Black Hole: a massive object in the universe whose gravitational field distorts all space-time in its vicinity to such an extent that neither matter nor light can escape that vicinity.

In 1967, the physicist John Wheeler coined the phrase "black hole" in acknowledgment of the fact that such an object, due to its entrapment of all light waves, would present itself to the naked eye as entirely black. Since that time, dozens of black holes have been discovered in the universe. There is one right at the center of our Milky Way; others are at the centers of other galaxies. In fall 2005, astronomers discovered a black hole floating through the universe all on its own, far beyond all galaxies, at a distance of about 5 giga-lightyears.

A literary description of a black hole can be found in Stanislaw Lem's novel Fiasco.

Birth of a Monster

A black hole is created whenever matter exceeds a certain degree of density. General relativity theory tells us that massive objects, by their gravitational fields, distort all space in their vicinity. This can be noticed by the fact that light rays in the vicinity of such an object no longer run in straight lines. Rather, they are shifted in the direction of the object as if passed through a convex lens. This effect is noticeable even in our own sun, but is even more pronounced in neutron stars or in still denser objects, according to their respective degrees of density. If the massive object happens to be very small, it can force the light onto its orbit, thus "strangulating" space entirely. In this way a black hole evolves. At its center it contains a so-called singularity, a point of infinite density and gravity.

The size beneath which an object can become a black hole is called the object's Schwarzschild radius, after the physicist Karl Schwarzschild. What an object's Schwarzschild radius is depends on its mass. In the case of our sun — which, however, has too small a mass ever to become a black hole in reality — the Schwarzschild radius is 3 kilometers.* For an object of the Earth's mass it comes to about 9 millimeters.

The Schwarzschild radius is at the same time the black hole's event horizon, that is, the boundary that cannot be transgressed by any matter or radiation from within its region. This boundary can be traversed only in one direction. It therefore acts as a kind of one-way causal barrier: events beyond it cannot have causes located within (with the exception of the black hole itself), though it is possible for events within to have causes located beyond it.

Gravitational field surrounding a black hole

In this graphic, the bottom end of the funnel constitutes the event horizon. Underneath it there is an infinitely deep tunnel leading right into the singularity at the center of the black hole. But this region is in principle not observable from outside the black hole. Because at this point the singularity still eludes all description, there is speculation that it has the shape of a wormhole leading into another universe or another dimension. Since, however, it is not advisable to actually enter a black hole so as to verify this hypothesis, it will probably remain mere conjecture.

Due to the strong gravitational field, the gravity in the vicinity of a black hole is subject to dramatic effects. A space traveler would feel these effects long before hitting the event horizon. Those of his body parts that face toward the hole would accelerate more rapidly — and hence would fall toward the hole at a faster speed — than those facing the opposite direction. Thus, if he were to fall head first toward the hole, the difference in acceleration between head and feet would stretch him apart, and this — in the case of a black hole with the sun's mass — with a force that reaches 100 billion kg at the event horizon itself. At the same time, the massive forces would press his body together at the sides. Thus, on his way into the hole our unfortunate space traveler would acquire a rather slim and stretched-out shape — a process that the physicist Stephen Hawking has graphically termed spaghettification.

Black Holes Need Not be Black

Just like an elementary particle, a black hole has no individual characteristics whatsoever. Black holes differ only in their mass, charge, and rotating speed. Moreover, they are by no means always black. For one thing, as Hawking showed in 1981 by means of theoretical calculations, their event horizon itself does emit weak radiation due to the extremely strong gravitational field (Hawking radiation).** For another, the matter attracted by the gravitational field is heated up just prior to falling into the hole to such an extent that the hole actually shines brightly toward the outside, as long as there is a sufficient amount of matter in its vicinity.

Supermassive black hole, radiograph (Satellit Chandra / NASA)

Black holes can be separated into several types on the basis of their sizes and histories:

Stellar black holes constitute the terminal state in the development of heavy stars with a mass at least ten times that of our sun. They explode at the end of their lives as supernovas, thereby ejecting part of their matter in the form of a gas cloud. The remainder collapses under the influence of gravity until all their atomic nuclei touch. Such an extremely compressed structure is called a neutron star.

A neutron star is not yet a black hole. However, beyond a certain amount of mass the atomic nuclei continue to be compressed further and the radius of the neutron star diminishes accordingly. Finally, the star collapses completely and becomes a black hole. This entire process, from the collapse of the star's remnants through the creation of the black hole, takes a very short time — a few seconds to a few minutes.

Intermediate-mass black holes can have several hundred times the mass of our sun and evolve by the collision of several stars — as, for instance, in double or multiple star systems. However, for reasons not yet known to us, there are few black holes of this intermediate-mass type. It seems that some mechanism in the universe that we do not yet know of inhibits the fusion of stars or of several black holes.

Supermassive black holes with a mass billions of times our sun's sit at the centers of most galaxies just like spiders in their webs. They have typically acquired their sizes by collecting interstellar gas and dust. One such black hole, the one in our Milky Way, is estimated to be 3.6 million times the mass of the sun. Due to the heating of the matter falling into them, these supermassive black holes were the brightest objects in the sky during the early history of the universe. We can still see them shining brightly in the form of quasars at a great distance and in their distant past. Meanwhile, however, they have already sucked up most matter in their vicinity, so that the luminosity of the galaxies is largely determined by their stars.

The spaghettification effect mentioned above does not occur in supermassive black holes, since the larger radius of such a hole renders its gravitational field less susceptible to changes (in spite of that field's greater strength). For this reason, black holes located at centers of galaxies could in principle be utilized for decelerating and accelerating spacecraft. They could thus play an important part in future space travel.

Primordial black holes evolved soon after the big bang in regions of space with high mass and energy concentrations. Since at that time there was extremely high pressure in the universe and matter was compressed accordingly, primordial black holes can theoretically contain only relatively little mass, amounting to no more than a few thousand kilograms. However, such "miniature holes" have probably disappeared by now, since they lose mass continuously due to Hawking radiation until they are completely dissolved by it.

Black Holes on Earth

Up to this point, we can only speculate about black holes. Physicists have planned for years to create and examine a black hole on Earth. This sounds like a daredevil undertaking, but physicists are really serious about it. After various hiccups, the LHC (Large Hadron Collider) in Geneva, the largest particle accelerator on Earth, is scheduled (at this writing) to be finally launched in 2009; it could in principle produce black holes the size of an elementary particle.*** By examining such micro-holes and their decay, scientists hope to acquire new insights about the number of dimensions in our space and hence also about string theory, which is expected to become the final, definitive theory of physics.

* The formula is R = 2Gm/c2, where R is the Schwarzschild radius, G the gravitational constant, m the mass of the object, and c the speed of light. When we apply the values we obtain R = 1.5·10-27·m, so that the Schwarzschild radius of an average scientist of 70 kg body weight is approximately 10-25 meters. The Schwarzschild radius of the visible universe amounts to approx. 15 giga-lightyears and hence is not that far beneath the universe's actual size.

** Admittedly, in identifying this radiation Hawking used a combination of relativity and quantum theory, even though these theories apply to different space-time domains. It is therefore not entirely certain whether Hawking radiation really exists.

*** Don't worry: such small black holes cannot pose a danger to Earth. They quickly decay due to Hawking Radiation (see also the above foot note ;).

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