Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black hole and virtual gnomes

  1. Feb 3, 2012 #1
    Hello.

    On the "Ask an Astrophysicist" page on the NASA site, one can read the following:
    This raises three questions for a layman such as me:
    1. Do the scientific community agrees with this answer?
    2. Can the virtual gnomes break any rule and fulfill any wish as long as we don't detect them ?
    3. Should I trust the rest of the site?


    EDIT: Somehow the virtual photons transformed into virtual gnomes. I guess it corroborates that they can pretty much "do" whatever they like.
     
  2. jcsd
  3. Feb 3, 2012 #2

    tom.stoer

    User Avatar
    Science Advisor

    Sorry to say that but I've rarely read more mischief; nothing (no gravitational waves, no gravitons, no virtual particles) needs to escape from the event horizon in order to "create the gravitational attraction"; the gravitational curvature (and therefore attraction) outside the horizon has already been there before the collaps; in case of a radially symmetric collaps of a massive star to a black hole the spacetime doesn't change at all during the collapse; and neither in GR nor in (Q)ED it is necessary to explain the force entirely due to the exchange of virtual particles
     
    Last edited: Feb 3, 2012
  4. Feb 3, 2012 #3

    Bill_K

    User Avatar
    Science Advisor

    Doubly surprising, since one of the two authors of this comment is one of Bryce DeWitt's students.
     
  5. Feb 3, 2012 #4
    I read a recent article by Matt Strassler who commented that he didn't like the name virtual particles because it was misleading as to what they really were. In more or less his own words they were general disturbances that can be treated like the particles to whose field they belonged in order to perform calculations. Personally, my only real experience of the word "virtual" is via virtual reality. As such I've always taken the word to mean "not really, (but sort of)". Anyway, in view of a general annoyance amongst the QM community when "normal" people talk about virtual particles, I can only suggest this is a tongue and cheek nod to it.

    Virtual gnomes mediating blackhole reactions would be pure win, though.

    EDiT: Isn't the quoted paragraph how hawking radiation works?
     
  6. Feb 3, 2012 #5
    @Tom Stoer: To be fair, please read the linked article, the gravity is explained as you say. The quoted part with virtual photons is about a charged black hole and electromagnetic radiations.
     
  7. Feb 3, 2012 #6
    The electric fields of charged black holes are frame-independent so they aren't actually changing from before and after a black hole is formed...according to relativity. It's the same with gravity.
     
  8. Feb 3, 2012 #7
    I don't understand your answer, but since we're at it, I may as well formulate the paradox (at least to my layman's eyes) for which I was looking for an answer on the nasa's site:
    the mass and charge are within the event horizon of the black hole, yet they have an influence outside of the event horizon. What's the mechanic?
     
  9. Feb 3, 2012 #8

    atyy

    User Avatar
    Science Advisor

    They do say that for classical gravity. "In a classical point of view, this question is based on an incorrect picture of gravity. Gravity is just the manifestation of spacetime curvature ... Once formed, they and their gravity just are."

    I think the rest of the answer is for an electrically charged black hole in semiclassical gravity.

    :rofl::rofl::cry::rofl::rofl::cry::rofl::rofl:
     
    Last edited: Feb 3, 2012
  10. Feb 3, 2012 #9
    Basically with frame independence, it is what it is frame whatever frame of reference that you'd measure using time, how its measured isn't determined by time passing, but rather just by the physics itself which isn't relative.
    With the event horizon paradox, there's either this upper notion, or the concept that's what's carrying the force in the first place are virtual particles or virtual photons which carry forces like gravity.
     
  11. Feb 4, 2012 #10

    tom.stoer

    User Avatar
    Science Advisor

    I'll check the article again.
     
  12. Feb 4, 2012 #11

    Bill_K

    User Avatar
    Science Advisor

    Not everything has to be visualized of in terms of photons flying about. Classical electromagnetism and Maxwell's Equations have not yet been repealed! And a charged black hole is best described as a classical object. Tom's remarks about the gravitational field existing before the collapse apply equally well to the Coulomb field. The object that collapsed to form the hole had a net charge to begin with, and a Coulomb field surrounding it, and thanks to charge conservation this field persisted after the collapse. The Coulomb field is not something that needs to come flying out through the event horizon, it was outside to begin with.
     
  13. Feb 4, 2012 #12
    So even if the matter inside the black hole would disappear or teleport away, even if the black hole's inside would be emptied, the gravity field and coulomb field would remain?
    Those fields are not linked to the current content of the black hole, they're just remnants? They just are, without an active object causing them?
     
