giovedì 3 gennaio 2019

Black Holes

Black holes are a phenomenon predicted by Albert Einstein's General Theory of Relativity, which was published in 1916. In fact, the idea of a black hole was proposed as early as 1783 by the amateur British astronomer John Michell (and independently by the Frenchman Pierre-Simon Laplace in 1795).
Ironically, Einstein himself did not believe in the existence of black holes, and he strongly resisted the idea, even though his own theory predicted them. The general scientific consensus is now that black holes do in fact exist, and that they are actually one of the most important features of our universe. Astronomers have detected them indirectly in enough different ways that there is little doubt of their existence.
black hole (the phrase is usually credited to the American physicist John Wheeler in 1967, and is certainly a distinct improvement on the original label of “gravitationally completely collapsed objects”) is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation such as visible light, can escape its pull - a kind of bottomless pit in space-time.
Artist's impression of a star torn apart by the gravity of a black hole - click for larger version
(Click for a larger version)
Artist's impression of a star torn apart by the gravity of a black hole
(Source: Chandra X-Ray Observatory:http://chandra.harvard.edu/resources/
illustrations/blackholes2.html
)
At its center lies an infinitely small, infinitely dense singularity, a place where the normal laws of physics break down. As the comedian Steven Wright once remarked: “Black holes are where God divided by zero”.
Einstein’s work was also at the heart of the theory of wormholes, or “bridges” as he called them. The idea of a hypothetical topological feature of space-time that is essentially a short-cut through space and time, potentially linking widely separated parts of the universe (or even different universes), has been understandably much loved by science fiction writers over the years, although there is also much theoretical work to support them.
To better understand how black holes might be formed, a little background knowledge of the life cycle of stars is useful (which will be covered in the following section), as well as an understanding of general relativity and curved space-time (which is a separate topic in its own right).
star begins its life as a cloud of dust and gas (mainly hydrogen) known as a nebula. A protostar is formed when gravity causes the dust and gas of a nebula to clump together in a process called accretion. As gravity continues to pull ever more matter inward towards the core, its temperature, pressure and density increases. If a critical temperature in the core of a protostar is reached, then nuclear fusion begins and a star is born. If the critical temperature is not reached, however, it ends up as a brown dwarf, or dead star, and never attains star status.
A typical star like our own Sun (technically a yellow dwarf star), then, is fuelled by nuclear fusion, the conversion of hydrogen (the simplest atom, with a nucleus consisting of just one proton) into helium (the second simplest, with two protons and two neutrons in its nucleus). The nucleus of a helium atom actually weighs only 99.3% as much as the two protons and two neutrons that go to make it up, the remaining 0.7% being released as heat and light energy. This 0.7% coefficient, which is essentially due to the extent to which the strong nuclear force is able to overcome the electrical repulsion in the atoms, turns out to be a critical one in determining the life-cycle of stars and the development of the variety of atoms we see in the universe around us.
The process of star formation - click for larger version
(Click for a larger version)
The process of star formation
(Original Source N/A: ssc.spitzer.caltech.edu/
documents/compendium/galsci/)
The Sun's own gravity traps and squeezes this ultra-hot gas into a confined space, thus generating enough heat for the fusion reaction to take place. The process remains in equilibrium as long as it retains enough fuel to create this heat- and light-producing outward energy which counteracts the inward pressure of its gravity (known as hydrostatic equilibrium). This is the period known as the main sequence of the star.
Already about 4.5 - 5 billion years old, when the Sun's hydrogen fuel starts to run out (in an estimated further 5 billion years or so), its main sequence comes to an end, and it starts to cool down and collapse under its own gravity. However, energyfrom the collapse then heats up the core even more, until it is hot enough to start burning helium and, under the extra heat of the helium burning, its outer layers expand briefly (for a "mere" 100 million years) into a massive red giant star.
