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Understanding of a Black Hole

  1. Jul 6, 2011 #1
    Can someone tell me the answers to these questions.

    1. Does a black hole only pull in stuff that falls into the event horizon or is it actually pulling in the space around it as well. E.g. is it like pulling the plug in the bath where the water i.e. space is being sucked in which is pulling the planets as well?

    2. Can a black hole grow? - My understanding of a black hole is that it's size depends on the mass of the original star. Then once the BH has been created it will remain the same size unless it's not sucking any matter in which will make it shrink. Meaning that it can only stay the same size or get smaller?
  2. jcsd
  3. Jul 6, 2011 #2


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    It doesn't really make much sense to talk about 'sucking' space, at least not in the way the word is normally used. Sometimes people talk about rotating black holes 'dragging' space around them, but this terminology can be quite confusing unless you know precisely what is meant by the words. So, as a general rule, I would answer your question as black holes merely curve space (as a bowling ball will curve a taut sheet of rubber). The planets and everything around the hole then just follows the geometry of the sheet. (This is a very common analogy, but don't try to push it too far!

    Black holes can, and most certainly do, grow. Any infalling mass will add to the mass of the black hole, and even when two black holes merge the area of the final black hole will be larger than the sum of the two merging holes! There is a quantum mechanical process known as Hawking radiation, which causes black holes to lose a very small amount of mass. The amount of energy radiated through this process is inversely proportional to the mass of the hole -- that is, larger holes radiate less. If you run the numbers, the amount of mass lost by any stellar mass black hole (or more massive) is far less than the energy which falls into the black hole from the Cosmic Microwave Background radiation -- that is, these black holes are always growing.
  4. Jul 6, 2011 #3
    Ok so what your saying is that they don't pull in the space like in the bath when you pull the plug then. The Black hole can only effect the event horizon around itself yes?

    So if BH's are growning bigger and bigger then what are the chances of the Supermassive BH's dominating their galaxies and destroying all of their stars?

    If a micro BH was created by that CERN machine then they wouldn't evaporate straight away like they said they would then because immediately they can pull in the Earth and grow just as big?
  5. Jul 6, 2011 #4


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    A black hole is no different than any other object with mass to anyone outside the event horizon: all objects with mass attract all other objects in the universe.
  6. Jul 6, 2011 #5


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    Zero. As russ notes, a black hole is essentially the same as any other gravitating body when you are far away with it, so don't expect black holes to gobble everything up.

    No, these would evaporate very quickly (certainly within a fraction of a second). The Hawking Radiation is only negligible for astrophysical black holes. Also, as with all gravity, the Earth would pull on any small black hole the same as the black hole pulls on the earth. And recalling from simple mechanics, the Earth has much more mass (therefore inertia), so the Earth will not move any more than it does in response to you throwing a baseball.
  7. Jul 6, 2011 #6


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    One of the big misunderstandings with Black Holes is the belief that they suck in everything that they influence with their gravity. But gravity has an infinite range, so that can't be true. The reality, just like has been said twice now, is that the amount of gravitational force produced by an object is solely due to it's mass. If we could replace the sun with a black hole of equal mass, (Which isn't possible) nothing in the solar system would change orbits or get sucked in.
  8. Jul 8, 2011 #7
    Black holes are not cosmic plugs and this is a common misconception.

    Outside the event horizon a black hole has the same effects on spacetime as any other rest mass - the EH is basically an information hole, in that in non esoteric theoreticals no information can be discerned from anywhere past the EH.

    So with regards to your questions:

    It exerts the same gravitational effect as any other rest mass outside of the boundary of the EH. Within the boundary of the EH all world lines point further "down".

    Black holes that consume matter will grow proportionally to the matter they consume, if they dont consume matter they are asssumed to evaporate via hawkwing radiation. As larger BHs evaporate less hawkwing radiation they will evaporate slower.

    Thats pretty much what everyone has stated.
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