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Why don't black holes disconnect themselves from the universe?

  1. Apr 18, 2010 #1
    Gravitons are considered to be the mediating particles of gravitational force just as photons are considered to be the mediating particles of electromagnetic force. Both have zero rest mass. If extreme gravity of black holes does not allow photons of light to escape, why does it not do the same with gravitons?

    In other words : Observations of the cosmos show that black holes exert tremendous influence on the nearby stars to the extent of deforming them. It means in spite of extreme gravity, gravitons are able to ‘operate’ between a black hole and a nearby star. Why doesn’t the black hole ‘suck’ the gravitons just like photons and disappear/disconnect from the universe?

    Thanks in advance.
  2. jcsd
  3. Apr 18, 2010 #2


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    Gravitons (if they exist*) propagate a change in gravity. No gravitons, or gravity waves, are required to move simply for gravity to exist.

    *I'm no expert, but I think that gravitons have neither been proven nor disproven to exist. Gravity waves are, however, a necessary part of general relativity, and propagate at the speed of light when there is a change of gravity.
  4. Apr 18, 2010 #3


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    I think you are 'over egging' the effect of black holes in general.
    You can expect black holes with masses the same as small stars or less. They will have no greater effect on a nearby star than an equivalent mass in the same place. I'm not sure what "observations" you refer to but they can't apply to all black holes.

    I think the way you are extending what you have found out about black holes into such a dramatic scenario is a bit speculative. Have you a serious reference to this?
  5. Apr 19, 2010 #4
  6. Apr 19, 2010 #5


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    This question goes to the heart of a theory of quantum gravity, which has yet to be solved. What we do know is gravitational waves exist and gravity has properties that distinguishes it from photons. For another less than satisfactory explanation see http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980601a.html
  7. Apr 19, 2010 #6


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    As I suspected, this link refers to Super Massive Black Holes. Not just any old black hole! If we had a black hole in place of our Sun, we'd go round in the same orbit as we do at the moment (just a bit more chilly).
  8. Apr 20, 2010 #7
    Your contention has raised a question in my mind.

    It's only the super massive black holes that 'tear apart' the nearby stars. Whatever was there before the formation of a massive black hole (an extremely big star etc.) would not have 'tore apart' the neighboring stars.

    It means when something turns into a black hole something special to its gravity happens. In other words, if our sun turns into a black hole, the earth would not revolve around it as it is doing at present. It may actually be sucked up by the black hole(of our sun).

    I leave full scope of my argument being wrong.
  9. Apr 20, 2010 #8


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    I suppose that would follow, unless there is some reason for a 'threshold' effect, for big enough black holes, which doesn't occur for 'normal' ones. My understanding stops way before that, though.:smile:
  10. Apr 20, 2010 #9
    No, nothing special happens...

    A supermassive black hole FORMS differently, and being "super massive" it has the capacity to "do more damage". That said, if you imagined an impossible star with the mass of HUUUUGE black hole, then allowed it collapse, you would still be able to orbit it at he same distance you would have orbited the original star.

    Of course, in real life that BH is spinning like mad, has a truly enormous Ergoregion (see my name), has a MASSIVE accretion disk (which is a pretty violent place), and is probably blasting radiation from its "poles" (see galactic jets).

    That being said, if I light a match, and you light 1 million matches it's still fire. A pound of C4 vs. a Ton of it... it's still the same deal.

    Finally, if you magically turned Sol into a black hole with PRECISELY the same mass (this doesn't happen in real life), and you don't have any transitional period... then yes, you would orbit it just as before. The changes would be: Loss of Solar Ejecta, light, heat, and we'd probably be bombarded by lethal radiation. That doesn't change the gravity however... but if you get too close.... then it changes.

