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EM field of black hole

  1. Apr 1, 2004 #1


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    If a black hole (BH) has a net charge, it should have an electric field. If it also spins, it should also have a magnetic field. The photon is the messenger particle of the EM field. Can these photons escape the BH's gravity to cause the fields to manifest? Do these photons only exist outside the event horizon?

  2. jcsd
  3. Apr 1, 2004 #2


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    This was covered once before, but I'll repost the info here:

    One thing that I have noticed, after reading very many pages of info I found on "classical" Hawking Radiation, is that it was conceived, and most often described as in its original form as applying to a static, non-rotating, non-accreting and chargless black hole in the original Schwartzchild configuration as a simple mass-only expression of the Rs. All of the virtual particle pair production scenarios are based on this and require one particle to "fall back" with the other escaping as a real particle causing a mass loss of the BH.

    However, many other sites and past PF threads have noted that it is almost impossible to form a BH with no angular momentum (spin). Even the "no hair" statement was that a BH has only three observable properties; (1) mass, (2) angular momentum and (3) charge (usually net zero). But a lot of recent discoveries (and older theories) have added one new property that is (4) magnetic field. At first, it was thought that a magnetic field would only surround a BH that was accreting matter, but Khanna and Zeldovich has shown that all black holes will have a magnetic field. There is also the term "Hawking Process" (Effect?) appearing which includes/combines the original HR work with work of others as mentioned as well as Thorn and especially Kerr (for spin) and Newman (for charge). The "Kerr-Newman" BH. (A source quote:) “Finkelstein's Black Hole, which shows how Mass curves SpaceTime by Gravity, can be generalized to deal with Spin and Electric Charge. The generalization, called a Kerr-Newman Black Hole, was developed by Kerr (who generalized to add angular momentum J to mass M in 1963) and by Newman (who generalized to add charge e in 1965), according to the book General Relativity, by Robert Wald (Chicago 1984).”

    Here are a few sites to peruse with some of the quotes provided. Any emphasis (bold and underline) was added by me to draw attention to particular phrases. Unfornunately for a PF reader, you would have to link to a link to a link and back again to get to the 50-60 points of significance, a long task which wouldn't be expected. Somewhere in this mess is a statement, with mathematical proof I don’t understand, that “A rotating black hole will act, at the event horizon, exactly like a rotating metal sphere.”

    "In his paper "Generation and Evolution of Magnetic Fields in the Gravitomagnetic Field of a Kerr Black Hole", Ramon Khanna says: "... a rotating black hole can generate magnetic fields in an initially un-magnetized plasma. In axisymmetry a plasma battery can only generate a toroidal magnetic field, but then the coupling of the gravitomagnetic potential with toroidal magnetic fields generates poloidal magnetic fields. Even an axisymmetric self-excited dynamo is theoretically possible, i.e. Cowling's theorem does not hold close to a Kerr black hole. Due to the joint action of gravitomagnetic battery and gravitomagnetic dynamo source term, a rotating black hole will always be surrounded by poloidal and toroidal magnetic fields (probably of low field strength, though). The gravitomagnetic dynamo source may generate closed poloidal magnetic field structures around the hole, which will influence the efficiency of the Blandford-Znajek mechanism [whereby coupling of the gravitomagnetic potential with a magnetic field results in an electromotive force that drives currents that may extract rotational energy from a black hole.” ….and:

    "in June 1971 ... Zeldovich announced a spinning black hole must radiate ... a spinning metal sphere emits electromagnetic radiation ... The radiation is so weak ... that nobody has ever observed it, nor predicted it before. However, it must occur. The metal sphere will radiate when electromagnetic vacuum fluctuations tickle it.. Zeldovich's mechanism by which vacuum fluctuations cause a spinning body to radiate. <His sketch> showed a wave flowing toward a spinning object, skimming around its surface for a while, and then flowing away. The wave might be electromagnetic and the spinning body a metal sphere ... or the wave might be gravitational and the body a black hole. The incoming wave is not a "real" wave ... but rather a vacuum fluctuation. ... the wave's outer parts are in the "radiation zone" while the inner parts are in the "near zone" ... the wave's outer parts move at the speed of light ... its inner parts move more slowly than the body's surface is spinning ... the rapidly spinning body will ... accelerate ...[the inner parts of the incoming wave] ... The acceleration feeds some of the body's spin energy into the wave, amplifying it. The new, amplified portion of the wave is a "real wave" with positive total energy, while the original, unamplified portion remains a vacuum fluctuation with zero total energy. Zeldovich proved that a spinning metal sphere radiates in this way; his proof was based on the laws of quantum electrodynamics.”

    "Hawking undertook the task of applying quantum mechanics to black hole dynamics. While his formulation is beyond the scope of this web page, a slightly quantitative and highly qualitative examination of the problem can yield a very good picture of what Hawking discovered. Hawking first attempted to examine the space-time outside the black hole using quantum field theory, which has a very different picture of empty space than the classical definition. His first step was to consider what happens when any field (for example, the electromagnetic field) is quantized in the space-time exterior to a black hole. The quantum mechanical description of a vacuum is space seething with virtual particles and antiparticles whose presence cannot be detected directly." ….. and:

    "At first glance, this process of virtual particle creation may seem a little phony. With that in mind, we can consider the more tangible case of electric field particle creation. (This process can actually be observed by applying a strong electric field across a capacitor in a vacuum.)" …. and:

    "The quantum mechanical description of the vacuum allows for the creation of the particle/antiparticle pairs, and the electric field tends to separate the charges. If the field is strong enough, the particles tunnel through the quantum barrier and materialize as real particles. The field necessary to accomplish this feat is achieved when the work done to separated the charges by a Compton wavelength equals the energy necessary to create the particles. It should be noted that conservation of energy is not violated, as the energy it took to create the particles would be precisely equal to the decrease in the energy of the weakened electric field.".. (Labguy Note: not necessarily just the BH mass loss.) … and:.. then goes on to explain the process from previous posts where:

    “"So far we have not introduced the possibility of a charged black hole, and will only consider an uncharged, non-rotating black hole." <clip> “When looking at the entire system, the total energy of the black hole has decreased and therefore, by Einstein's famous relation, the mass must have decreased. This is the process by which black holes radiate, which is now known as Hawking radiation.”

    Carrol, Bradley W. and Ostlie, Dale A. An Introduction to Modern Astrophysics. Reading: Addison-Wesley, 1996.

    Wald, Robert M. General Relativity. Chicago: University of Chicago, 1984.

    Eisberg, R. and Resnick, R. Quantum Physics. New York: John Wiley & Sons, 1985.

    Narlikar, J.V. Introduction to Cosmology. Cambridge: Cambridge University Press, 1993.

    Hawking, S.W. Hawking on the Big Bang and Black Holes. New Jersey: World Scientific Publishing Co., 1993.

    Hawking, S.W. A Brief History of Time. New York: Bantam Books, 1988.

    Shapiro, S. and Teukolsky, S. Black Holes, White Dwarfs, and Neutron Stars - The Physics of Compact Objects. New York: John Wiley & Sons, 1983.

    Thorne, Price, and Macdonald, eds. Black Holes: The Membrane Paradigm. New Haven: Yale University Press, 1986.

    Wald, Robert M. General Relativity. Chicago: University of Chicago, 1984.
  4. Apr 1, 2004 #3


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    Thanks. This should keep me busy a while.
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