Eventually, the outer layers blow off completely and the core settles down into a white dwarf star, a small cinder about the size of the Earth composed mainly of carbon and oxygen. Over a very long stretch of time, white dwarfs will eventually fade into black dwarfs, and this is the ultimate fate of about 97% of stars in our galaxy. The matter which makes up white and black dwarfs is largely composed of, and supported by, electron-degenerate matter, in which the atoms making up the starare prevented from further collapse by the effective pressure of their electrons, due to the Pauli Exclusion Principle (which states that no two electrons can occupy identical states, even under the pressure of a collapsing star of several solar masses).
However, a star significantly larger than our Sun is hotter and burns up its fuel more quickly and generally has a shorter but more dramatic life. A star of ten solar masses, for example, would burn fuel at about a thousand times the rate of the Sun, and would exhaust its hydrogen fuel in less than 100 million years (compared to the Sun's 10 billion year lifetime). A star 20 times the mass of our Sun would burn its fuel 36,000 times faster than the Sun, and might live only a few million years in total.
Larger stars are much hotter and the higher temperatures within such a star are sufficient to fuse even helium. The helium then becomes the star's raw fuel, and it goes on to release ever higher levels of energy as the helium is fused into carbon and oxygen, while the outer layer of hydrogen actually cools and expands significantly in the star's red giant phase.
Even larger stars continue in further rounds of nuclear fusion, each of successively increased violence and shorter duration, as carbon fuses into neon, neon into magnesium and oxygen, then to silicon and finally iron. So, although a star the size of our own Sun does not progress very far along this path, a larger star continues through a chain of transmutations to progressively heavier nuclei. Eventually, a star of sufficient initial mass becomes a red supergiant, which has a core layered like an onion, with a broad shell of hydrogen on the outside, surrounding a shell of helium, and then successively denser shells of carbon, then neon, then oxygen, then silicon, and finally a core of white-hot iron.
The iron in the star's core is very resistant to further fusing, however high the temperatures, and the heat from its nuclear fusion is no longer sufficient to support it against its own crushing gravity and it will suddenly and catastrophically collapse. The final collapse of a massive star under its own gravity happens incredibly quickly: in a thousandth of a second it can shrink from thousands of kilometers across to a ball of ultra-condensed matter just a few kilometers across.
This rapid collapse results in a massive rebound when the core reaches the density of an atomic nucleus, like a ball bouncing off a brick wall, resulting in ultra-hot shock-waves which are imparted to the rest of the star. In this way, the star ultimately ends its life in a cataclysmic explosion known as a supernova, and for a few short weeks it burns as brightly as several billion suns, briefly outshining the star's entire home galaxy. For example, the supernova whose remnants we see today as the Crab Nebula, was recorded by Chinese astronomers in the year 1054 as visible to the naked eye for several months, even in the daytime, and bright enough to read by at night, despite its being about 6,500 light years away. The visible light of a supernova, though, represents only about 1% of the released energy, the vast majority being in the form of ultraviolet light, x-rays, gamma rays and, particularly, neutrinos.
Evolution of high and low mass stars - click for larger version
(Click for a larger version)
Evolution of high and low mass stars
(Source: RedOrbit:http://www.redorbit.com/education/
reference_library/universe/
stellar_evolution/246/index.html
 - Credit: Thomas Learning)
The conditions in the blast of a supernova are even hotter and more violent than in the core of the old star and this finally allows elementseven heavier than iron to be created, such as radioactive versions of cobalt, aluminum, titanium, etc. In the process of its explosion, a supernova blows out into space a nebula of debris containing a mix of all of the naturally-occurring elements, in proportions which agree closely with those calculated to exist on earth. The variety of atoms in the dusty cloud from which our own Sun (and the Earth itself) were formed 4.5 billion years ago were essentially the ashes of generations of earlier starshaving run through their entire life-cycles. Supernovas are therefore ultimately responsible for providing the mix of atoms on Earth, and the building blocks for the intricate chemistry of life. Most of these building blocks (carbon, oxygen, iron, etc) were therefore not produced in the Big Bang at the start of the universe - at the time the very first stars were being formed, their composition would have been about 75% hydrogen and 25% helium with just traces of the next heaviest element, lithium - but much later in the center of starsand their supernova explosions. It is in this respect that people talk of humans as being composed of "stardust" (or, for the less romantically inclined, nuclear waste).
