woodchip wrote:So for all these years I thought anything near a black hole got sucked in and nothing escaped. It would seem not:
Only inside the event horizon. Objects outside the event horizon can sometimes be sling shotted AWAY from the black hole.
People often consider a black hole to have some kind of magically more powerful gravity. But gravity follows the exact same inverse square law, even for a black hole.
Consider, for example, if earth was somehow collapsed down into a black hole.
To begin with, someone standing on the surface of the earth experiences 9.8m/s^2 acceleration due to gravity. The shuttle, orbiting 300 miles further away, is accelerated at about 8.33m/s^2.
Now we start collapsing the earths mass. It all shrinks down to a point source and becomes a micro black hole. What will happen to the orbiting shuttle?
Nothing.
We have the exact same amount of mass beneath it, and it still exerts an acceleration of 8.33m/s^2 at that distance. The shuttle will continue orbiting.
What about the guy standing on what USED to be the surface of the planet? He is still being pulled down at 9.8m/s^2, no increase. BUT, since he is now unsupported, he actually begins to fall.
When earth was still 8000miles in diameter, going DOWN from the surface (down a deep sinkhole for example) did NOT result in an increase in gravity. As the falling man goes down the sink hole gravity actually DECREASES because mass that was previously below him pulling him down is now above him pulling him back up. Gravity would continue decreasing until you reached the center of the earth where you would have the same amount of mass pulling you in all directions, resulting in a total acceleration due to gravity of zero.
BUT, after we shrink the earth into a micro black hole, this changes. Now the person falling DOES experience an increase in the acceleration due to gravity because all of the mass is still beneath him. As a matter of fact, he can get arbitrarily close to the entire mass of the earth. The inverse square law still applies, but as we reduce the distance towards zero, the resulting acceleration due to gravity moves up towards infinity.
The distance at which the acceleration of gravity becomes so great that light can not escape is called the event horizon.
For a micro black hole with the mass of the earth, the event horizon would be pretty small. For a black hole with the mass of a star, the event horizon would be much larger. The event horizon for one of the huge black holes at the center of a galaxy would be much much larger.
But even with these enormous black holes, the inverse square law still applies. If you were orbiting a galaxy and were able to watch as a black hole devoured every sun in the entire galaxy, your orbit would not be changed. The amount of mass you are orbiting has not changed. It's just become a point source instead of a larger mass, but if you were calculating your orbit around the galaxy you would treat all the mass of the galaxy as a point source anyway.
So a huge black hole can "blow a bubble" because objects above the event horizon can be accelerated and ejected from the system at very high velocities. Just like happens with comets (or even planets) in a solar system.
