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Black Holes have no gravity below the event horizon? - How can that be?

  1. Mar 3, 2009 #1
    blechman's statement:

    Gravitons are emitted FROM THE SURFACE of the event horizon (remember: gravitons are massless, and therefore move at the speed of light). THAT's what we see. There is NO information (gravitational or otherwise) that can escape from INSIDE the black hole. This is not a contradiction of the existence of gravitons.

    dilletante's response:

    If a planet orbiting a black hole experiences the same gravity as it did before the star collapsed, it seems necessary and coincidental that the event horizon would emit the same quantity of gravitons as the original star before collapse. If the gravitons at this point are virtual particles, they certainly do not correspond well to the number of virtual photons emitted, which by virtue of "blackness" are apparently considerably less than originally emitted by the star.

    Also, before the star collapsed, where were the gravitons emitted from? The surface of the star, or perhaps the center of mass, or maybe the theoretical event horizon? I suspect there are mathematical calculations to explain these things but as a layman, I am confused.

    How are the gravitons above the event horizon in knowledge of what's beneath them, if there is no information passed beyond the event horizon to them?
  2. jcsd
  3. Mar 3, 2009 #2


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    The idea of a particle as quantum of a field works well and intuitively in flat or nearly flat spacetime.

    The quantum theory of fields when constructed on highly curved spacetime does not rely on particles. So it is a bad idea to think in terms of gravity being transmitted by gravitons in such extreme cases as the black hole.

    Therefore the discussion by blechman anbd dilettante that you cite is rather meaningless.

    The basic entity is the field, as always.
  4. Mar 4, 2009 #3


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    marcus has a point - it is misleading to think of "gravitons" near a black hole, since as I said in the thread this post comes from, quantizing gravity near a black hole is an open question.

    What I was trying to say from my post was that "information" about the black hole lives on the event horizon. The interior of a black hole is out of causal contact with an observer outside, and therefore the gravity field (just like the electromagnetic field) cannot transmit information from inside the black hole. This is the basis of the Bekenstein-Hawking entropy formula for a black hole. Dusty_matter: I don't know what you're level of expertise actually is (I should've asked earlier) but if you don't know about this result, I STRONGLY encourage you to look it up on Wikipedia or perhaps someone on this forum talks about it, or if you're higher-trained, find a textbook, because this theorem is deeply connected to the understanding of black holes, and it is hard to explain what's happening if you don't understand this.

    Armed with this result, I move on:

    This is the first step toward the "information paradox": due to the so-called "no-hair theorem" of black holes, all black holes look the same! The only descriptive numbers about a black hole are its mass, charge and spin (classical spin, now). The information paradox is basically the statement that as things fall into the black hole, they can go to changing these numbers, but you can imagine an infinite number of ways these numbers can change. Therefore the "history" of a black hole is not unique.

    This is a certifiable disaster when trying to quantize gravity! Quantum mechanics requires at its core the condition of "unitarity", which in layman's terms can be read as: "the sum of all probabilities must equal 1." But since material can fall into the black hole and "vanish" from sight, this violates the unitarity condition. This is a serious dilemma when trying to make sense of quantum gravity.

    Why? Because Hawking tells us that black hole event horizons radiate and shrink. And furthermore, they radiate like a "blackbody," that is an object whose radiation carries the LEAST information. So as the black hole radiates and shrinks, and ultimately fizzles out of existence, it takes whatever is inside with it (the radiation carries no information, so it must remain inside the black hole). And THIS is the information paradox: where did all the matter that fell into the black hole go?

    Answer this question, and you will be praised as a supergenius for life! :wink:
  5. Mar 4, 2009 #4
    Very good, blechman. I will look up some information on Bekenstein-Hawking entropy. I have no expertise. I am only a life long student, and at this time I am not taking any sides with these issues. I am only trying to learn. Thank you for letting me peek over your shoulder, and allowing me to try and grasp, what is to me some deep stuff. As far as: "Where did the matter go?" Sounds like some good material for the next Dr. Suess book. Thank you for answering my questions.
  6. Mar 4, 2009 #5
    <mode=Devil's advocate>
    One major reason for information loss by black holes is that they are characterized by 3 numbers : mass, charge and angular momentum. How do we outside observers know of the electric charge inside ?

