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Change the FAQ? Also, how can 0-volume objects collide?

  1. Feb 18, 2016 #1
    (1) Maybe should've messanged this, but given LIGO, the FAQ might need a light edit. It says,

    "Gravitational waves have never been detected directly, but the loss of energy from the Hulse-Taylor binary pulsar has been checked to high precision against GR’s predictions of the power emitted in the form of gravitational waves. Therefore it is extremely unlikely that there is anything seriously wrong with general relativity’s description of gravitational waves."

    (2) How can two zero-volume objects collide? Shouldn't they instead orbit one another infinitely?

    Alternate version: Asked differently, but intending to be the same question, If two black holes collide, aren't we obliged to assume they have both (A) a center of gravity and (B) stuff around that center?

    (I searched the backlog of posts and read the FAQ, but still, apologies if this has been asked and answered ad nauseam.)
  2. jcsd
  3. Feb 19, 2016 #2
    Unsure why you ask this: particles are routinely smashed together both experimentally and in nature. "Zero volume objects' have effects that are non zero, such as gravity and electromagnetism.

    In addition, at least some theorists believe that information inside a BH resides in a dual form just outside the event horizon [on a stretched horizon] which is a form of the 'holographic principle'.

    I've never seen any description of a BH as a 'zero volume' object. Have you a reference, a description, of what that means? The singularity at the center of a BH is but one portion of a BH whose effects at not point like.

    In general, gravity causes the elementary particles and elements from the origin of the universe to coalesce and form stars, planets, solar systems and galaxies.

    Black holes are formed via the power of gravity. Once a horizon forms, anything inside is causally disconnected from the rest of the universe. Outside the horizon and information which may reside there, is typically an accretion disk which consists of matter 'stuff' and radiation that has been attracted exterior to the horizon. Supposedly there is a singularity at the center of the BH but exactly what that is, and whether is is actually a singularity is unknown; It is the not clear that our model based on General Relativity is applicate at sub Planck [tiny,tiny,] scales.

    In addition, jets of matter are typically ejected along the axis of rotation of a BH but I believe these are generally thought to arise from interactions with the exterior disk, not from the interior of the BH [not inside the horizon].

    I checked that comment here: https://en.wikipedia.org/wiki/Astrophysical_jet seems ok for now.
  4. Feb 19, 2016 #3
    Here's why I'm asking.

    Somehow black holes seem different in my head than protons etc.

    You're saying, for the purposes of collision, not so?
  5. Feb 19, 2016 #4
    ok, and the earth is different than either.....so??

    One of the biggest differences is that a BH has an event horizon: causal effects, like the effects of mass, can only go in, not out.

    Did I say that? BH collide; protons can me made to collide although they have to be really fast moving, very energetic, to overcome the strong electromagnetic repulsion. BH are naturally attracted to each other by gravity.
  6. Feb 19, 2016 #5


    Staff: Mentor

    In Newtonian mechanics you are correct. The gravitational PE can decrease without bound as two Newtonian point particles approach each other. If they are radiating that energy away then an infinite amount of energy could be extracted.

    I don't know if that is resolved in GR, or if it requires a quantum theory of gravity to resolve.
  7. Feb 19, 2016 #6


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    I don't know of any solution in GR that could really represent a point particle, so it seems to me GR solves the problem by banning them. A BH with horizon is obviously not a point particle, and only finite energy can be released from merger. A naked singularity (e.g. on super-extremal Kerr BH) is not really a point particle either since the singularity is not a position in the manifold, and is definitely not just a missing point. I believe you would only get finite energy merging two naked singularities.

    [edit: for many purposes, point particles in GR are modeled as the limit of small bodies, taking mass and volume down together in such a way as never to be a BH. But this is really a formal definition of a test body, as the only mass you can give it is zero].
    Last edited: Feb 19, 2016
  8. Feb 19, 2016 #7

    Mister T

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    Are you thinking that a collision is surfaces of the objects making contact?
  9. Feb 19, 2016 #8

    I'm assuming that's incorrect?
  10. Feb 19, 2016 #9
    Maybe your response wasn't directed at me, but it's an assumption I was making, for sure.
  11. Feb 19, 2016 #10

    Are you thinking that a collision is surfaces of the objects making contact?


    I'm assuming that's incorrect?

    You'll have to decide which objects you wish to discuss: point particles, neutron stars or black Holes.
  12. Feb 19, 2016 #11
    Black holes. Like everyone, I care about LIGO for the time being.
  13. Feb 19, 2016 #12
    Wait wait wait.

    I care about the singularity causing the black hole. Yes. That.
  14. Feb 19, 2016 #13


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    Singularities are not points in space.
  15. Feb 19, 2016 #14
  16. Feb 19, 2016 #15
    oykl: let's not worry about that here, ok ??
    it's a theoretical entity and is hidden from our measurements behind the horizon of any black hole; that is, it is causally disconnected from our part of the universe as I noted in post #2.
  17. Feb 19, 2016 #16
  18. Feb 19, 2016 #17
    Oh, also—thank you so much for your attention! Seems to be a robust community here, and I appreciate the guidance.
  19. Feb 19, 2016 #18
    I don't know if this is too technical for me, but Wikipedia says . . .

    "The Final Parsec Problem

    "The natural separation of two supermassive black holes at the center of a galaxy is a few to a few tens of parsecs (pc). This is the separation at which the two black holes form a bound, binary system that must lose energy somehow before the black holes can merge.[13] To generate gravitational waves at a significant level, the binary must first shrink to a much smaller separation, roughly 0.01 - 0.001 pc. This is called the "final-parsec problem".[14] A number of solutions to the final parsec problem have been proposed; most involve the interaction of the massive binary with surrounding matter, either stars or gas, which can extract energy from the binary and cause it to shrink. For instance, gravitational slingshot ejection of passing stars can bring the two black holes together in a time much less than the age of the universe.[15]"
  20. Feb 19, 2016 #19


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    Zero volume objects aka point particles behave weirdly in Newtonian physics, the issue is more fundamental than General Relativity. See for instance "Off to infinity in zero time", http://www.ams.org/notices/199505/saari-2.pdf, and/or Baez's series of articles on "Struggles with the Continuum" here on PF. I'll quote a bit of the former reference since I suspect most won't bother to read it and it illustrates some of the problems with the idea of point particles even in Newtonian physics.

    I won't quote Baez's reference - there's about seven links, there worth a read if you have the interest, look them up. But to say a lot more would be a digression.

    The good news is that GR doesn't really have point particles (except perhaps for test particles) as others have mentioned - it has fields and continuous distributions of matter of finite density. So "point particles" don't have to collide in an inspiral, at least in any mathematical sense. From a purist point of view, what is analyzed instead is the limit of field interactions when the source of the field is "very small" so the internal details of the source can be ignored. This is usually popularized as "point particles" - but it's not, really.
  21. Feb 19, 2016 #20


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    Staff: Mentor

    Am I missing something obvious here or did other people misunderstand this? You are asking about black holes, right? And suggesting they are zero volume, right? Black holes are not zero volume. Their volume includes everything behind the event horizon - regardless of whether the singularity actually exists or not.
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