When a star explodes as a supernova, most of its matter is blown away into space to form a nebula (such as the Crab Nebula). The ultra-dense remnants of the imploding core which are left behind are known as a neutron star, as its electrons and protons are crushed together in the huge gravity to form neutrons. In 1935, the young Indian-American astrophysicist Subrahmanyan Chandrasekharestablished that there is in fact a limit, known as the Chandrasekhar limit, of about 1.4 solar masses above which a star must continue to collapse under its own gravity into a neutron star rather than settling down into a white dwarf (a similar discovery was made around the same time by the Russian scientist Lev Davidovich Landau).
neutron star is typically between 1.4 and 4 times as massive as our own Sun, but is squeezed into a volume only about twenty kilometers in diameter, and so has an extremely high density. Given that, as Sir Isaac Newton pointed out as long ago as the 17th Century, gravity is subject to an inverse-square law (so that as the distance from the source decreases, gravity increases by the square of that amount), the gravitational pull of a small, dense neutron star is much greater than that around a normal star of many times its size. In fact, the gravitational force on a massively dense neutron star is about a million million times fiercer than on the Earth, and a projectile would need to attain almost half the speed of light in order to escape its gravity. Under conditions of such powerful gravity, Sir Isaac Newton's Law of Universal Gravitation (which generally works well enough in our own Solar System) becomes redundant, and the more sophisticated model of Albert Einstein's General Theory of Relativity is needed. Thus, clocks on a neutron star would run 10 - 20% slower than those on Earth, and any light from its surface would be so strongly curved that, viewed from afar, part of the back of the neutron star would be visible as well.
Because neutron stars retain the angular momentum of the original much larger star, they usually rotate at very high speed (as fast as several hundred times per second in a newly formed neutron star), in the same way as an ice skater spins faster as she tucks in her arms. In some cases, their intense magnetic fields sweep regular pulses of radio waves across the universe, for which they are known as pulsars. We know of about 2,000 neutron stars in our own Milky Way galaxy, the majority of which were detected as radio pulsars.
A particular type of large neutron star known as a magnetar has a particularly powerful magnetic field (up to a hundred trillion times the strength of the Earth's magnetic field), which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma rays, as it decays over a period of around 10,000 years. Perhaps 1 in 10 neutron starsdevelop as magnetars.
A slightly different kind of supernova explosion occurs when even larger, hotter stars (blue giants and blue supergiants) reach the end of their short, dramatic lives. These stars are hot enough to burn not just hydrogen and helium as fuel, but also carbon, oxygen and silicon. Eventually, the fusion in these stars forms the element iron (which is the most stable of all nuclei, and will not easily fuse into heavier elements), which effectively ends the nuclear fusion process within the star. Lacking fuel for fusion, the temperature of the star decreases and the rate of collapse due to gravity increases, until it collapses completely on itself, blowing out material in a massive supernova explosion.
If the mass of the compressed remnant of the star exceeds about 3 - 4 solar masses, then even the degeneracy pressure of neutrons is insufficient to halt the collapse and, instead of forming a neutron star, the core collapses completely into a gravitational singularity, a single point containing all the mass of the entire original star. The gravity in such a phenomenon is so strong that it overwhelms all other forces, to the extent that even light can not escape from it, hence the name black hole. Thus, the gravity of a body just a few times denser than a neutron star would result in its inevitable further collapse into a black hole.