    My point is really : why bother using gravitons here ? You already have a problem with photons. This is well known.
    How does the gravity get out of the black hole?
  7. Mar 4, 2009 #6


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    When talking about black holes, one must be very careful about time. To outside observers black holes never quite form, i.e. they take an infinite amount of time. The gravitons "seen" by the outside observer come from the black hole to be.
  8. Mar 4, 2009 #7
    I have heard mathman, that time supposedly is a problem when it comes to the creation of Black Holes, but visually it seems to be a different matter? Do we not have a mega large BH at the center of our own galaxy which emits no light? Does not the Cygnus X-1star system contain an invisible partner? I believe that they have even seen a star that got too close to our Milky Way's central BH completely disappear. It never came around the back side. It seems to me that BH's act in real time to the outside environment, all the while emitting no light. If something took forever to spiral into a BH, then how would the jets form, that we see shooting out from some of them?
  9. Mar 4, 2009 #8
    The matter in BH jets never crossed the horizon. It did not spiral into the black hole, it did not need to take forever to be collimated in the jet. Mathman is perfectly right. If you looked at something fall towards a BH, you would never see it reach the horizon. It would slowly freeze while approaching the horizon from your outside point of view.
  10. Mar 4, 2009 #9
    How can BHs merge then? The BBC news has an article in their science section on two BHs merging. If BHs can never fully form, then they shouldn't be able to merge then either, right?
  11. Mar 4, 2009 #10
    The theory says that you can not see an object cross the horizon when being an outside observer. There is no valid reason to doubt this theoretical point. It's pretty simple.
  12. Mar 4, 2009 #11
    So you're saying that BHs can't merge?
  13. Mar 4, 2009 #12
    No I am not.
  14. Mar 5, 2009 #13
    I am having a hard time grasping this. Two BHs, equal two event horizons in which nothing is supposed to cross over through either of the horizons. Time suposedly stops at the horizon and so neither one can penetrate the other. It would look like two balls stuck to each other with neither of them being able to merge into the other. That is if nothing can cross over the event horizon. You would have two inpenetrable shells. Is that not so?

    Also, if the matter or information behind the event horizon is lost then how would the two horizons be able to add up?
  15. Mar 5, 2009 #14


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    Hey guys! Hold up a second. There is some serious confusion going on here.

    First of all, let's be clear. Things CAN fall into a black hole. PHYSICALLY! OK, what humanino and mathman are talking about is that in the test particle approximation (which means that we are standing very far away from the black hole, watching a point particle fall into the black hole) it is true that we do not actually see the particle entering the black hole itself. HOWEVER, this is precisely because the test particle approximation breaks down! That is, we have been illegally ignoring the gravitational field of the test particle itself! If you take this into account, you can show that indeed, the particle DOES fall into the black hole in finite time.

    FOR THE EXPERTS: Think of it like this. As an object of mass m approaches a black hole of mass M, then as the object gets closer, the composite object (since we can no longer think of them as two separate objects) now has mass M+m, and so the event horizon expands to swallow up the test particle! So even in Schwarzschild coordinates, this is true!!

    Now if we think about black holes merging, you can certainly see that the "test particle approximation" is ludicrous! This is the proper resulution to the paradox this thread has stumbled onto.

    P.S.: Thanks to my friend (a string theorist) for helping me with this post.
  16. Mar 5, 2009 #15
    blechman.. For me it sounds like fresh and one step level up explanation. thanks.
  17. Mar 6, 2009 #16
    Yes, That does make a bit more sense.
  18. Mar 7, 2009 #17
    When we say the time dilation is infinity at the event horizon we assume that the event horizon is a mathematical surface, the space is infinitely continuous and the falling matter is a mathematical point object, so the matter can approach the event horizon infinitely close, but these assumptions are not all true. This assumption is too mathematical, for example all falling object have a size. Also there is good explanation by blechman above.

    So, we are changing our view of BH:
    The outside scientist will observe that the falling object to BH cross the event horizon, and black holes are formed and merge.
  19. Mar 7, 2009 #18
    Your points are irrelevant. The property that we can not see an object crossing an event horizon is a topological property : you can not break this property without breaking the event horizon itself.
  20. Mar 7, 2009 #19
    Humanino I agree with your statement, if we are to go with the assumption that time stops at the event horizon of a BH. This assumption would therefore make mergers an impossibility.
    But BHs do form, mergers do happen, BHs are black, and matter does cross over the event horizon of a BH. So then the assumptions must be wrong. Time must not stop at the event horizon of a BH.
    Gravity then may not be quantum in nature, but it may simply be the a warping of the spacetime fabric, which would mean that gravity comes not from the event horizon itself, but from the mass that is beneath it, which would also allow for the masses of BH mergers to continue to add up, and would also mean that the information of a BH is not really lost at all. It continues to exist. Maybe the true nature of BH’s has not been fully realized yet. Could we yet be still in our infancy of the understanding of things?
  21. Mar 7, 2009 #20
    One prob of our old assumption on event horizon is we think it is quite steady, but reality it may not. Big scientists say BH will release energy slowly and the event horizon will shrink in the long term, but for some time it can grow by sucking more materials. In this changing event horizon situation i think the falling object does not have to freeze to outsider, because the situation on event horizon has changed, it is not time dilation issue.
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