Can black holes grow on their own?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by kaneda, Sep 3, 2009.

  1. kaneda Actual Cynic Registered Senior Member

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    We are told particles come from literally nothing, and normally return to nothing. Most of the inside of a black hole is nothing (other than gravity). Could particles form between the core and the event horizon and before they have a chance to vanish, be dragged to the core at almost light speed and become part of the core, so not vanish, causing a black hole to slowly accumulate mass without the addition of infalling material?
     
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  3. prometheus viva voce! Registered Senior Member

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    No. Quantum effects cause black holes to lose mass, not gain it. In your idea that particles are create inside the horizon, you have to remember that energy is still conserved - you can't get something for nothing. If if makes it easier you can think about it like this - on particle has positive energy and the other negative. when they hit the singularity it gains mass from the positive energy particle and loses it from the other. Net effect zero.

    Quantum effects can change the mass of a black hole, but the important local is the horizon where the pairs (or more) of particles can get separated. When you do the calculation you find the black hole emits particles and loses mass, since the energy of the particles has to come from somewhere. The only possibility is from the black hole.
     
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  5. kaneda Actual Cynic Registered Senior Member

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    Don't you only get an anti-particle too where it is down to charged particles, so conserving charge?

    As to Hawking radiation, you have two particles which would normally annihilate each other atomically close in an incredibly powerful gravitational field. One escapes and one is pulled inside. Surely "atomically close" would make sure that both went in or both escaped (or both annihilated)?
     
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  7. AlphaNumeric Fully ionized Registered Senior Member

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    Anti particles will have opposite charge due to charge conservation but uncharged particles can still have antiparticles, it's just that if a particle is charged then it'll appear in pairs.

    If they both go in the smae direction then they will annihilate and nothing changes, but every so often one of them goes a different direction to the other. Just because something is unlikely or rare doesn't mean it's impossible.
     
  8. prometheus viva voce! Registered Senior Member

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    Also, virtual pairs of particles have a total energy of zero, just like a total charge of zero. a virtual pair that hits the singularity will have no effect on the mass of the black hole since one particle will raise it and the other will lower it by the same amount.
     
  9. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    I thought this was the mechanism behind Hawking Radiation? If this is the case, then somehow the mass should decrease.
     
  10. D H Some other guy Valued Senior Member

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    Like Vegas, anything that happens inside the event horizon stays inside the event horizon. Since the total energy of a virtual pair is zero, virtual pair production inside the event horizon has no effect.

    Hawking radiation pertains to a virtual pair that pop into existence just outside the event horizon. If one of those particles has enough energy to escape the black hole and the other does not, the particles will become real. The one with positive energy will escape the black hole and the one with negative energy will fall into the black hole -- thereby reducing the mass (energy) of the black hole.
     
  11. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Ahh right. This I knew---I thought this is what we were talking about. This will teach me to read the thread, as opposed to just the last post

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  12. prometheus viva voce! Registered Senior Member

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    Exactly DH, although the original calculation by Hawking says nothing about virtual particles. Instead you put in a scalar field in the vacuum state from the point of view of some observer, and an observer at infinity sees a thermal spectrum who's temperature is inversely proportional to the black holes mass. Hawking speculated that it was caused by pairs of particles that we created near the horizon, one of which falls into the black hole and the other escapes.

    As is quite often the case, this isn't quite the end of the story - since the particle that falls in effectively has negative mass it cannot fall in because it is repelled by the black hole. Instead it must quantum tunnel through the horizon. If you don't like the idea of negative mass then you can think of a pair of virtual particles created inside the horizon and the real particle tunnels out - both methods are equivalent. The tunnelling calculation was first done by Wilczek and Parikh - it's a very nice paper that's surprisingly accessible. If you haven't read it I recommend that you do.

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  13. DRZion Theoretical Experimentalist Valued Senior Member

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    At a risk of sounding somewhat backwards, I ask for some experimental proof of quantum fluctuation. One good link please.

    I'm beginning to like quantum fluctuations, so I'd really like a reason to believe they are real.

    Does this mean that the black hole radiates more as it's mass shrinks?
     
  14. prometheus viva voce! Registered Senior Member

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    The Casimir effect.

    Yep. What happens when the mass of the black hole goes to zero has been studied fairly recently as well. The temperature doesn't go to infinity but smoothly goes to a maximum value. I don't know too much about this but when the horizon becomes of the order of the size given by the uncertainty principle the semi classical description that was being used to calculate the inverse proportionality no longer holds. The black hole still disappears but the temperature never diverges. I heard that in a conference talk - If I can find a reference I'll post it here.
     
