Black hole question

  1. I am assuming that some current theories believe that there is a singularity inside a black hole.

    My question is this. Would there be a way to tell if the matter in a black hole was a singularity or not? Would there be a difference?

    My guess is that there should be some observable difference if the matter had volume and was not a singularity. Like maybe the diameter of the event horizon.
     
  2. jcsd
  3. Danger

    Danger 9,879
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    As far as I know, the singularity is what defines a black hole. I've never seen anything that would allow for a >c escape velocity without one. I'm not all that 'up' on the subject, though.
     
  4. There is a contention that if particles are emminating from a BH then there is NO singularity, its the concept of Singularity that "traps" anything that encounters it?
     
  5. hellfire

    hellfire 1,047
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    As far as I know there are singularity theorems in general relativity which state that the existence of an event horizon is always related to the existence of a singularity behind it. This means that if there is no singularity then no event horizon will exist. Thus if one could observe the event horizon one would know about the existence of the singularity. This could be achieved indirectly detecting Hawking radiation which is very faint. However, being more realistic I guess one has to rely on the estimations of the mass of the object to know whether there exists a singularity or not. If the mass is greater than the Oppenheimer-Volkoff limit, then general relativity predicts a black hole (and therefore an event horizon and a singularity) instead of a smooth distribution of matter.
     
  6. The even horizon is not a physical surface. It does not exist physically anymore than the sphere that our GPS satellites orbit on around the earth. All the matter is concentrated in the singularity.
     
  7. I don't see the need for a singularity to create an event horizon, just a certain amount of gravity. Are you saying that if enough matter is gathered in one spot that exceeds the theoretical mass required to create an event horizon and no singularity is formed then no event horizon is formed? That doesn't seem right.
     
  8. Danger

    Danger 9,879
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    I just don't see how enough mass to form an event horizon can avoid the gravitational collapse into a singularity, even if it happens after the horizon is formed.
     
  9. Labguy

    Labguy 734
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    True about the EH, but all we really know about the mass is that it is inside the influence "sphere" of the EH.
    I agree with this 100%. There is less insistence on the need for a singularity than there was just a few years ago. Even Quark Stars are being discussed again by physicists at the Harvard-Smithsonian Center for Astrophysics, among others. I copied a recent blurb to Wordpad so I don't remember the source.
    So, if we consider Quark stars, who has yet published the degree of compression it would take? Is it possible that the mass compressed into a quark star could be enough to fall within 2GM/c2 ? Either of these warrant consideration? I think so, and maybe stars of the more massive Top Quark, much more massive than the other 5 types.
    http://www.iop.org/EJ/article/1367-2630/4/1/314/nj2114.html
    http://spaceflightnow.com/news/n0204/11newmatter/
    ....:confused:
     
  10. hellfire

    hellfire 1,047
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    May be I misinterpreted the singularity theorems, but my current understanding does not allow me to agree with this. Take for example this rough formulation of one of the singularity theorems of Hawking and Penrose:


    We only have to agree that an event horizon is a trapped null surface (as far as I know it is) and we will conclude that event horizons always have (under realistic conditions) singularities within them. This is described with more detail in Wald's GR book pp. 239 - 241 (also a definition of trapped surface is given there).

    To argue that there can be mass within it's Schwarzschild radius, that without a singularity leads to a body from which light does not escape, seams to me a "newtonian" argument that might not be necessarily valid.

    I would appreciate corrections if I misunderstood this.
     
    Last edited: Nov 5, 2005
  11. I'm not a physicist. I don't know what a null surface is. What I do know is that an event horizon is nothing more than the point in a field of gravity where the force of gravity exceeds a certain level. This level can be exceeded by the presence of a certain amount of matter whether it collapses or not.

    My contention is that if that certain amount of matter is present there should be some observable difference between the matter being a singularity or not, like the diameter of the event horizon.
     