Simulated black hole in front of the Milky Way - click for larger version
(Click for a larger version)
Simulated black hole in front of the Milky Way
(Source: Space Time Travel:http://www.spacetimetravel.org/
galerie/galerie.html
 - Credit: Ute Kraus)
Although singularity at the center of a black hole is infinitely dense, the black hole itself is not necessarily huge, as is sometimes assumed. A black hole with the mass of our Sun, for example, would have a radius of just three kilometers (roughly two hundred million times smaller than the Sun), while one with the mass of the Earth would fit in the palm of your hand! Having said that, black holes can grow to great size over time as they assimilate more and more matter and even other black holes, and some do become extremely massive.
Contrary to popular belief, a black hole does not just "suck up" everything around it in an uncontrolled orgy of destruction: it actually exerts no more gravitational pull on the objects around it than the original star from which it was formed, and any objects orbiting the original star (and which survived the supernova blast) would now orbit a black hole instead (an object would need to approach quite close to a black hole before being sucked in). The very largest blue stars may skip even the supernova stage, so that even their outer shells become incorporated into the singularity.
By definition, we cannot observe black holes directly, but they can be detected by the gravitationaleffect they exert on other bodies or on light rays. This is especially easy to spot in the case of binary star systems where an ordinary star is orbiting around a black hole. In the early 1990s, Reinhard Genzel pioneered this work, using the then new technique of adaptive optics to plot and track the motions of stars near the center of our own Milky Way galaxy, to show that they must be orbiting a very massive, but invisible, object. From the immense speed with which the stars closest to the center of the galaxy are orbiting - millions of kilometers per hour - we know that there is a "supermassive black hole" (known as Sagittarius A) at the center of the Milky Way, with a mass of around 2 - 4 million times that of our Sun. In addition, in the Milky Way galaxy alone, there are many millions of black holes of at least ten solar masses each.
Supermassive black holes lurk in the centers of most galaxies, forming the hubs around which the galaxies rotate. In fact, from observations of the intense radiation of gases swirling around them at close to the speed of light, we can infer that there are much larger supermassive black holes in the centers of other galaxies, some of them weighing as much as several billion suns. The black hole at the center of a galaxy known as M87 has a mass estimated at around 20 billion solar masses, and may be as large as our entire Solar System.
It seems likely that the early universe, in which very large, short-lived stars were the norm, was scattered with many, many black holes, which gradually merged together over time, creating larger and larger black holes. Observations have shown that is not uncommon for two black holes to swirl around each other in a kind of cosmic dance as their gravitational fields interact. The ripples in space-time caused by two black holes orbiting around each other - typically in a three-leaved clover shape or more complex multi-pass configuration, rather than the simple orbit of an electron within an atom, and ever-smaller and faster as the two objects inevitably approach each other - can be recorded visually and even audibly.
Long and short gamma ray bursts - click for larger version
(Click for a larger version)
Long and short gamma ray bursts
(Source: Internet Encyclopedia of Science:http://www.daviddarling.info/
encyclopedia/G/gamma-ray_burst.html
 - Credit: Ute Kraus)
In the case of the largest events, moments after the creation of a black hole, the heat and the hugely amplified magnetic field of the collapsing star combine to focus a pair of tight beams or jets of radiation, perpendicular to the spinning plane of the accretion disk. These beams focus vast amounts of particles and energy (of the order of a billion billion times the energy output of our Sun) away from the black hole at close to the speed of light. The shock waves of this massively energetic beam cause gamma rays to be emitted in a phenomenon known as a "gamma ray burst" or "hypernova" event (so named because its energy and brightness dwarfs even that of a supernova, by a factor of up to a hundred million times). Gamma ray bursts are by far the brightest electromagneticevents occurring in the universe, and can last from mere milliseconds to nearly an hour - a typical burst lasts a few seconds - usually followed by a longer-lived “afterglow” emitting at longer wavelengths (x-ray, ultraviolet, visible, infrared and radio waves). It is likely that collisions between neutron stars, or between a neutron star and a black hole, can also cause gamma ray bursts.
