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Absorb all sort of matter and radiations and still not releasing out?

  1. Jun 10, 2008 #1
    Blackhole is a little bit confusing topic for me.Can anybody tell me,whats the reason and principle behind it that it absorb all sort of matter and radiations and still not releasing out?
  2. jcsd
  3. Jun 10, 2008 #2


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    The simplest (classical) explanation is the idea of escape velocity. Any body (for example the earth) exerts a gravitational pull. In order for anything to leave the earth, it has to go at least 25000 mph (I think that's correct, but it doesn't matter). For a back hole, the escape velocity is greater than the speed of light.

    The full explanation requires general relativity, but the principle is the same.
  4. Jun 12, 2008 #3
    yea, basically it is an object that is a possible end result of a star's life. The stars that become black holes are typically 3+ Solar Masses.

    What happens is that when the star stops producing energy, its gravity causes it to collapse in on itself. The black hole is the extreme case: it collapses on itself so much that it becomes a singularity - a point that is infinitely small, and thus has infinite density.

    We know that the greater the mass of on object, the greater its gravitational field. Well, a black hole has infinite mass, so the escape velocity is greater than the speed of light.

    I could do some calculations, but there is the rough theory behind it anyway.
  5. Jun 13, 2008 #4


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    A black hole does not have infinite mass. It is a black hole because its size (radius) is very small for its mass.
  6. Jun 13, 2008 #5
    I think the word he was looking for was infinite density.

    The first idea of a black hole was stumbled upon by someone wondering what would happen if you had a star the size of the sun, but 500 time its mass. Its density would then be so great as to be greater then the speed of light.

    That was using newton's laws of gravity.

    Now that we have relativity, we actually do not even need an object to have such massive mass, just the correct density, as when it reaches a certain threshold, the gravity of the object bends all of spacetime in such a way to lead every patch out of the black hole toward itself.

    So simply speaking, the earth is too small to collapse into a blackhole, however if by some divine intervention you were to collapse all its mass it would form a black hole with an event horizon on the order of 9millimiter. (roughly 2/5 of an inch).

    Naturally, it is currently believe to be possible to have a black hole under only 3 conditions.

    1- 3-100 sun masses, we're talking a pretty big star that goes super nova, and then collapse further then a neutron star to form a black hole. In this category, another event might create a black hole in those mass range, such as the merging of two neutron star.

    2- supermassive blackhole, currently known to be in the center of galaxy, we're talking several thousand to several billion solar masses. Also hypothesise to be the power behind quasars, and various active galaxy.

    3- micro black hole, below 1.44 solar mass, and way smaller. Those black hole are currently undiscovered mostly because of their small size, and their origin, as they could only have formed during the big bang. They would atm if current theory hold, be emiting massive amount of radiation. (read a brief history of time for more details or wikipedia is a great help).

    Lastly, a black hole is in theory a singular point with zero for all three normal dimension, thus when you try and divide its mass by its volume, you get a very bothersome result, as math currently can not deal very elegantly with division by zero.
  7. Jun 13, 2008 #6
    yea my bad, first year Astro doesn't give us much to play with, and as I had just finished my exams, my knowledge just left my head. Sorry about that :P
  8. Jun 19, 2008 #7
    Hi, I'm jumping into this thread to ask a few things about the formation of black holes.

    Is this right? :
    basically when a star reaches the end of it's live, it explodes in a supernova but the core remains because the power of the explosion is not strong enough to drive the core apart, because of the high gravitation there. At one point, the gravitation of this remaining core becomes so high, that all atoms basically merge into 1 point , not able to "escape" the insane gravity.

    I also have 2 questions here: why does the gravitational force suddenly become so strong inside the core? What changes inside the core during the supernova to make gravitational force rise?

    What keeps neutrons and protons from collapsing into each other, meaning, what keeps them at distance (since they have a mass -> gravitational force, they should pull at each other right?)

  9. Jun 19, 2008 #8
    Actually the matter approaches a certain part and then become frozen above the event horizon. The material is redshifted so far that it becomes impossible to observer it. It then becomes impossible to measure the mass distribution and the gravitational field becomes identical to the field that would result if all the matter was confined to a point.

  10. Jun 20, 2008 #9
    The gravitational force doesn't change at all, but the star itself can no longer fuse elements(because there's none left that it has the capacity to fuse), and so there is no longer enough pressure inside the star to resist gravity. With the pressure resisting gravity gone, the only thing left for the star to do is to collapse under it's own weight.

    Just think of an airplane in the air... As long as there's fuel to burn, it'll stay in the air. As soon as that fuel runs out, it's going to succumb to the force of gravity. It's not that gravity changed at all, just that the forces resisting ran out.
  11. Jun 21, 2008 #10
    Correct me if I'm wrong but Einstein's algebraic formula for gravity is g = energy density + 3 x pressure.

    Based on this a neutron star with an average equation of state of ~1/10 (1/20 at the surface, 1/3 at the core) would actually produce a gravity field 1/10th more than it's mass (this doesn't necessarily mean the denser the matter, the higher the equation of state). If the core of this neutron star was to collapse into quark matter which has an equation of state ~1/6 (the pressure drops due to the fact that the neutrons break down into smaller components i.e. quarks and even though the density increases, the pressure drops) the overall equation of state drops from ~1/10 to ~1/13 which it appears would also affect the gravity of the star, 1/13th more as apposed to 1/10th more. It might be said this change in state could be signified by gravity waves as the pressure changes in the core, which in turn changes the gravity of an object.
  12. Jun 21, 2008 #11


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    How fast would it happen?

    "It might be said this change in state could be signified by gravity waves as the pressure changes in the core, which in turn changes the gravity of an object."

    This might determine how detectable such a wave might be.
  13. Jun 22, 2008 #12
    I got my fractions slightly wrong. Based on Einsteins equation for gravity (g = energy density + 3 x pressure), if the average equation of state is 1/10 then the gravity field would be ~1/3rd more than the mass; if the equation of state dropped to 1/13 then the gravity field would be ~1/4 more than the mass.

    Last edited: Jun 22, 2008
  14. Jun 27, 2008 #13
    Re: Blackhole

    I'm pretty sure the change of state would happen in the fraction of a second. Regarding gravity waves, it would be interesting to know (if they exist) what there range is. Do they behave like electromagnetism or are they short range? I'm almost tempted to believe they occur within a radius defined by what the compact object would have had if it had been a regular star. When looking at coordinate acceleration, gravitational redshift and frame dragging, I realized the effects of these phenomena tail off to virtually zero (ca/gr become 1, fd becomes 0) at a radius based on the mass of the compact object (i.e. white dwarfs, neutron stars & black hole) having the density of a main sequence star; (say for a 3 sol mass black hole, all the effects of frame-dragging, gravitational redshift and co-ordinate acceleration become zero at a radius of 1 million km, the radius of a 3 sol mass star based on the average density of our Sun). It's as if anything out of the ordinary was confined within this zone, which might include gravity waves.
    Last edited: Jun 27, 2008
  15. Jul 4, 2008 #14
    Re: Blackhole

    I just quickly browsed through the previous replies, but I believe there are two possible answers to this question:
    1) As others have stated the escape velocity in a black could be infinte, because of the desinty
    2) We now believe that black hole's do radiate (through Hawking radiation), also they may emit others type of undectable energy....
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