  14. Feb 4, 2012 #13

    tom.stoer

    User Avatar
    Science Advisor

    Matter / energy, angular momentum and charge of the black hole do not disappear during or after the collaps; they are still there. One can describe the black hole in terms of these entities. Far away from the black hole we cannot distingush its spacetime geometry from the geometry of an ordinary star of the same mass and with the same angular momentum.

    The problem we have is that we don't know in which sense they continue to exist, b/c we do neither understand in which sense pure geometry can carry these entities, nor do we have a theory wich resolves the singularity and tells us in which strange state matter and energy do exist.

    Anyway, what GR tells us is that there is a singularity surrounded by vacuum; this signals the breakdown of GR itself (b/c of the the singularity), but it does not tell us that something (physically) disappears in the singularity.

    (perhaps string theory and the fuzzball proposal provide a glimpse of an answer)
     
  15. Feb 4, 2012 #14
    Thank you, I understand a little better now (mainly, that I know nothing!).
     
  16. Feb 4, 2012 #15
    Agreed that gravity is due to the curvature of space time, however I think you've ignored the fact that changes in that curvature of space time propagate at c. Unless you are willing to postulate that super massive black holes are the result of the collapse of equally super massive stars then we must conclude that the gravity of black holes changes over time. There seem to be only two possibilities. Either matter passes through the event horizon, adds its mass to that of the singularity and that change in mass causes a change in the curvature of the space time of the black hole which must have traveled faster than c and backwards in time to escape the EH.

    The second possibility is that somehow as the matter approached the EH, the change in the curvature of space time became trapped at the EH. This begs the question of how, if matter passes through the EH, how does its gravitational field remain trapped at the EH?
     
  17. Feb 4, 2012 #16

    Bill_K

    User Avatar
    Science Advisor

    The gravitational field of a mass extends to spatial infinity, where it may be treated as a linearized field, and it's convenient to think of it as resolved into multipoles. The monopole part of the Riemann tensor contains the mass m and goes as m/r3, while the dipole goes as (d cos θ)/r4. Both m and d are constant, by conservation of energy and momentum. The quadrupole and higher moments can vary with time, and their time derivatives go as 1/r at infinity, leading to gravitational radiation.

    As one mass mA approaches another mass mB (say they are at a distance R) then for any r > R the coefficient in the monopole part of their combined field is the total mass, M = mA + mB. There is no "propagation at c" or "propagation at infinite speed" required for this, the fields always extended to spatial infinity to begin with, and being linear this is how they superpose. It's the gravitational analog of Gauss's Law. The higher multipoles grow smaller as the masses approach one another, producing radiation. Through nonlinear terms in the field equations the radiation feeds back into the monopole, reducing it slightly, reflecting the radiative energy loss.

    Eventually the field settles down and the monopole with mass M is all that's left. In nonlinear terms the two black holes have merged, leaving a Schwarzschild solution of mass M. Nothing is ever "trapped at the hole's surface" during this process.
     
  18. Feb 4, 2012 #17

    tom.stoer

    User Avatar
    Science Advisor

    I didn't ignore that.

    Certainly not. Distortion of spacetime due to infalling matter is already present when the matter is still outside the horizon.

    Gravity is not trapped inside the horizon. The horizon itself is distorted and eventually 'radiates away the deformation' forming a Kerr black hole with larger mass and changed angular momentum. Mass is strictly additive.
     
  19. Feb 5, 2012 #18
    Where in your first post did you address the velocity of changes in space time?

    Of course, anyplace there is matter there is distortion of space time, however as that matter moves towards the EH, both the position in space and time of that distortion changes.

    You seem to have jumped over a few steps making the last part of your statement not follow from the first part. The deformation at the horizon is a distortion in space time, not energy. Energy may radiate away but deformation doesn't.

    Also you fail to mention whether the "deformation" radiates away before or after the matter passes through the event horizon and how the mass of the energy radiated away compares to the mass that caused the deformation in the first place.
     
  20. Feb 5, 2012 #19

    tom.stoer

    User Avatar
    Science Advisor

    Nowhere, b/c it's not necessary. As I said there is no need to use virtual gravitons, gravitational waves or something like that to describe the static part of the gravitational curvature; it's not necessary to use virtual photons to describe the static part of the Coulomb potential; the latter problem can easily be solved in (Q)ED using Coulomb gauge w/o ever referring to (virtual) photons; you immediately get a 'static' 1/r potential - just do it

    Did I say that there's distortion in energy? I don't think so. Yes, it's distortion in space-time and it certainly radiates away; eventually the black hole approaches a symmetric Kerr solution w/o distortion.

    After mass passes through the event horizon the mass of the hole doesn't change; nevertheless it 'radiates away' the distortion.
     
    Last edited: Feb 5, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Black hole and virtual gnomes
  1. Black Holes (Replies: 2)

Loading...