A brief history of black holesA black hole is an object or region of space where the pull of gravity is so strong that nothing can escape from it, i.e. the escape velocity exceeds the speed of light. The term was coined in 1968 by the physicist John Wheeler. However, the possibility that a lump of matter could be compressed to the point at which its surface gravity would prevent even the escape of light was first suggested in the late 18th century by the English physicist John Michell (c.1724-1793), and then by Pierre Simon, Marquis de Laplace (1749-1827).
Black holes began to take on their modern form in 1916 when the German astronomer Karl Schwarzschild (1873-1916) used Einstein's general theory of relativity to find out what would happen if all the mass of an object were squeezed down to a dimensionless point – a singularity. He discovered that around the infinitely compressed matter would appear a spherical region of space out of which nothing could return to the normal universe. This boundary is known as the event horizon since no event that occurs inside it can ever be observed from the outside. Although Schwarzschild's calculations caused little stir at the time, interest was rekindled in them when, in 1939, J. Robert Oppenheimer, of atomic bomb fame, and Hartland Snyder, a graduate student, described a mechanism by which black holes might actually be created in the real universe. A star that has exhausted all its useful nuclear fuel, they found, can no longer support itself against the inward pull of its own gravity. The stellar remains begin to shrink rapidly. If the collapsing star manages to hold on to a critical amount of mass, no force in the Universe can halt its contraction and, in a fraction of a second, the material of the star is squeezed down into the singularity of a black hole.
Stellar black holes
Artist's impression of Cygnus X-1
In theory, any mass if sufficiently compressed would become a black hole. The Sun would suffer this fate if it were shrunk down to a ball about 2.5 km in diameter. In practice, a stellar black hole is only likely to result from a heavyweight star whose remnant core exceeds the Oppenheimer-Volkoff limit following a supernova explosion.
More than two dozen stellar black holes have been tentatively identified in the Milky Way, all of them part of binary systems in which the other component is a visible star. A handful of stellar black holes have also been discovered in neighboring galaxies. Observations of highly variable X-ray emission from the accretion disk surrounding the dark companion together with a mass determined from observations of the visible star, enable a black hole characterization to be made.
Among the best stellar black hole candidates are Cygnus X-1, V404 Cygni, and several microquasars. The two heaviest known stellar black holes lie in galaxies outside our own. One of these black hole heavyweights, called M33 X-7, is in the Triangulum Galaxy (M33), 3 million light-years from the Milky Way, and has a mass of 15.7 times that of the Sun. Another, whose discovery was announced in October 2007, just a few weeks after that of M33 X-7, is called IC 10 X-1, and lies in the nearby dwarf galaxy, IC 10, 1.8 million light-years away. IC 10 X-1 shattered the record for a stellar black hole with its mass of 24 to 33 times that of the Sun. Given that massive stars lose a significant fraction of their content through violent stellar winds toward the end of their lives, and that interaction between the members of a binary system can further increase the mass loss of the heavier star, it is a challenge to theorists to explain how any star could retain enough matter to form a black hole as heavy as that of IC 10 X-1.
The microquasar V4641 Sagittarii contains the closest known black hole to Earth, with a distance of about 1,500 light-years.
Supermassive, intermediate-mass and mini black holes
Supermassive black holes are known almost certainly to exist at the center of many large galaxies, and to be the ultimate source of energy behind the phenomenon of the active galactic nucleus. At the other end of the scale, it has been hypothesized that countless numbers of mini black holes may populate the universe, having been formed in the early stages of the Big Bang; however, there is yet no observational evidence for them.
In 2002, astronomers found a missing link between stellar-mass black holes and the supermassive variety in the form of middleweight black holes at the center of some large globular clusters. The giant G1 cluster in the Andromeda Galaxy appears to contain a black hole of some 20,000 solar masses. Another globular cluster, 32,000 light-years away within our own Milky Way, apparently harbors a similar object weighing 4,000 solar masses. Interestingly, the ratio of the black hole's mass to the total mass of the host cluster appears constant, at about 0.5%. This proportion matches that of a typical supermassive black hole at a galaxy's center, compared to the total galactic mass. If this result turns out to be true for many more cluster black holes, it will suggest some profound link between the way the two types of black hole form. It is possible that supermassive black holes form when clusters deposit their middleweight black hole cargoes in the galactic centers, and they merge together.