  12. pervect

    pervect 7,901
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    Where did you get this defintion of an "Event horizon"? This does not match anything I've read. See for instance the Wikipedia defintion of an event horizion

    http://en.wikipedia.org/wiki/Event_horizon

    Wald's defintion of "Event horizon" in "General Relativity" is much more technical, but it also involves the ability of light to escape "to infinity".

    MTW doesn't list "Event horizon" in the index (I suspect it's mentioned somewhere in the 1200 pages, but I don't recall where exactly - the poor indexing of the book is a chronic problem).

    I can't find my copy of Taylor & Wheeler "Black Holes & Time Warps" which would probaly have a good non-technical defintion of the term.

    Anyway, I've looked at serveral sources, and none of them are at all similar to your defintion of "event horizon", which I think is incorrect.
     
  13. pervect

    pervect 7,901
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    Global methods in GR are unfortunately one of my weak points, but as Hellfire mentions it has in fact been proven that given a number of reasonable assumptions, black holes in fact must contain singularities.

    Probably the most arguable of these assumptions is the non-existence of exotic matter, i.e. the statement that matter obeys the "strong energy conditions".

    If one has exotic matter, it would be possible to imagine a black hole containing a core of exotic matter (with pressure exceeding its density) supporting normal matter with a positive energy density that satisfies the "strong energy condition", creating a black hole that has an event horizon without a central singularity.

    Without exotic matter, the singularity theorems apply and show that according to the laws of GR, black holes must contain singularities. At least that's my understanding. This statment only applies to GR, I believe quantum gravity can still avoid the existence of true singularities.
     
  14. hellfire

    hellfire 1,047
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    Thank you for the clarification pervect. Singularity theorems are often formulated in very technical terms. I have found a slide-set for black hole lectures here in which this is stated in very easy and clear terms (see slide nr. 17):

     
  15. Here is a definition I found in a dictionary:

    "The region, usually described as spherical, marking the outer boundary of a black hole, inside which the gravitational force is strong enough to prevent matter or radiation from escaping."

    That sure sounds like what I said.
     
  16. Danger

    Danger 9,879
    Gold Member

    Keep in mind, though, that a dictionary is not a physics text. Their definitions are geared to the general public with barely a passing interest in the subject, such as someone reading a newspaper article or doing a puzzle. As an example of that, the event horizon is not the outer boundary of a black hole... it's the inner one.
     
  17. You have lost me.
    Fact: gravity can bend light. This has been observed.
    Deduction: enough gravity can trap light.
    Deduction: a certain amount of matter can create that amount of gravity
    Deduction: enough matter can be in one place to exceed that amount
    Deduction: since we don't know singularities exist, that amount of matter may or may not be a singularity but will not be limited to the amount of gravity it produces

    According to you guys at least one of those deductions is wrong. It sounds like you are saying if there is no singularity then there is an upper limit to gravity no matter how much matter is there and that limit is less than the amount needed to trap light or no amount of gravity can trap light without a singularity. :confused:

    Unfortunately I don't have Powerpoint to watch that slide show. :mad:
     
    Last edited: Nov 5, 2005
  18. hellfire

    hellfire 1,047
    Science Advisor

    General relativity is not easy and may defy "common sense" (based on newtonian conception of gravity) sometimes. The "amount" of gravity you are looking for, is given only by the existence of a singularity. Otherwise you will get no event horizon. This is the claim of that singularity theorem.
     
  19. The event horizon is just the radial distance from the center of mass where the speed of light is the escape velocity, and the radius of the object does not extend beyond this radius. Then all particles have an event horizon by that definition if you solve with newtonian gravity, so that makes me ask a question. Is there some radial distance and/or some mass where GR fails to compute the even horizon? Does this have anything to do with the uncertainty principle?
     
  20. Danger

    Danger 9,879
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    My contention here is that you cannot assemble that much mass in one place without it collapsing into a singularity due to its own gravity.

    Can't help with your question, Jonny. It's way beyond me.
     
  21. pervect

    pervect 7,901
    Staff Emeritus
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