Quote (Azrad @ Feb 20 2010 08:17pm)
Every object exerts gravitational force on its surroundings. The Earth exerts more force on its surface and the moon does (this is why the video of the astronauts jumping around looks funny). The Earth also exerts this force on the moon which keeps it locked in orbit around the Earth (the moon also exerts force on the Earth but lets ignore this in this case). If we were to take a giant vice and crush the earth to 1/2 of its current volume. It would have just as much mass. So it would exert just as much force on the moon as it currently does. But to a person standing on the surface of the Earth, they would feel much heavier since they would be significantly closer to the entire earth than they were before the vice crushed the Earth. The velocity needed to leave the Earth (and say fly to the moon) would become much greater than it was before it was crushed, you would have to build a much more powerful rocket.
If we kept crushing the Earth into a smaller and smaller size. A person standing on the Earth would continue to feel heavier and heaver, and you would need more and more powerful rockets to escape the Earth. Eventually you would reach a point where no rocket no matter how strong could escape. In fact you eventually reach a point where not even a beam of light moves fast enough to escape. The radius at which this happens for any object is called its Schwarzschild radius. All objects have a Schwarzschild radius. The Schwarzschild radius of the Earth would be about 1/2 an inch. So if you managed to crush it to that point, no light would be able to escape the Earth, and it would be referred to as a black hole. A black hole is simply any object whose actual radius is smaller than its Schwarzschild radius. Objects with lots of mass like stars have rather a rather large Schwarzschild radius, while smaller objects have a smaller Schwarzschild radius. An interesting thing is that if the Earth was crushed to this size, it would have little effect on the moons orbit, since the actual mass of the Earth has not changed.
Now it isn't very likely that the Earth could ever get crushed like this, but for a very large star, this is actually possible because of the complex series of nuclear reactions that take place. Basically a massive star has so much gravity that it can eventually crush itself down to this size.
Once an object has been crushed to the size where it is a black hole (remember this means the Schwarzschild radius is larger than the actual radius), it has what is called an event horizon. The event horizon is a radius around the black hole where the velocity required to escape the black hole is equal to the velocity of light. Since nothing can go faster than light this is the point of no return. Anything that crosses this point is doomed. It will be attracted with an irresistible force to the black hole. Any point outside the event horizon has an escape velocity less than the speed of light, so in principle you could move close to an event horizon and still escape. (In reality the tidal forces of a typical black hole would kill you long before you reached the event horizon, but lets not worry about that). This event horizon is the shape of the surface of a sphere around the center of mass of the black hole.
Once an object crosses the event horizon and falls into the black hole, the mass of the black hole increases, and its event horizon grows.
Now I know people will point out problems with this explanation, and yes it does have problems. It is a very simplified version but hopefully it will help the original poster.
you should point out its being crushed towards its gravitational center and not its geometric center[earth being mostly spherical].
/e though it seems impossible for any object to retain a spherical shape while having its center of gravity not equal to its geometric center.
only posted b/c of second bold
Quote (ECK9PA7 @ Mar 2 2010 01:17pm)
According to the general theory of relativity, a black hole is a region of space from which nothing, including light, can escape. It is the result of the deformation of spacetime caused by a very compact mass. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it, reflecting nothing, just like a perfect black body in thermodynamics. Under the theory of quantum mechanics black holes possess a temperature and emit Hawking radiation.
Despite its invisible interior, a black hole can be observed through its interaction with other matter. A black hole can be inferred by tracking the movement of a group of stars that orbit a region in space. Alternatively, when gas falls into a stellar black hole from a companion star, the gas spirals inward, heating to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and Earth-orbiting telescopes.
Astronomers have identified numerous stellar black hole candidates, and have also found evidence of supermassive black holes at the center of galaxies. After observing the motion of nearby stars for 16 years, in 2008 astronomers found compelling evidence that a supermassive black hole of more than 4 million solar masses is located near the Sagittarius A* region in the center of the Milky Way galaxy.
may appear different in this simulation of gravitational lensing
Quote (Azrad @ Feb 24 2010 05:56am)
The closest known black hole is like 1600 light years from Earth. Meaning it would take a prob 1600 years to get to it, if it could be travel at near light speed, and another 1600 years for the telemetry from the probe to get back to Earth. People were not launching space probes 3200 years ago........
time dilation
This post was edited by majorblood on Jun 3 2010 06:44pm