    As terrible as the "rubber sheet" analogy is, in this case it's useful. The incline does become more extreme as you approach the BH... the difference is that eventually escape becomes impossible. The absolute range of the "dent" in the sheet never changes, only its geometry.
  11. Apr 20, 2010 #10


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    I feel it necessary to spell out explicitly that the gravity of the object DOES change were it to transform from a sun to a black hole. What Frame Dragger is saying (and is commonly quoted) is that this change is negligibly small at the distance of the Earth's orbit. It is not as if there is some magical cutoff beyond which there is no difference, it is simply that we can safely ignore any changes in gravity at this distance. This is what we mean by the gravity changes (appreciably) when you get "too close". Of course, this "too close" limit is defined by how precisely you are making your measurements.
  12. Apr 20, 2010 #11
    All true, but to be fair I was presenting an impossible scenario in which our sun intantly and without emission or perturbation, becomes a BH. Obviously the real thing is quite violent, and I have doubts as to how well we'd orbit a BH that was... well... Frame Dragging.

    Bottom line, sure, changing the geometry is going to have an effect, but that's why the caveat is, "from a stable orbit". This also does nothing to distinguish a supermassive BH, from a stellar-mass BH. Yes, you get a mushroom cloud from the MOAB, but not from lighting a puddle of gasoline... still.. the basics are unchanged.
  13. Apr 20, 2010 #12
    The question asked by Deepak is a very good one, and one that I have asked myself. I suppose the obvious answer is that gravitons are not affected by gravity because their exchange is responsible for the gravitational force (in accordance with a quantum theory of gravity, which of course we don't have). Truly, though, I don't know the answer, even though I'm a physicist, and I wish one of the high-powered physicists on PF woud supply an answer, or at least speculate about it. The replies to the question I have viewed so far seem to skirt the issue.
  14. Apr 20, 2010 #13
    Welcome to PF!

    Which question posed? There have, in fact, been several iterations now. Anyway, what branch of physics are you involved in, and how do you feel that people here, myself included have skirted this issue?

    For a first post, you certainly don't mince words, but you also haven't offered much in the way of detail for your critique. It's a bit unusual to join a site, and your first post dismisses all present and asks for a PF Mentor. A less charitable person than myself would be suspicious that you are not new here at all. :smile:
  15. Apr 21, 2010 #14
  16. Apr 21, 2010 #15
    Thanks for your detailed answer.
  17. Apr 21, 2010 #16
    My pleasure.

    @crager: Hmmm... my first reaction is: "who the hell knows?!". I still find the notion of gravitons + geometric gravity = Headache. I'd love to know the answer however.
  18. Apr 21, 2010 #17


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    The OP asked - I think - the following question:

    If a BH is called a BH because "not even photons can escape from it", then how come that gravitons can escape from it ?

    What has been discussed in this thread is that, indeed, at a sufficient distance from a BH, gravity acts in the same way as if the mass making up the BH was made up by a less dense, non-BH object. So yes, the earth will orbit the sun-turned-into-a-BH in the same way as when the sun was still there in its normal, "diluted" form. But the question is WHY ?

    In general relativity, that's fairly straightforward to show. But that is a classical theory. But in the quantum theory of gravity which we don't have yet, the question can be posed of how the heck do the virtual gravitons get out. They have to be virtual because, as someone noticed, real gravitons are supposed to describe quanta of gravitational waves. In fact, gravitons are the quanta of a *linearized*, "weak-field" theory of gravity waves (and are purely hypothetical). We don't know what is supposed to get out of a BH in a quantum context. (for sure, *I* don't know it :smile: ).

    BTW, you can also ask: how does an electric field get out ? Because a BH CAN be electrically charged a priori. And even though *real* photons won't get out (light will not get out), the Coulomb field (and so the virtual photons which make up the static coulomb interaction) do "get out".
  19. Apr 22, 2010 #18
    Do magnetic fields interact with gravitational fields . Can you bend a magnetic field with a gravitational field .
  20. Apr 22, 2010 #19
    Sure, just consider a gravitational lens. Light = EM radiation after all.
  21. Apr 22, 2010 #20
    so then gravitons interact with magnetism .
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