There’s something puzzling about black holes, if you stop to consider it. On the one hand, they’re objects so massive and dense — compacted into such a small region of space — that nothing can escape from it, not even light. That’s the definition of a black hole, and why “black” is in the name.

Image credit: James Provost,

Image credit: James Provost,

But gravity also moves at the speed of light, and yet the gravitational influence of a black hole has absolutely no problem extending not only beyond the event horizon, but infinite distances out into the abyss of space.

Image credit: Henze, NASA.

Image credit: Henze, NASA.

What’s the resolution to this puzzle? Jillian Scudder has the answer on Astroquizzical today!


  1. #1 David
    June 30, 2015

    Thanks for tackling this, Ethan! It’s a question I’ve wondered on for a while. In a non-quantum theory, the curvature of spacetime at one point influences the curvature at nearby points (hence, the Einstein field equation is a differential equation with local effects). “Nearby” refers to 4-d intervals. So, no problem propagating the gravitational field across the event horizon. But in a quantized theory, the interaction is mediated by exchange of gravitons between distant objects. How does the a graviton travel out of the black hole? There is no lightlike geodesic that connects the interior with the exterior. In other words, for the central mass to attract distant objects, the graviton has to escape from the black hole and cross the event horizon. This puzzles me. Thanks again, and, PS: love your blog.

  2. #2 Denier
    United States
    June 30, 2015

    I’ve reread this article now 4 times and don’t see where the question is ever answered. Is the second to last line containing the phrase “we don’t know exactly how” the answer? Or am I missing something?

    My takeaways are this:

    1 – If the Sun completely vanished, the Earth would continue to orbit the spot where the Sun used to be for another 8 minutes because gravity is information and it takes that long for the information to travel to us.

    2 – The Event Horizon represents the point in space where the gravity well becomes so steep that information would need to exceed the speed of light in order to escape.

    3 – Information about the mass of the black hole is being conveyed from inside the Event Horizon to outside the event horizon despite gravitational information traveling no faster than the speed of light.

    The article talks about rubber sheets, information speed limits, and what a black hole is, but never gets around to reconciling points 1, 2, and 3 above. Then at the end of the article is a line about how we really don’t understand it very well. Is my understanding of the article correct?

  3. #3 Omega Centauri
    June 30, 2015

    Though he didn’t say it, I think Ethan’s information speed limit gives at least a crude back of the envelope explanation: The information, that the mass has “vanished” can’t get any further than the event horizon because at that point space is moving towards the BH’s singularity at the speed of light. So the “outside” universe still “remembers” the original mass because it can’t get any info from inside the EH.

    I suspect in reality, you just have to solve the equations for general relativity, and handwaving explanations probably breakdown.

  4. #4 Sinisa Lazarek
    June 30, 2015

    Nothing has to escape from within event horizon. The gravity of BH to the outside observer comes from event horizon, not beyond it. And there is no need to tackle gravitons in order to understand this, just regular GR. In other words, just like from POV of observer outside looking at something falling in.. it would never actually cross the event horizon, but would take asimptoticaly an infinite amount of time to cross, thus all the curvature beyond the horizon and towards you, comes from the horizon, not from inside.

  5. #5 Denier
    United States
    June 30, 2015

    @Omega Centauri #3

    The universe isn’t “remembering” the original mass. If it were, adding additional mass wouldn’t add more gravity. The addition couldn’t communicate that it had been added but that is not what happens. The black hole seems to be relaying information about the current aggregate mass-energy residing inside the Event Horizon to the outside universe gravitationally. The question remaining is the same one sitting at top of the page: How?

  6. #6 PJ
    Perth, west Oz
    June 30, 2015

    The guest writer of this article is Jillian Scudder, with Ethan as host.

  7. #7 Denier
    United States
    June 30, 2015

    @Sinisa Lazarek #4

    Would that also hold true for angular momentum? Contracting bodies increase their spin rate to conserve angular momentum. Does a black hole spin at a rate consistent with all mass existing as a hollow shell on the Event Horizon?

  8. #8 Bootdinker
    Horathenon 5
    June 30, 2015

    That was a fantastically unclear answer to the question, but I think the actual answer might have been here:

    “The ‘movement’ of gravity at the speed of light limit can be considered entirely separately; while gravity has a broad extent across a wide swath of space, the speed of light imposes a limit on how quickly information about changes can travel across that space.”

    So gravitational information isn’t bound to the black hole, it’s only delivered at the speed of light limit. Going into why light information but not gravitational information is limited to the event horizon would have been helpful.

