# Black Holes

3671 Words8 Pages

Black Holes

Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a black hole. Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein's general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, by thinking about gravity under fairly simple circumstances.

Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don't throw it too hard, it will rise for a while, but eventually the acceleration due to the planet's gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet's gravity is called the "escape velocity." As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the escape velocity . The Earth's escape velocity is 11.2 kilometers per second (about 25,000 M.P.H.), while the Moon's is only 2.4 kilometers per second (about 5300 M.P.H.). We cannot see it, but radiation is emitted by any matter that gets swallowed by black hole in the form of X-rays. Matter usually orbits a black hole before being swallowed. The matter spins very fast and with other matter forms an accretion disk of rapidly spinning matter. This accretion disk heats up through friction to such high temperatures that it emits X-rays. And also there is some X-ray sources which have all the properties described above. Unfortunately it is impossible to distinguish between a black hole and a neutron star unless we can prove that the mass of the unseen component is too great for a neutron star. Strong evidence was found by Royal Greenwich Observatory astronomers that one of these sources called Cyg X-1 (whic...

... middle of paper ...

...detect this radiation was Joseph Weber. He eventually came up with the first bar gravity-wave detector. This was a long aluminum cylinder, 2m by ½m, that should be compressed with an incoming gravity wave. To detect this compression he wired piezoelectric crystals, which respond to pressure by generating an electric current, to the outside surface of the bar. Although it didn't work, other bar detectors were built that used a device called a stroboscopic sensor to filter out random vibrations. This was an ingenious device, but it too proved to be a non-contributor in the advancement of learning more of the galaxy. Just as X-ray astronomy went from simple detectors in the noses of rockets to full fledged X-ray telescopes housed in orbiting satellites, and radio astronomy went from crude dishes to continent spanning arrays, gravity wave detectors may show a completely new spectrum. And, just as X-rays brought a completely new universe into focus, one can hardly imagine what a gravitational view of the universe will reveal. At the very least, we will have definitive proof or denial of black holes, but we may find that black holes are some of the more subtle features of the universe.