end their existence.

Bunn (1995) notes that “Back in the 1970’s, Stephen Hawking came up with theoretical arguments showing that black holes are not really entirely black: due to quantum-mechanical effects, they emit radiation. The energy that produces the radiation comes from the mass of the black hole. Consequently, the black hole gradually shrinks. It turns out that the rate of radiation increases as the mass decreases, so the black hole continues to radiate more and more intensely and to shrink more and more rapidly until it presumably vanishes entirely (p. 1)” This is a mere theory.

There have been no proof or scientific conclusions as to how black holes really diminish. Black holes are just one of the many things included in the vast universe we are a part of. Thanks to science and the technological advancements at present, we can have greater awareness and knowledge of what is within our universe but beyond our reach. References Bunn, T. (1995). Black Hole FAQ List. Retrieved December 13, 2007, from http://cosmology. berkeley. edu/Education/BHfaq. html. Lochner, J. (2007). Black Holes. Retrieved December 13, 2007, from http://imagine. gsfc. nasa. gov/docs/science/know_l2/black_holes. html.

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