Black hole

A black hole is a theoretical prediction of the theory of relativity. It is impossible to prove that a black hole does not exist, and thus it fails the falsifiability requirement of science. Nevertheless, the theoretical black hole remains a popular topic for magazines, the science page of the New York Times, and armchair followers of science.

Artist's conception of a binary system consisting of a black hole and a main sequence star. The black hole is drawing matter from the main sequence star via an accretion disk around it, and some of this matter forms a gas jet.

Nature of a Black Hole

A black hole is a mass with an infinitesimal volume, and therefore an infinite density. This gives them an escape velocity greater than the speed of light, meaning that light cannot escape from them due to their steep “gravity wells.” As this renders black holes invisible by traditional means of observation, scientists discover their locations through indirect means, such as the effect of their gravitational pull on nearby stars. Stars that are near black holes, e.g. by being part of a binary star system that contains one, show wobbles in their orbits similar to the tidal effects of the moon on Earth’s oceans. While matter and energy, even light, may not escape a black hole, Stephen Hawking has shown that they should emit Hawking radiation, which absent of an influx of mass-energy would lead to the evaporation of the black hole in a burst of gamma rays. Scientists are currently working to pick up one of these bursts, or the radiation itself, with any of several land- and space-based telescopes. However, the matter falling into black holes as well as the cosmic microwave background from the Big Bang obscures the radiation and inhibits its detection. The outer surface of a black hole is an event horizon, a non-material dimensional boundary, beyond which nothing can escape. The center of a black hole, where the mass exists, is called the singularity. The distance from the singularity to the event horizon is known as the Schwarzschild radius. All matter that falls into a black hole becomes part of the singularity, and as such is effectively destroyed.

Origins of Black Holes

Black holes form when supermassive stars more than ten times the mass of the Sun run out of fuel and die. The process of death occurs when supermassive stars that have fused the products of their own fusion into larger and larger elements, up to iron. The star then tries to fuse the iron core that forms as a result, but this does not produce enough energy to hold the outer layers of the star apart against the pull of gravity. When this happens, the iron core at the center of the star implodes in a supernova, and the outer layers of the star are blasted into space in one of the most energetic events in the universe—one star going out in a supernova can give off as much light as an entire galaxy. Not all supernovae result in black holes, but if the mass of the core is large enough, about 1.2 times the mass of the Sun, the leftover gravity of the shrinking core stalls the outward rush of the initial blast, and crushes the core into a point of infinite density: a black hole.

Properties of Black Holes

Black holes only have three properties by which one differs from another: mass, electric charge, and spin. Mass describes the amount of matter in the singularity. It increases when matter falls into the hole, and decreases as the hole emits Hawking radiation and shrinks. Spin refers to whether the hole is stationary or spinning around an axis. While the singularity of a non-spinning hole is an infinitely small point, the singularity of a spinning hole is in the shape of an infinitely thin ring. Matter entering a spinning hole is first swirled around by the hole’s gravity, causing it to heat up and emit x-rays, which can be used to detect the hole. In the supermassive black holes at the centers of galaxies, some of the matter does not fall into the hole. Instead it is blasted into space in twin jets of hot gas perpendicular to the accretion disc, in a phenomenon known as an Active Galactic Nucleus.

Potential Future Exploration

Scientists have speculated that if a spinning black hole is large enough, a person could pass through the center of the ring-shaped singularity and possibly enter a wormhole. However, it would have to be a very large hole, for if it were not, the hypothetical astronaut would never survive to reach the event horizon.

Matter coming close to the event horizon of a small black hole undergoes a process called spaghettification. Because the hole is so dense, its gravitational pull on the near end of an object is much greater than the hole’s pull on the object’s far end. This causes the object to be stretched out in a way resembling a piece of spaghetti, and generally torn in two.

Scientific Understanding of Black Holes

Schwarzschild discovered that black holes were possible under the theory of relativity. Albert Einstein tried to re-work the whole theory to eliminate the need for these singularities. However, Roger Penrose and Stephen Hawking proved the first of many Singularity Theorems, which states that singularities must form under certain conditions. This demonstrated that, rather than mathematical oddities, singularities are a fairly generic feature of realistic solutions to relativity. According to relativity, any mass with radius less than its Schwarzschild Radius is a black hole.

Contrary to popular myth, a black hole is not a cosmic vacuum cleaner. In other words, a one-solar-mass black hole is no better than any other one-solar-mass object (such as, for example, the Sun) at "sucking in" distant objects. However, a one-solar-mass black hole has the same amount of matter as any other one-solar-mass object but compressed into a much smaller space, making it impossible to move fast enough to leave a black hole once there. If a spaceship could land on a black hole, it would never be able to take off again.

External links

• Black Holes
• Black Holes (Frequently Asked Questions)