Interestingly, it appears to be easier for stars with fewer heavy elements to turn hypernova and generate gamma ray bursts. That, and the fact that larger, more short-lived stars were more common earlier in the life of the universe, mean that the phenomenon of gamma ray bursts is actually rarer today than it was. Having said that, NASA's Swift Probe, launched in 2004 with a mission specifically to locate gamma ray bursts throughout the universe, is recording at least one such event each day, so these are not rare incidents. (It should be remembered that any supernovasor gamma ray bursts we observe today in galaxies, say, 9 billion light years away, actually occurred 9 billion years ago.)
The simplest type of black hole, in which the core does not rotate and just has a singularity and an event horizon, is known as a Schwarzschild black holeafter the German physicist Karl Schwarzschild who pioneered much of the very early theory behind black holes in the 1910s, along with Albert Einstein. In 1958, David Finkelstein published a paper, based on Einstein and Schwarzschild’s work, describing the idea of a “one-way membrane” which triggered a renewed interest in black hole theory (although the phrase itself was not coined until a lecture by John Wheeler in 1967).
In 1963, the New Zealander Roy Kerr discovered a solution to Einstein’s field equations of general relativity which described a spinning object, and suggested that anything which collapsed would eventually settle down into a spinning black hole. It spins because the star from which it formed was spinning, and it is now thought that this is actually likely to be the most common form in nature. A rotating black hole would bulge outward near its equator due to its rotation (the faster the spin, the more the bulge).
Spinning and non-spinning black holes - click for larger version
(Click for a larger version)
Spinning and non-spinning black holes
(Source: Chandra X-Ray Observatory:http://chandra.harvard.edu/
photo/2003/bhspin/
)
In the mid-1960s, the young English mathematician Roger Penrose devoted himself to the study of black holes and, in 1965, he proved an important theorem which showed that a gravitational collapse of a large dying star must result in a singularity, where space-time cannot be continued and classical general relativity breaks down. Penrose and Wheeler went on to prove that any non-rotating star, however complicated its initial shape and internal structure, would end up after gravitationalcollapse as a perfectly spherical black hole, whose size would depend solely on its mass.
In the late 1960s, Penrose collaborated with his Cambridge friend and colleague, Stephen Hawking, in more investigations into the subject. They applied a new, complex mathematical model derived from Einstein's theory of general relativity, which led, in 1970, to Hawking's proof of the first of several singularity theorems. Such theorems provided a set of sufficient conditions for the existence of a gravitational singularity in space-time, and showed that, far from being mathematical curiosities which appear only in special cases, singularities are actually a fairly generic feature of general relativity.
Although it may seem a very complex, peculiar and perhaps counter-intuitive object, a black holecan essentially be described by just three quantities: how much mass went into it, how fast it is spinning (its angular momentum) and its electrical charge. This came to be known as the “No Hair Theorem”, after John Wheeler’s comment that “black holes have no hair”, by which he meant that any other information about the matter which formed a black hole (for which "hair" is a metaphor) remains permanently inaccessible to external observers within its event horizon, and is all but irrelevant.
Brandon Carter and Stephen Hawking proved the No-Hair Theorem mathematically in the early 1970s, showing that the size and shape of a rotating black hole would depend only on its mass and rate of rotation, and not on the nature of the body that collapsed to form it. They also proposed four laws of black hole mechanics, analogous to the laws of thermodynamics, by relating mass to energy, area to entropy, and surface gravity to temperature.
Hawking radiation as particle pairs are created near a black hole - click for larger version
(Click for a larger version)
Hawking radiation as particle pairs are created near a black hole
(Source: University of St Andrews:http://www.st-andrews.ac.uk/~ulf/fibre.html)
In 1974, Hawking shocked the physics world by showing that black holesshould in fact thermally create and emit sub-atomic particles, known today as Hawking radiation, until they exhaust their energy and evaporate completely. According to this theory, black holes are not completely black, and neither do they last forever.
