wOOt, I have this to say...
Black holes are extremely dense stellar phenomena with an intense gravitational pull ¡V a singularity in space time. The escape velocity is dependant on gravity, and in the case of a black hole it is greater than the speed of light ¡V if you were to fall into a black hole, light is bent to such an extent you would be able to see the back of your own head. This makes black holes extremely hard to detect, or prove their existence, however there is evidence that suggests they occur.
They are formed by collapsed stars, varying in size from many kilometres to 10-33 m across, and consist primarily of two parts; the event horizon (Schwarzschild radius) and the singularity.
Formation Of A Black Hole
Black holes are a product of the collapse of a massive star; to understand this it is useful to know about how stars form and the processes which occur within.
Interstellar material is evenly distributed in space with random concentrations; due to gravity alone, or a trigger such as the blast wave from a supernova, this matter can start to compress. This process continues due to gravity resulting in a protostar of predominantly hydrogen molecules. As compression continues and temperature rises these molecules disintegrate into atoms (atomisation) and then ions (the protons and electrons separate) ¡V forming plasma. With further raises in temperature any Helium ionises (leaving the star at 1/400 its size as a cloud) and nuclear fusion becomes possible.
Fusion occurs when two (or more) nuclei combine to make a larger one. The energy as a result of this process is calculated by the product of the difference in mass and the speed of light squared (E=mc2). The type of fusion present in a star is determined by its mass ¡V in small stars the only possible type is hydrogen to helium (proton-proton fusion), in medium size stars, late in their lives, they convert helium into oxygen and carbon (carbon cycle fusion). In massive stars when hydrogen becomes depleted, they can convert helium atoms into carbon and oxygen, followed by the fusion of carbon and oxygen into neon, sodium, magnesium, sulphur and silicon (triple-alpha process). These reactions can continue and form calcium, iron, nickel, chromium, and copper, and at supernovae they create heavy elements (above the mass of iron).
For example, the simplest form of fusion (that of the sun), is believed to occur in steps. The combination of four protons produces one atom, two positrons, and two neutrinos, as shown below:
When a star is formed, the expanding force of the fusion is counteracted by gravity, leading to stability. There are several types of stars as shown in the Hertzsprung-Russell Diagram.
The main sequence stars; where the luminosity is proportional to the heat, form the majority of stellar material in the universe.
As stars age, they gets cooler, moving towards the bottom right of the Hertzsprung-Russell Diagram. They can then form giants once their hydrogen supply has been depleted. The core then contracts as the outer layers expand to 100 times their original size, while cooling to under 6,500 K (in the case of a Red Giant, for example Betelgeuse).
If the temperature does not fall the star forms a Blue Giant, and starts burning Helium. When these eventually cool they form Supergiant¡¦s, which can be almost as large as our entire solar system. All such stars will eventually explode to become a planetary nebulae or supernovae (when a star loses its inner balance and explodes) and then become white dwarfs, neutron stars, or black holes(depending on their mass).
On the other end of the scale are the faint, virtually dead stars. The White Dwarf is an example of such a star; small, very dense and hot, and principally consisting of carbon. They are the remains of a red giant after the loss of its outer layers. These will eventually cool and become a cold, dark black dwarf. In a Neutron Star (a very small, super-dense star composed predominantly of tightly-packed neutrons), the gravitational pull is so high the electrons are pulled into the nucleus where they combine with protons to form neutrons. There are other types of less common stars such as the Brown Dwarf (a "star" in which nuclear fusion does not occur due to insufficient mass), Pulsar (a rapidly spinning neutron star that emits energy in pulses) and many different types of Binary Star (a system of two stars orbiting a common centre of mass).
If a star is above a critical mass (in general a neutron star would be the only type to have one high enough), it can form a black hole. Initially the gravity of the huge mass is balanced by the opposing forces of nuclear fusion, but as the elements enabling such fusion are exhausted the star cannot support itself any more, collapsing to form a black hole. This can happen in two ways; below the critical mass the ¡§Strong Force¡¨ (that holds atomic nuclei together) can stop the implosion abruptly, leading to a supernova. If this happens and the energy is too low to completely eradicate the stellar envelope, a significant part of the star can fall back and form a black hole. However, if the star is over the critical mass (two to three times that of the sun) it will collapse to form a black hole directly. The entire process takes between a tenth and half a second. The second possible cause is a neutron star accreting too much from, or merging with a companion star (in a binary system) and it mass becoming too high making it unstable. The star again collapses to form a black hole, and while the accretion can take millions of years, the actual collapse takes less than a second.
Upon formation an "apparent" event horizon forms which separates light rays that are trapped inside a black hole from those that can move away from it. However, some light rays that are moving away at a given instant of time may find themselves trapped later if more matter or energy falls into the black hole, increasing its gravitational pull. The event horizon is traced out by "critical" light rays that will never escape or fall in.
The Anatomy Of A Black Hole
If you were falling into a black hole the first thing you would encounter is the event horizon, located at the Schwarzschild radius (named after Karl Schwarzschild who found a solution for the equations of relativity in empty space). As it¡¦s the point at which the star collapsed into a black hole it is proportional to the stars mass ¡V
for a black hole whose mass equals 10 suns, this radius is about 30 kilometres. Behind the event horizon, the inward pull of gravity is overwhelming and no information about the black hole's interior can escape to the outer universe.
We do not know what occurs or exists between the event horizon and the centre as it is impossible to find out. The singularity lies at the centre of the black hole, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and spacetime has infinite curvature. At the singularity, space and time cease to exist in their usual form, so talking about them in relation to a singularity is pointless, as is talking about the conventional laws of physics. Hence laws based on quantum gravity have been suggested for this area where cause and effect cannot be unravelled.