  9. #9 Wow
    July 1, 2015

    “1 – If the Sun completely vanished, the Earth would continue to orbit the spot where the Sun used to be for another 8 minutes because gravity is information and it takes that long for the information to travel to us.”

    We’d lose the light, not the gravity. So 8 minutes later, we see the sun go out. We don’t see the mass disappear.


  10. #10 Wow
    July 1, 2015

    “The universe isn’t “remembering” the original mass. If it were, adding additional mass wouldn’t add more gravity. ”

    Indeed. It doesn’t see the mass leave. No need to remember it: it’s still there.

  11. #11 Sinisa Lazarek
    July 1, 2015

    @ Denier #7

    I honestly do not know how fast BH spin, and if that spin in only related to mass, or are there other factors. Would have to research a bit in order to answer. But do know that the size of event horizon is directly proportional to the total mass. Charge, spin, mass… of BH are all calculated from event horizon and around it. And yes, in general, you can treat the event horizon as a hollow sphere holding all the mass and information on it about the things which fell in.

  12. #12 Wow
    July 1, 2015

    SL, spin is energy and that energy adds in just like any other does.

    Sufficient spin may spread out the shape of the singularity and thereby expose it.

    Here, though, is yet another case where denier has stated some phenomena and the *impression* is that this is somehow relevant. But nothing indicates what it is relevant *to*.

    I believe denier is just JAQing off here.

  13. #13 david
    July 1, 2015

    A rotating black hole has angular momentum in addition to total mass. The Kerr metric describes the situation. THere’s still an event horizon, which behaves just like the event horizon of a non-rotating hole. Outside the event horizon is an “ergosphere” which is a region of space that co-rotates, at speeds greater than light (relative to a distant observer). Objects in the ergosphere are dragged along, and radiate energy away. This is the reason many black holes emit xrays. The radiation reduces both the angular momentum and the total mass of the BH, which then approaches a non-rotating Schwarzschild metric.

  14. #14 Wow
    July 1, 2015

    Thanks for that, David.

    And it doesn’t seem to refute or complicate to breaking point SL’s points.

    So maybe you can point out what you mean, denier? We wait with baited breath.

  15. #15 G
    July 1, 2015

    Guess from before clicking on the article (just taking the title as a quiz question):

    In Einsteinian relativity, gravity is a distortion of the fabric of spacetime. Whatever means exist that mediate it, are related to the nature of spacetime rather than to gravity waves. Gravity waves are produced by gravity but are not identical with the effects of gravity on spacetime.

    OK, now I’ll read the article & comments and see how badly I’ve embarrassed myself. On the flipside, it’s good to encourage a social atmosphere where laypeople are willing to take chances by making guesses and learning.


  16. #16 Wow
    July 1, 2015

    Oh, no problem with people trying to learn. Too many internets have people who aren’t for doing that, though.


    No JAQing off in public. If you aren’t going to do anything with an answer, don’t ask the damn question.

    When you’ve thought of a problem, remember that the specialists will have thought of that already, don’t assume they’re all dumbasses just because you didn’t hear them when they looked into it

    Look before posting. As I posted on another thread, if you look and fail, when you’re given the answer you’ll remember it a hell of a lot more easily than if you didn’t bother trying. See:

    Don’t prosetylize. If you want to preach your version of reality, get your own show. If you don’t want people to take the massive piss out of you in public for your, hmmm, *unique* perspective on reality, don’t go to a public place and shout them out. And either keep NOMA or admit that science has disproven all claims of god so far tested.

    Hell, a good way of learning is to put the discovery in your terms and ask how close it is to the officially understood one. It checks if you’re steering off course too far.

  17. #17 Rich Feldenberg
    July 1, 2015

    It’s an interesting question that I hadn’t thought about before.
    I have always wondered how physicists reconcile the concept of gravity as the warping of space time with the theoretical graviton model of gravity. Maybe that is where relativity and quantum theory don’t go together so well?

  18. #18 G
    July 1, 2015

    OK, so I get approx. 25% for that, which isn’t quite a passing grade to put it mildly. First sentence, gravity well, yep, that was easy. Second sentence, confused handwaving. Third sentence, quasi-OK but not really because saying what it isn’t doesn’t say what it is. Information, missed that part entirely.

    ‘Tis better to live & learn, than to do neither;-)

    Though, I don’t see the basis for those who are criticizing the article for not having a complete explanatory mechanism: if it did, we would also have unified field theory and we’d all know at least something about it.

  19. #19 Sinisa Lazarek
    July 1, 2015

    @ G

    there isn’t much issue with the article. it might not give a precise explanation, but in general.. there is no gravity escaping from inside event horizon in the first place.