Hawking showed how the strong gravitational field around a black holecan affect the production of matching pairs of particles and anti-particles, as is happening all the time in apparently empty space according to quantum theory. If the particles are created just outside the event horizonof a black hole, then it is possible that the positive member of the pair (say, an electron) may escape - observed as thermal radiation emitting from the black hole - while the negative particle (say, a positron, with its negative energy and negative mass) may fall back into the black hole, and in this way the black hole would gradually lose mass. This was perhaps one of the first ever examples of a theory which synthesized, at least to some extent, quantum mechanicsand general relativity.
A corollary of this, though, is the so-called “Information Paradox” or “Hawking Paradox”, whereby physical information (which roughly means the distinct identity and properties of particles going into a black hole) appears to be completely lost to the universe, in contravention of the accepted laws of physics (sometimes referred to as the "law of conservation of information"). Hawkingvigorously defended this paradox against the arguments of Leonard Susskind and others for almost thirty years, until he famously retracted his claim in 2004, effectively conceding defeat to Susskind in what had become known as the "black hole war". Hawking's latest line of reasoning is that the information is in fact conserved, although perhaps not in our observable universe but in other parallel universes in the multiverse as a whole.
Unfortunately, Susskind's proposed solution is even more difficult, and almost impossible to envisage or explain in an understandable way. He suggests that, as an object falls into a black hole, a copy of the information that makes it up is sort of scrambled and smeared in two dimensionsaround the edge of the black hole. Furthermore, Susskind believes that a similar process occurs in the universe as a whole, which raises the rather alarming idea that what we think of as three-dimensional reality is in fact something like a holographic representation of a "real" reality, which is actually contained in two dimensions around the edge of the universe.
It is also theoretically possible that "primordial" or "mini" black holes could have been created in the conditions during the early moments after the Big Bang, possibly in huge numbers. No such mini black holes have ever been observed, however - indeed, they would be extremely difficult to spot - and they remain largely speculative. It is anyway likely that all but the largest of them would have already evaporated by now as they leak away Hawking radiation. According to Hawking's theory, the amount of mass lost is greater for small black holes, and so quantum-sized black holes would evaporate over very short time-scales. But it is hoped that such mini black holes might be experimentally re-created in the extreme conditions of the Large Hadron Collider at CERN, which, among other things, would lend much-needed credence to some of the current theoretical predictions of superstring theory regarding gravity.
black hole’s mass is concentrated at a single point deep in its heart, and clearly cannot be seen. The “hole” that can, in principle, be seen (although no-one has ever actually seen a black hole directly) is the region of space around the singularity where gravity is so strong that nothing, not even light, the fastest thing in the universe, can escape, and where the time dilation becomes almost infinite.
black hole is therefore bounded by a well-defined surface or edge known as the “event horizon”, within which nothing can be seen and nothing can escape, because the necessary escape velocity would equal or exceed the speed of light (a physical impossibility). The event horizon acts like a kind of one-way membrane, similar to the "point-of-no-return" a boat experiences when approaching a whirlpool and reaching the point where it is no longer possible to navigate against the flow. Or, to look at it in a different way, within the event horizon, space itself is falling into the black hole at a notional speed greater than the speed of light.
Event horizon and accretion disk of a black hole - click for larger version
(Click for a larger version)
Event horizon, accretion disk and gamma ray jets of a black hole
(Source: Internet Encyclopedia of Science:http://www.daviddarling.info/
encyclopedia/E/event_horizon.html
 - Credit & ©: Astronomy / Roen Kelly)
The event horizon of a black hole from an exploding star with a mass of several times that of our own Sun, would be perhaps a few kilometers across. However, it could then grow over time as it swallowed dust, planets, stars, even other black holes. The black hole at the center of the Milky Way, for example, is estimated to have a mass equal to about 2,500,000 suns and have an event horizon many millions of kilometers across.
Material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon called the accretion disk (or disc). Although no-one has ever actually seen a black hole or even its event horizon, this accretion disk can be seen, because the spinning particles are accelerated to tremendous speeds by the huge gravity of the black hole, releasing heat and powerful x-rays and gamma rays out into the universe as they smash into each other.
