Well according to hawking then that statement would be false and a black hole indeed looses matter by ejecting it. Im not sure myself why this would happen but it is still pretty cool

http://images.google.ca/imgres?imgurl=http://www.guidetothecosmos.com/images/slide03.jpg&imgrefurl=http://www.guidetothecosmos.com/present_stars.htm&usg=__pHc23KPZlvA3FJgqFoFnvod6g6s=&h=300&w=400&sz=35&hl=en&start=24&um=1&tbnid=EVZpHNMxN5id4M:&tbnh=93&tbnw=124&prev=/images%3Fq%3Dblack%2Bhole%2Bejection%26ndsp%3D18%26hl%3Den%26sa%3DN%26start%3D18%26um%3D1



I googled ''why does a black hole eject matter?'' and came across two pages. They don't directly tell you WHY they eject the matter just that they do

http://chandra.harvard.edu/chronicle/0105/bh_ce/index.html
http://en.wikipedia.org/wiki/White_hole

http://images.google.ca/imgres?imgurl=http://www.nasa.gov/images/content/283365main_2blackholes_microquasar2_HI.jpg&imgrefurl=http://www.nasa.gov/topics/universe/features/blackhole_lesson.html&usg=__z2y-dzCI1zRg16USBRhrCpo3GjY=&h=958&w=1224&sz=407&hl=en&start=4&um=1&tbnid=wC85mqcR-4ukLM:&tbnh=117&tbnw=150&prev=/images%3Fq%3Dwhy%2Bdoes%2Ba%2Bblack%2Bhole%2Beject%2Bmatter%26hl%3Den%26sa%3DN%26um%3D1

That link above provides a better explanation but also has this quote



This post was edited by short_circut on May 20 2009 01:34pm

we know nothing about how much mass a black hole actually has because we know nothing about gravity. we dont even know if mass is ever sucked past the event horizon, it is possible that it orbits the body indefinately until the black hole ceases to exist (if they do fizzle out that is).

a black hole does lose mass in the form of gamma rays at its poles.

Something i did not know was that black holes eventually faded away, i just assumed they roamed the universe for all existance.

Okay well I can explain this a bit more in depth. Hawking discovered that light rays that are sucked into a black hole should never approach each other when they spiral toward the center, and therefore should never ever meet, instead they would just "hover" around the event horizon always trying to "escape" and never quite succeeding (imagine you running away from something and keeping ahead of it, but never escaping). Thus, Hawking also realized that the event horizon could stay the same size or increase in size but never decrease, as that would mean some light rays would end up approaching if not touching each other.

He noticed that an increase in the event horizon could happen when one black hole merged with another or when one threw matter with a lot of entropy inside of the black hole (i.e. A box of hot gas)... now this means that black holes themselves obey the second law of thermodynamics, which is, all objects tend to go towards disorder. But theres a catch, anything that has entropy has temperature, which means it radiates some kind of electromagnetic particles. So to put that last sentence in other words, if the entropy inside the event horizon went up, to counter balance that, the entropy outside must go down (in form of giving off some kind of energy/radiation).

Now we know that blackholes by definition must swallow up everything if it can swallow light, so where would this radiation needed to keep the law of entropy intact come from? Hawking realized it came from virtual particles, JUST outside of the event horizon. Heisenberg's principle of Uncertainty allows for extremely tiny PAIRS of particles called virtual particles to exist for a very short amount of time in a complete vacuum (they just appear and then disappear). In this short time period, if the pairs of particles are observed (observed means, if the particles interact with our "real" matter), then they can no longer be virtual and by the laws of quantum mechanics are forced to become real (yes, this does violate the law of conservation of energy).

one particle in a VP pair is positive, and one is negative. In order for the particles to "disappear" again, the negative VP must seek out its pos. partner and annihilate with it. But in the presence of a black hole, the positive particle may be ripped away from its counter part due to the massive gravitational pull and thus loses energy and any ability to attract its negative counterpart. The negative particle then becomes real! and it may or may not escape the black hole, if it does, to an outside observer it would seem as if the black hole is emitting radiation.

The final thing that solidifies this conclusion for hawking is that the rate at which black holes emit radiation is exactly the minimum rate it needs to be in order for it not to violate the laws of entropy. Wow that was a lot... if you want even more I'm sure you can dig up some lectures from college classes or read some books by hawking / feynman etc... Hope it clarified a few things.

Edit: Julius, we can predict the mass of a blackhole... it has to be the same mass as the star it was formed from and from all the matter that entered it. By looking at its size, I believe it is determinable. And it does not lose anything at its poles, all radiation is lost just outside the event horizon called the acceleration disk

This post was edited by BovineDesi on May 20 2009 08:48pm