    It’s more the question itself that might lead one to be puzzled.. if it’s happening, how is it happening.

    Of course, this is when you take GR as it is.. a semi-classical field theory. The problem, and a big question mark, comes if you try to give the same explanation, but treat gravitational field as a a bunch of graviton particles. With all the quantum effects and an extreme scenario of a black hole.. But that picture is not necessary in order to explain how BH curves spacetime.

  20. #20 Denier
    United States
    July 1, 2015

    @Sinisa Lazarek #11 & david #13

    Thank you for your responses. After taking the night to think about it I realized the size of the collapsed object inside the Event Horizon was irrelevant. The spin of a BH is inferred by the properties of the accretion disc, but unless I’m mistaken what is really being shown is the effect of the frame dragging on the accretion disc. Being that it would be the same amount of mass underfoot and the same amount of angular momentum, the frame dragging at the given altitude of the accretion disc would be the same regardless of the true diameter of the collapsed object. So once again, the BH isn’t giving up anything that isn’t gravity.

  21. #21 Someone
    July 1, 2015

    Wow only gets on this website to try and start arguments over things that no one in their right mind would argue about. What a lonely miserable little child.

  22. #22 PJ
    Perth, west Oz
    July 1, 2015

    Then why add fuel to the fire?

  23. #23 Wow
    July 2, 2015

    Because he wants to silence words he doesn’t like. Because he wants to be a “Hero” fighting for others.

    Because he wants his voice to have power and meaning and he doesn’t have any other way to do it but to tone troll someone out of the way.

    Because it’s the only way to get godbotherers out of their “worldwide persecution”. Because there’s a War On Christmas!!!!!!

    It’s pretty obvious.

  24. #24 Jim Bot
    July 2, 2015

    “But gravity also moves at the speed of light …”

    Has this ever been measured? I think not, yet is stated as fact here.

  25. #25 PJ
    Perth, west Oz
    July 2, 2015

    Well, I guess we are all pretty aware empty cans make the most noise. Ignoring same usually causes that kind to leave after a while when they realize their words hold no weight or meaning.

  26. #26 Vinod Sehgal
    July 6, 2015

    In GR, gravity has not been described as a force but distortion in space is itself gravity. Distortions are in space ( space-time?) itself due to high dense material mass and they proceed ahead in space. In view of this , speed of movement of distortions of space in space is not comparable with movement of light in space. Light in space may move with some upper limit since that is the movement of e.m energy in space but why movement of distortions of space ( comprising of space only) should be limited by light speed ?
    Further, this also our assumption that distortions move forward like light energy. Since gravity in GR has not been described as some energy ( or force), therefore, there should be no question of transmission of distortions . It could be that dense mass creating distortions in space without any time lag

  27. #27 Vinod Sehgal
    July 6, 2015

    Since gravitation around a BH ( distortions of space) does not allow anything, matter or even light to escape event horizon, question of gravitation ( distortions in space) escaping from gravitation of a BH is superfluous. For escaping from gravitation, there should be some thing else

  28. #28 Wow
    July 7, 2015

    Please prove your claim.

  29. #29 Bill S.
    Harwich, UK
    July 7, 2015

    “Has this ever been measured? I think not, yet is stated as fact here.”

  30. #30 Wow
    July 8, 2015

    OK, Bill. Lets do some *science* here!

    Proposition: gravity goes faster than light. What are the consequences?

    Causality broken.

    Just like FTL Neutrinos, assuming they’re not tachyon events is the default until someone shows otherwise, and then the scientific route is to see if that measurement is wrong, first. Then if that measure manages to hold against all disproofs, it becomes a theory.

  31. #31 Wow
    July 8, 2015

    Oh, sorry, I see that you were quoting another, not asking yourself.

  32. #32 John Poppenhusen
    February 1, 2016

    Gravity is created by a black hole when it formed by the implosion of a giant star and then grows larger as it continually pulls in all matter within it’s reach which would include planets, moon’s and even smaller stars, as it grows. All forms of matter, including the gravitational energy would transform, as it crossed the Event Horizon, into the Spiritual energy state or reverse creation. This event is an interdimentional shift. Black holes are an integral part of this ongoing infinite process of death and recreation in the universe. This could theoritically be an interdimentional process between Galaxies.

  33. #33 John Poppenhusen
    February 1, 2016

    The last sentence from my previous comment should have stated Universes and not Galaxies. Sorry about that.

  34. #34 PJ
    Perth, west Oz
    February 2, 2016

    @32 I take it you have proof of the ‘event’ being an ‘interdimentional shift’ ? Reverse creation ? Hmmm ……

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