These accretion disks are also known as quasars (quasi-stellar radio sources). Quasars are the oldest known bodies in the universe and (with the exception of gamma ray bursts) the most distant objects we can actually see, as well as being the brightest and most massive, outshining trillions of stars. A quasar is, then, a bright halo of matter surrounding, and being drawn into, a rotating black hole, effectively feeding it with matter. A quasar dims into a normal black hole when there is no matter around it left to eat.
A non-rotating black hole would be precisely spherical. However, a rotating black hole (created from the collapse of a rotating star) bulges out at its equator due to centripetal force. A rotating black hole is also surrounded by a region of space-time in which it is impossible to stand still, called the ergosphere. This is due to a process known as frame-dragging, whereby any rotating mass will tend to slightly "drag" along the space-time immediately surrounding it. In fact, space-time in the ergosphere is technically dragged around faster than the speed of light (relative, that is, to other regions of space-time surrounding it). It may be possible for objects in the ergosphere to escape from orbit around the black hole but, once within the ergosphere, they cannot remain stationary.
Also due to the extreme gravity around a black hole, an object in its gravitational field experiences a slowing down of time, known as gravitational time dilation, relative to observers outside the field. From the viewpoint of a distant observer an object falling into a black hole appears to slow down and fade, approaching but never quite reaching the event horizon. Finally, at a point just before it reaches the event horizon, it becomes so dim that it can no longer be seen (all due to the time dilation effect).
In the center of a black hole is a gravitational singularity, a one-dimensionalpoint which contains a huge mass in an infinitely small space, where densityand gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate. As the eminent American physicist Kip Thorne describes it, it is "the point where all laws of physics break down".
Current theory suggests that, as an object falls into a black hole and approaches the singularity at the center, it will become stretched out or “spaghettified” due to the increasing differential in gravitational attraction on different parts of it, before presumably losing dimensionality completely and disappearing irrevocably into the singularity. An observer watching from a safe distance outside, though, would have a different view of the event. According to relativity theory, they would see the object moving slower and slower as it approaches the black hole until it comes to a complete halt at the event horizon, never actually falling into the black hole.
A gravitational singularity is hidden within a black hole - click for larger version
(Click for a larger version)
A gravitational singularity is hidden within a black hole
(Source: Northern Arizona University:http://www4.nau.edu/meteorite/
Meteorite/Book-GlossaryS.html
)
The existence of a singularity is often taken as proof that the theory of general relativity has broken down, which is perhaps not unexpected as it occurs in conditions where quantum effects should become important. It is conceivable that some future combined theory of quantum gravity(such as current research into superstrings) may be able to describe black holes without the need for singularities, but such a theory is still many years away.
According to the "cosmic censorship" hypothesis, a black hole's singularity remains hidden behind its event horizon, in that it is always surrounded by an area which does not allow light to escape, and therefore cannot be directly observed. The only exception the hypothesis allows (known as a “naked” singularity) is the initial Big Bang itself.
It seems likely, then, that, by its very nature, we will never be able to fully describe or even understand the singularity at the center of a black hole. Although an observer can send signals into a black hole, nothing inside the black hole can ever communicate with anything outside it, so its secrets would seem to be safe forever.
Like black holeswormholes arise as valid solutions to the equations of Albert Einstein's General Theory of Relativity, and, like black holes, the phrase was coined (in 1957) by the American physicist John Wheeler. Also like black holes, they have never been observed directly, but they crop up so readily in theory that some physicists are encouraged to think that real counterparts may eventually be found or fabricated.
In 1916, the Austrian physicist Ludwig Flamm, while looking over Karl Schwarzschild's solution to Einstein's field equations, which describes a particular form of black hole known as a Schwarzschild black hole, noticed that another solution was also possible, which described a phenomenon which later came to be known as a “white hole”. A white hole is the theoretical time reversal of a black hole and, while a black hole acts as a vacuum, drawing in any matter that crosses the event horizon, a white hole acts as a source that ejects matter from its event horizon. Some have even speculated that there is a white hole on the "other side" of all black holes, where all the matter the black hole sucks up is blown out in some alternative universe, and even that what we think of as the Big Bang might in fact have been the result of just such a phenomenon.