Inside a black hole
According to the general theory of relativity, the material inside a black hole is squashed inside an infinitely dense point, known as a singularity. This is surrounded by the event horizon at which the escape velocity equals the speed of light and that thus marks the outer boundary of a black hole. Nothing from within the event horizon can travel back into the outside universe; on the other hand, matter and energy can pass through this surface-of-no-return from outside and travel deeper into the black hole.
For a non-rotating black hole, the event horizon is a spherical surface, with a radius equal to the Schwarzschild radius, centered on the singularity at the black hole's heart. For a spinning black hole (a much more likely contingency in reality), the event horizon is distorted – in effect, caused to bulge at the equator by the rotation. Within the event horizon, objects and information can only move inward, quickly reaching the singularity. A technical exception is Hawking radiation, a quantum mechanical process that is unimaginably weak for massive black holes but that would tend to cause the mini variety to explode.
Three distinct types of black hole are recognized:
* A Schwarzschild black hole is characterized solely by its mass, lacking both rotation and charge. It possesses both an event horizon and a point singularity.
* A Kerr black hole is formed by rotating matter, possesses a ring singularity, and is of interest in connection with time travel since it permits closed time-like paths (through the ring).
* A Reissner-Nordstrom black hole is formed from non-rotating but electrically-charged matter. When collapsing, such an object forms a Cauchy horizon but whether it also forms closed time-like paths is uncertain.
The equations of general relativity also allow for the possibility of spacetime tunnels, or wormholes, connected to the mouths of black holes. These could act as shortcuts linking remote points of the universe. Unfortunately, they appear to be useless for travel or even for sending messages since any matter or energy attempting to pass through them would immediately cause their gravitational collapse. Yet not all is lost. Wormholes, leading to remote regions in space, might be traversable if some means can be found to hold them open long enough for a signal, or a spacecraft, to pass through.
How black holes work
Escape Velocity
If ball is thrown upwards from the surface of the Earth it reaches a certain height and then falls back. The harder it is thrown, the higher it goes. Laplace calculated the height it would reach for a given initial speed. He found that the height increased faster than the speed, so that the height became very large for a not very great speed. At a speed of 40000 km/h (25000 mph, only 20 times faster than Concorde) the height becomes very great indeed - it tends to infinity, as the mathematician would say. This speed is called the `escape velocity' from the surface of the Earth, and is the speed which must be achieved if a space craft is to reach the Moon or any of the planets. Being a mathematician, Laplace solved the problem for all round bodies, not just the Earth.
He found a very simple formula for the escape velocity. This formula says that small but massive objects have large escape velocities. For example if the Earth could be squeezed and made four times smaller, the escape velocity would need to be twice as large. This surprisingly simple derivation gives exactly the same answer as is obtained from the full theory of relativity.
Light travels at just over 1000 million km/h (670 million mph), and in 1905 Albert Einstein proved in the Special Theory of Relativity that nothing can travel faster than light. The above Laplace formula can be turned around to tell us what radius an object must have if the escape velocity from its surface is to be the speed of light. This particular radius is called the `Schwarzschild radius' in honor of the German astronomer who first derived it from Einstein's theory of gravity (General Theory of Relativity). The formula tells us that the Schwarzschild radius for the Earth is less than a centimeters, compared with its actual radius of 6357 km.
Apparent versus Event Horizon
As a doomed star reaches its critical circumference, an "apparent" event horizon forms suddenly. Why "apparent?" Because it 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. Even before the star meets its final doom, the event horizon forms at the center, balloons out and breaks through the star's surface at the very moment it shrinks through the critical circumference. At this point in time, the apparent and event horizons merge as one: the horizon. For more details, see the caption for the above diagram. The distinction between apparent horizon and event horizon may seem subtle, even obscure. Nevertheless the difference becomes important in computer simulations of how black holes form and evolve. Beyond the event horizon, nothing, not even light, can escape. So the event horizon acts as a kind of "surface" or "skin" beyond which we can venture but cannot see. Imagine what happens as you approach the horizon, and then cross the threshold.
