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Escape velocity of a body from a black hole.

  1. May 9, 2006 #1
    i read that the escape velocity from black hole is greater then C, my question is: is that true for all black holes, or we could have some black holes which have smaller escape velocity than C, there's the equation v=sqrt(2MG/R) which implies that M/R>c^2/2G, but does the star before it becomes a black hole cannot change its mass (i think that infact before it becomes a black hole, it shrinks into neutron star which is radius is much smaller. but does its mass stays as it was?).

    p.s
    1.when a black hole "consumes" (if this term is correct) mass from its surrondings, does it not expand? and if it does does its event horizon increase as well?
    2. i heard that perhaps black holes' end in vaporization, how is this idea is being conveyed?
     
  2. jcsd
  3. May 9, 2006 #2

    tony873004

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    It can't have escape velocity < c or it wouldn't be a black hole. Neutron stars have an escape velocity close to c, but they're not black holes.

    The mass doesn't have to change. It just gets smaller which changes R in your equations. If you could compress the entire Moon into a sphere about 1 or 2 inches, it would become a black hole. As it is now, the closest you can get to the Moon is 1 Moon radii, or about 1600km. Using 1600km as R, the escape velocity for the Moon is in the low km/s. But if you could get 1 cm from the Moon, its escape velocity would exceed C.

    When it "consumes" it becomes more massive, and that expands the event horizon. The singularity will not grow. It's called a singularity because its size is 0.

    Black holes can end in evaporation. Small black holes are thought to evaporate very quickly, while massive ones would require timescales that exceed the life of the universe.
     
  4. May 9, 2006 #3
    for a star to become a black hole it must loose mass
    as a super nova is the only way I know of for that to happen
    and a good bit of the star is blowen off in the process
    there is no known way to colapes a moon or other non star

    non star formed black holes are not proven and as yet unfound
     
    Last edited: May 9, 2006
  5. May 9, 2006 #4
    Not exactally what happens is that it collapses into a very condsen region in space and the mass is left but it's all on a single point in space and that what causes it's gravity be so strong.
     
  6. May 9, 2006 #5
    On this note, we have never "seen" a stellar mass black hole, only inferred its existance. Just as we have inferred the existance of the supermassive black hole at the centre of the Milky Way.
     
  7. May 10, 2006 #6
    yes BUT you need a super nova to power the star collapse
    and that blows off a good part of the star as a result
    if 2 to 3 solar masses are left only then do you get a black hole
    so you cannot get a luner mass black hole
    it just will not go beyond a neutron matter state

    well we will never see a black hole
    but have not even inferred a less then 2 to 3 solar mass one
    nor do we have a real method to make one smaller then that
     
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