BHs Matter

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wolram

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For a BH to form it must have a mass boundary, but does this boundary
vary with differing types of matter?
 

mathman

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Black holes have only three properties: mass, charge, and angular momentum. What matter went into it doesn't matter.
 

Labguy

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mathman said:
Black holes have only three properties: mass, charge, and angular momentum. What matter went into it doesn't matter.
Black holes have four properties:

Mass
Charge
Angular momentum
Magnetic field

All blacks holes have to have a magnetic field.
 

mathman

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Is the magnetic field independent of the other three properties? (I don't think so, but I am not sure.)
 

Labguy

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mathman said:
Is the magnetic field independent of the other three properties? (I don't think so, but I am not sure.)
It would be independent of any net charge (usually=0), but the mass and especially angular momentum would contribute significantly to the strength of the magenetic field. See an older post on this at:

https://www.physicsforums.com/showthread.php?p=254126#post254126

EDIT: For some reason, that link didn't work (on this computer) so here is the prior post:

Here is just a part of an older post about virtual particle production at a BH event horizon. Both Hawking radiation and magnetic "quantum tunelling". It shows that virtual particles are produced and released at the EH by two methods. This has nothing to do with matter and energy release from an accretion disk.
__________________________________________________ ______________
One thing that I have noticed, after reading many pages of info on "classical" Hawking Radiation (HR), 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 (BH) in the original Schwarzchild configuration as a simple mass-only expression of the Schwarzchild Radius (Rs) where Rs = 2GM / c^2. 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 research sites and past papers have noted that it is almost impossible to form a BH with no angular momentum (spin). Even the "no hair" statement by Hawking 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 Ramon Khanna and Yakov Zeldovich have shown that all black holes will have a magnetic field. There are also the terms "Hawking Process" and “Hawking Effect” appearing, which include/combine the original HR work with work of others such as Thorn and especially Kerr (for spin) and Newman (for charge). The "Kerr-Newman" BH. (A source quote:) “David 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).

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.”

In June of 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 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] ... <and this> 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.”

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 BH mass loss as with Hawking radiation).

Sources:
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.
 
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mathman

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This is something I was not aware of. It looks like the magnetic field results from the interaction of the spinning black hole with the surrounding media. In that sense it is a derived thing, not basic as the three other items. The magnetic field doesn't seem to be an independent intrinsic property.
 

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