Care to take a one-way trip into a black hole?
The Singularity
At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and space-time has infinite curvature. Here it's no longer meaningful to speak of space and time, much less space-time. Jumbled up at the singularity, space and time cease to exist as we know them.
The Limits of Physical Law
Newton and Einstein may have looked at the universe very differently, but they would have agreed on one thing: all physical laws are inherently bound up with a coherent fabric of space and time. At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizarre realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity.
It's no surprise that throughout his life Einstein rejected the possibility of singularities. So disturbing were the implications that, by the late 1960s, physicists conjectured that the universe forbade "naked singularities." After all, if a singularity were "naked," it could alter the whole universe unpredictably. All singularities within the universe must therefore be "clothed." But inside what? The event horizon, of course! Cosmic censorship is thus enforced. Not so, however, for that ultimate cosmic singularity that gave rise to the Big Bang
If ball is thrown upwards from the surface of the Earth it reaches a certain height and then falls back. The harder it is thrown, the higher it goes. Laplace calculated the height it would reach for a given initial speed. He found that the height increased faster than the speed, so that the height became very large for a not very great speed. At a speed of 40000 km/h (25000 mph, only 20 times faster than Concorde) the height becomes very great indeed - it tends to infinity, as the mathematician would say. This speed is called the `escape velocity' from the surface of the Earth, and is the speed which must be achieved if a space craft is to reach the Moon or any of the planets. Being a mathematician, Laplace solved the problem for all round bodies, not just the Earth.
He found a very simple formula for the escape velocity. This formula says that small but massive objects have large escape velocities. For example if the Earth could be squeezed and made four times smaller, the escape velocity would need to be twice as large. This surprisingly simple derivation gives exactly the same answer as is obtained from the full theory of relativity.
Light travels at just over 1000 million km/h (670 million mph), and in 1905 Albert Einstein proved in the Special Theory of Relativity that nothing can travel faster than light. The above Laplace formula can be turned around to tell us what radius an object must have if the escape velocity from its surface is to be the speed of light. This particular radius is called the `Schwarzschild radius' in honor of the German astronomer who first derived it from Einstein's theory of gravity (General Theory of Relativity). The formula tells us that the Schwarzschild radius for the Earth is less than a centimeters, compared with its actual radius of 6357 km.
Apparent versus Event Horizon
As a doomed star reaches its critical circumference, an "apparent" event horizon forms suddenly. Why "apparent?" Because it 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. Even before the star meets its final doom, the event horizon forms at the center, balloons out and breaks through the star's surface at the very moment it shrinks through the critical circumference. At this point in time, the apparent and event horizons merge as one: the horizon. For more details, see the caption for the above diagram. The distinction between apparent horizon and event horizon may seem subtle, even obscure. Nevertheless the difference becomes important in computer simulations of how black holes form and evolve. Beyond the event horizon, nothing, not even light, can escape. So the event horizon acts as a kind of "surface" or "skin" beyond which we can venture but cannot see. Imagine what happens as you approach the horizon, and then cross the threshold.
Care to take a one-way trip into a black hole?
The Singularity
At the center of a black hole lies the singularity, where matter is crushed to infinite density, the pull of gravity is infinitely strong, and space-time has infinite curvature. Here it's no longer meaningful to speak of space and time, much less space-time. Jumbled up at the singularity, space and time cease to exist as we know them.
The Limits of Physical Law
Newton and Einstein may have looked at the universe very differently, but they would have agreed on one thing: all physical laws are inherently bound up with a coherent fabric of space and time. At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizarre realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity.
It's no surprise that throughout his life Einstein rejected the possibility of singularities. So disturbing were the implications that, by the late 1960s, physicists conjectured that the universe forbade "naked singularities." After all, if a singularity were "naked," it could alter the whole universe unpredictably. All singularities within the universe must therefore be "clothed." But inside what? The event horizon, of course! Cosmic censorship is thus enforced. Not so, however, for that ultimate cosmic singularity that gave rise to the Big Bang
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