  15. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    I thought about something last night.

    If the time-like and space-like directions switch inside the event horizon, then shouldn't a virtual particle on the inside be able to tunnel out of the horizon?

    This always confused me. I don't see how you get virtual particles from the Bogliubov transformations.

    For what it's worth, a friend of mine maintains that that paper is flawed, see here.
     
  16. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I bet it still make one hell of a bright UV or gamma ray flash as the last millia gram becomes radiant energy.
     
  17. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    Ben: I reviewed that paper. Quite interesting. You have an interesting friend!
     
  18. kurros Registered Senior Member

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    This has been confusing me recently, perhaps you can help me out. You see, the picture of pair production I had in my head was that there is NOT a total energy of zero in the pair, which is why they are virtual and have to annihilate within a short time to "repay" the energy to the vacuum. However, I have since been doing some tree-level calculations and have learned that we do these by treating the anti-particles as have negative energy, so in effect a particle-antiparticle pair DOES have zero total energy. This seems wrong to me though, since, for instance, one cannot produce electron-positron pairs from gamma rays unless the gamma ray has an energy greater than 1022 MeV (admittedly these are real and not virtual). I suspect I'll learn the answer properly when I get to calculating loop-level diagrams, but perhaps you can shed some light on it for me in the meantime.

    I was wondering about this too. I know they are planning an experiment at the LHC to try and see if gravity affects matter and anti-matter differently, but I have no idea what any theory says on this issue. I suspect it is tied into the above question about how we treat anti-particles and whether they "really" have negative energy, but perhaps you can tell me something solid.
     
  19. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    Minor correction. Electron/positron annihilation releases two gammas of 1022 KeV total, or 1.022 MeV. Each gamma is a 511 KeV photon. Thus, one needs at least a 1022 KeV photon to liberate an electron/positron pair from the "Dirac Sea" of virtual particle pairs.
     
  20. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I do not Think that is correct.

    True to PRODUCE an electron / positron pair out of the vacuum you need to lose energy from some source equal to 1.022MeV, but you do not seem to understand that Dirac as I do - As postualting a completely filled set of electron state with negative energy to explain why that solution of QM equations was not observed.

    Electron being fermions obey the Pauli exclusion principle. If the negative states were completely filled then not one could make any transisiton within the negative sea states - they would be undetectable. If there were to be an absence of one of the negative electrons that would look like / behave like / a positive electron and be detectable.

    How could such an absence happen? Well one of the higher energy negative states (near zero energy, say with only E = -80ev) could be exceited into a postive energy solution state. Then with 100ev you make the both an absence in the negative sea (thing that acts like/ is indistinguishabel from / ) a positron with 80eV and an electron with 20eV. You do not need to make any mass - the electon was already existing in the filled negative sea.
     
    Last edited by a moderator: Sep 7, 2009
  21. Walter L. Wagner Cosmic Truth Seeker Valued Senior Member

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    BillyT:

    I'm not certain where you are coming from on this. The correction was that he referred to 1022 MeV, not 1022 KeV. Almost a typo [leaving out the decimal point of 1.022 MeV].

    To create an electron from the Dirac Sea requires an input of 1022 KeV of energy, so as to make an electron [511 KeV rest mass] and a positron [511 KeV] rest mass. The energy of the photon is converted into the energy-equivalent of the rest-mass of the two particles. This is the exact opposite reaction of electron/positron annihilation, which converts the rest-mass of the electron and the positron into two opposite-direction gamma photons of 511 KeV energy each. We make use of this every day with our PET/CT scanners when we inject our patients with positron emitting radioisotopes such as (almost always, though we do use some other radioisotopes on occasion) F-18, typically tagged to glucose for metabolic studies, or sometimes to other chemicals for DNA synthesis studies, etc.
     
  22. kurros Registered Senior Member

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    Yes I missed a decimal point, my bad. Does anyone have a non "Dirac sea" way of looking at this? I don't think anyone really believes such a thing exists these days and it always sounded like crap to me anyway.
     
  23. kurros Registered Senior Member

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    Oh I see now. the properties of virtual particles prom was talking about are specific to the environment around a black hole. I was confusing this with a statement about virtual particles in general. I still don't know why virtual particles can be created with net-zero energy at the event horizon of a black hole, but this is a good start. Going to read now...
     

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