A black hole is a gravitationally dense astronomical object with such strength that no form of energy can escape its influence. A black hole creates a warp, or gravity well, on the space it occupies. From the point of approach, a black hole exhibits a visibly flat structure that contains the condensed matter pulled into the black hole: the accretion disk. The size of an accretion disk is dependent on how large the black hole is. Additionally, a black hole may exhibit plumes of gas, called jets, perpendicular to the accretion disk. Neither accretion disks nor gas jets are proof of a black hole, as other objects such as neutron stars and quasars have the same characteristics.
Typically, a black hole forms by the collapse of a massive object, such as a star. A star needs to be at least 20 solar masses, a neutron star, or a white dwarf to have enough influence to bend space. When a star nears the end of its fuel supply, it swells to a red giant. Once the star has burned its remaining gases, the shell of the star collapses in on itself and shrinks to a white dwarf. Given time, the pressure and mass may be so intense that the star will continue to degrade and collapse inward. The gravitational signature of the star becomes a singularity, and a black hole forms.
Upon approach to the black hole, the effects of the gravitational well begin at the event horizon, or the boundary between normal space and the black hole itself. Here, the velocity needed to escape the influence of the black hole is equivalent to the speed of light. Studies of the event horizon are called black hole thermodynamics. In theory, time itself is affected at the event horizon; however, only theory can explain what happens once an object crosses. At the very center of the black hole lies the only physical part of a black hole, a singularity. The singularity is the point source of the gravitational anomaly; it is what remains of the former object that created the black hole.
The quantum physics leading up to a black hole are quite discernable, from Albert Einstein’s theory of relativity to Stephen Hawking’s theory of radiation emission. However, once the event horizon is breached, modern quantum physics no longer applies. As Einstein noted, the curvature of gravity around a collapsing star pulls other particles with it, and continues to do so even as acceleration reaches a constant state.
Types of Black Holes
There are four types of black holes, each derived from two components: charge and rotation. Electric charge influences the black hole near its singularity, while a massive spinning object, such as a pulsar, causes the rotation of a black hole. These types of black holes are listed in Table 1. In 1963, New Zealand mathematician Roy Kerr additionally suggested that these black holes may also be gateways to parallel universes. Kerr was the first mathematician to solve and apply Einstein’s general relativity theory to a rotational star. Since then, additional theories suggest that the existence of an object opposite to a black hole must exist, as the matter pulled in must be pushed out. This is called a white hole and would then be connected to the black hole via a “wormhole.” However, until many black holes are studied in closer detail, no concrete evidence exists to support the existence of white holes.
Table 1 Types of Black Holes
Several physicists have attempted to describe the many processes that lead up to the event horizon. Einstein’s relativity theory states that as an observer approaches a black hole, the very warping of space itself will cause time to warp too and, in effect, slow down until it “freezes” at the event horizon. However, time at the vantage point of the observer’s origin will remain constant, and if the observer never breaches the event horizon, the point of origin will have advanced into a much distant future.
If a probe were sent to investigate a black hole, clocks on board the probe would act as normal, even though the probe had begun to enter the region of the event horizon. If the probe were commanded to return, it would simply just turn around and eventually be away from the influence of the gravity well. After approximately 1 minute of clock time to the probe, the probe would be on its way again through the “normal” universe.
However, back at the International Space Station, the probe would appear to slow down and freeze as though no longer moving. The simple act of sending a reverse command would take very little time to transmit but would take much longer to execute as the probe experiences the warp. Although it never fully crossed the event horizon and managed to climb out of the black hole region, it will have taken thousands of years to do so. When the probe returns to “normal” space, it may be thousands of years later for Earth, but just mere moments for the probe. The two clocks would then be asynchronous.
Hawking determined that black holes emit radiation due to quantum effects and, as such, allow the black holes to lose mass. A loss of mass suggests that the black hole is capable of losing mass and therefore dissipating given time. Because time does not apply within the black hole, though, it is unknown how this effect would occur.
The First Findings
In 1971, Tom Bolton of the David Dunlap Observatory at the University of Toronto, Canada, noted a star in the constellation Cygnus whose binary companion was causing the main star (called HDE 226868) to exhibit anomalous behavior. Too massive to be a mere neutron star, this area, known as Cygnus X-1, became the first object named a black hole. Since then, many more stellar regions have been found exhibiting similar peculiarities. Even now, there is a controversy surrounding the definition of a black hole and what should and should not be included as aspects in defining a region as a black hole. Although Hawking originally denied that this particular region was a black hole, he has since conceded that there is now enough evidence to support the theory.
In 1994, NASA scientists working with the Hubble Space Telescope suggested that a super massive black hole exists at the center of the elliptical galaxy M87. This was the first evidence that black holes may be at the center of all galaxies. On February 29, 2000, astronomers using the Chandra X-Ray Observatory (an orbiting X-ray telescope) discovered what appeared to be a massive object at the center of the Milky Way galaxy, which the Keck observatory confirmed shortly afterward. Scientists reduced the size of the accretion disk of the central black hole in 2004 to less than the distance between the earth and the sun using the Very Long Baseline Array of radio telescopes. Many universities and individual astronomers maintain several Internet sites devoted to lists of black holes, found by utilizing a search engine.
See also Big Bang Theory; Big Crunch Theory; Cosmogony; Cosmology, Inflationary; Einstein, Albert; Entropy; Hawking, Stephen; Light, Speed of; Pulsars and Quasars; Quantum Mechanics; Relativity, General Theory of; Relativity, Special Theory of; Singularities; Spacetime, Curvature of; Stars, Evolution of; Time Warps; Universe, Origin of; White Holes; Wormholes
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