Physics Of Black Holes
At the event horizon, ¡§Hawking Radiation¡¨ is emitted. Vacuum fluctuations, pairs of virtual particles that occur together at some point in space time, can move apart, and then come back together and annihilate each other. Neither can be directly observed, but indirect effects can be detected. If this happens near a black hole, one member of a pair of particles could fall into it leaving the other to escape to the outside world. From our frame of reference, it would appear this had been emitted by the black hole. The radiation from a black hole is that which we would expect from a hot body ¡V the temperature would depend on the gravitational field at the horizon, i.e. its size, the smaller it is the hotter it would appear. As a black hole of three times the mass of the sun would have a temperature of around a millionth of a degree (a larger black hole¡¦s temperature would be even smaller) it is very hard to detect this against the universe¡¦s 2.7K temperature remaining from the big bang.
A problem with the theory of black holes is the fact that light shouldn¡¦t have a mass. It has been proven experimentally that the nearer an object with mass gets to the speed of light the more massive it gets, by accelerating electrons to near the speed of light. Therefore at the speed of light an object would have infinite mass and would need infinite energy to get it there. This suggests that photons have no mass, and should therefore not be affected by gravity. However mass and energy are the same thing according to the theory of relativity, so how does light travel at the speed of light? The mass of a particle which travels with a velocity v is:
(m0 = rest mass, v = velocity, c = speed of light)
The equation above shows if something is travelling at the speed of light, the lower half of the equation will equal zero (v2/c2=1) and the equation will become undefined, suggesting this equation applies only to sub-light speed particles.
Relativity says that every particle with mass m is equal to an energy E given by the equation:
E=mc2
This equation suggests an amount of energy (E) has a mass of E/c2 ¡V very small in the case of a photon. Photons don¡¦t have a rest mass, but have energy, given by the equation:
E=hf
(h = Planck's constant, f = frequency)
Upon the combination of the two equations the following formula gives us the mass of a photon :
mc2 = hf ¡÷ m=
Therefore photons must have a mass, but no rest mass top travel at c. The fact they have energy necessitates a mass ¡V despite the fact they are wave particles. The duality of the nature of light (and of every particle) means that while a wave, light can also be have like a particle to a small extent - they have mass and frequency.
Another interesting feature of black holes is that when matter falls in such a gravitational field, its speed (and therefore) energy, increases. If a large amount of matter is falling at the same time, swirling around the black hole in a disk, friction between the various pieces of matter will turn much of the kinetic energy is has gained into heat, which than gets radiated away. This way, the matter surrounding a supermassive black hole can radiate more energy per gram of fuel than can be released by any other mechanism, including nuclear fusion ¡V a possible energy source for the future. Such energy, while hard to harness, would be free, efficient and long lasting.
Evidence For The Existence Of Black Holes
As I have discussed earlier, it is very hard to detect the presence of black holes themselves, even with the presence of Hawking radiation. It is however possible to detect their effect on surrounding stars - astronomers have found half a dozen binary star systems where one of the stars is invisible, yet its gravitational effect on the other is enough to make that star orbit around their common centre of gravity (and the mass of the invisible star is considerably greater than 5 solar masses).
Another suggestion is that supermassive black holes (about 1 billion solar masses) form the centres of many galaxies. This could lead to the eventual destruction of many galaxies if they are pulled into the black hole, which is an important consideration when researching them. A further possibility for detection is a gas, such as that from a nearby star, falling into a black hole would heat up and glow. Not only visible light, but more energetic photons like X-rays could be emitted and subsequently detected.
A third method to find black holes is to try and locate small but massive objects such as the astronomical object called M87, which weighs three billion times more than our sun, but takes up a volume no larger than our solar system. A further means of detection of black holes is to look for an acceleration of matter; anything in the proximity of a black hole would accelerate towards it, an occurrence which could be observed by looking for Doppler shifts in the light given off by an accelerating object.
These are the only practical ways in which a black hole could be detected, and there is no conclusive proof they exist, so much so that some people doubt they do. A good example is the author of the following website:
http://www.perceptions.couk.com/uef/nblckhls.html
I chose, however, not to believe this source for several reasons. Primarily the author is in an overwhelming minority, and while no conclusive proof exists for the existence of black holes, theoretically their existence is almost a certainty, and we do have some evidence for their existence. Whoever wrote the above website never adequately substantiates what he writes (such as explaining the UEF theory), in-fact often what he says doesn¡¦t make any sense at all and at no point does he give believable proof for the lack of existence of black holes. Any other sources I have used I have deemed to be reliable, such as Stephen Hawking¡¦s ¡V The Universe In A Nutshell, which I trust being a recognised and published work, or with websites I am less sure about I have only used information I have seen replicated in at least two different sources to try and ensure it is correct.
Therefore, while no conclusive proof of black holes exists I am choosing to believe they do. Theoretically their existence seems more than likely; without them there is no explanation for the fate of neutron stars, or the extremely massive but invisible objects in our galaxy. They would no longer be supported (as far as we know) by any type of fusion, but could grow by absorbing further stellar matter, and their destruction could only occur over trillions of years by the gradual evaporation or ¡§leaking¡¨ of the matter within through Hawking radiation. This means even if all the matter in the universe ended up as a massive black hole (a logical conclusion) over a long time this would escape, and evenly disperse itself throughout space, allowing the formation of further stars then black holes etc. This process would take well over 1¡1066 years. Black holes also raise questions about the possibility of time travel, while in theory this could be possible using black holes and cosmic strings in practice it is still unachievable. It is unlikely we will discover a great deal more about singularities, as while in theory possible, it is extremely unlikely that we could ever find a naked singularity ¡V one without an event horizon which would allow us to study them in more detail. So while in theory we may be able to work out much more about black holes, in reality only a small amount can be proven.