Flamm also noticed that the two solutions, describing two different regions of space-time could be mathematically connected by a kind of space-time conduit, and that, in theory at least, the black hole "entrance" and white hole "exit" could be in totally different parts of the same universe or even in different universesEinstein himself explored these ideas further in 1935, along with Nathan Rosen, and the two achieved a solution known as an Einstein-Rosen bridge (also known as a Lorentzian wormhole or a Schwarzschild wormhole).
A wormhole is a theoretical short-cut between distant regions of space-time - click for larger version
(Click for a larger version)
A wormhole is a theoretical "short-cut" between distant regions of space-time
(Source: Wikipedia:http://commons.wikimedia.org/
wiki/File:Worm3.jpg
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To better visualize a wormhole, consider the analogy of a piece of paper with two pencil marks drawn on it (to represent two points in space-time), the line between them showing the distance from one point to the other in normal space-time. If the paper is now bent and folded over almost double (the equivalent of drastically warping space-time), then poking the pencil through the paper provides a much shorter way of linking the two points, a short-cut through space-time much like a wormhole.
Some theorists are encouraged to think that real counterparts may eventually be found or fabricated and, perhaps, used as a tunnel or short-cut for high-speed space travel between distant points or even for time travel (with all the potential paradoxes that might entail). However, a generally accepted property of wormholes is that they are inherently highly unstable and would probably collapse in a much shorter time than it would take to get through to the other side. At any rate, it is predicted that they would collapse instantly if even the tiniest amount of matter (even a single photon) attempted to pass through them.
Although some possible theoretical ways around this problem have been suggested (for example, using “cosmic strings” or “negative matter” or some other exotic matter with “negative energy”) to prevent the wormhole from pinching closed, the idea remains largely in the realm of science fiction for the time being. It has, however, still not been mathematically proven beyond all doubt that some kind of exotic matter with negative energy density is an absolute requirement for wormholes, nor has it been established that such exotic matter cannot exist, so the possibility of a practical application of the theory still remains.
Because a wormholes is a conduit through 4-dimensional space-time, and not just through space, Stephen Hawking and others have also posited that wormholes might theoretically be utilized for travel through time as well as through space, although it is widely believed that time travel into the past will never be possible due to the potential for paradoxes and self-destructive feedback loops.

Neither wormholes or black holes have actually ever been seen directly, even with the sophisticated equipment in use today, but both follow inevitably from Albert Einstein’s General Theory of Relativity, and plenty of indirect evidence has been obtained (at least for black holes). The ideas have certainly been more than readily accepted by the science fiction community, for whom they suggest intriguing possibilities.
One of the most famous black hole theorists, the British physicist Stephen Hawking, proposed the four laws of black hole mechanics back in the 1960s, and calculated in 1974 that black holes should thermally create and emit sub-atomic particles, known today as Hawking radiation, until they eventually exhaust their energy and evaporate. Yet, as recently as 2004, he admitted to losing a bet he made with the Caltech physicists Kip Thorne and John Preskill, and overturned his long-held belief that any “information” crossing the event horizon of a black hole is lost to our universe, and is now convinced that black holes will eventually transmit, albeit in a garbled form as we perceive it in our observable universe, information about all matter they swallow (“information” in this sense may be loosely defined as “that which can distinguish one thing from another”, and essentially refers to the identity of a thing and all of its properties).

This is a good indication that the theory is far from cut and dried, and research (both theory and observation) into this challenging area proceeds unabated. As an example of the complexity of the subject matter, a short quote from Professor Hawking’s 2004 presentation may suffice: “The Euclidean path integral over all topologically trivial metrics can be done by time slicing and so is unitary when analytically continued to the Lorentzian”. Wow!