by Moonshadow » Fri Apr 10, 2009 7:20 pm
Chris Peterson wrote:Qev wrote:Wouldn't a black hole that has a net electric charge simply repel like charges and attract opposites, until its charge was neutralized, though?
I'm not sure I follow. Where are these opposite charges going to come from? If you take a ball bearing, put a charge on it, and stick it in a vacuum, it will hold that charge forever. Even in air, it may hold it for a long time.
A black hole actually behaves like a single particle, described entirely by mass, charge, and angular momentum (and possibly by magnetic charge). A black hole can no more spontaneously lose its charge than an electron can.
I'm not sure that's totally true. The electric field and magnetic fields associated from black holes arise from the region outside the black hole's event horizon. Any signal from inside the EH does not have the energy to inform the rest of the universe of its existance or lack thereof. In fact, you can't honestly say that black holes even have mass since one could easily imagine two black holes colliding, one constructed of matter and the other constructed of anitmatter. No mass could exist after the collision, only energy which would be contained within the now bigger event horizon. The new black hole would seem to have more mass but in reality has none. It truly only has energy density. If an electron and a proton combined into a neutron within the event horizon, the charge would "neutralize" however the electrostatic energy of each particles' charge would be added to the energy of the black hole. The charge "seen" on a black hole is really the time dilated images of any charges crossing the event horizon.
A charged particle in motion does not have the same charge distribution as one in a rest frame. In fact Gauss's law can not be applied to a moving charge in the same way as a stationary charge. Instead of the spherical shape you see in the rest frame, the field takes on a kind of rotated peanut shape, with the axial dimension along the path of travel becoming smaller and the radial dimension becoming larger. As the charge approches c, the parallel (axial) field approches zero while the perpendicular component (radial) approches infinity. So a charge falling into a black hole would have a diminishing field and one orbiting a black hole would have an increased field. This relativistic change in the electric field also applies to the related magnetic field too.
Magnetism is simply (and only) the relativistic effect of moving charge (just as an object's energy is the relativistic effect of moving mass). I say that with the assumption that the magnectic monopole does not exist in our universe as it would be so massive it should have easily been experimentally observed -- but you never know...
A spinning black hole does not have a spherical event horizon (Kerr black hole) so one would expect a different distribution of rotating charge in its accreation disk but even a nonrotating black hole could have a magnetic field since it comes from the accretion disk and not the black hole itself.
[quote="Chris Peterson"][quote="Qev"]Wouldn't a black hole that has a net electric charge simply repel like charges and attract opposites, until its charge was neutralized, though?[/quote]
I'm not sure I follow. Where are these opposite charges going to come from? If you take a ball bearing, put a charge on it, and stick it in a vacuum, it will hold that charge forever. Even in air, it may hold it for a long time.
A black hole actually behaves like a single particle, described entirely by mass, charge, and angular momentum (and possibly by magnetic charge). A black hole can no more spontaneously lose its charge than an electron can.[/quote]
I'm not sure that's totally true. The electric field and magnetic fields associated from black holes arise from the region outside the black hole's event horizon. Any signal from inside the EH does not have the energy to inform the rest of the universe of its existance or lack thereof. In fact, you can't honestly say that black holes even have mass since one could easily imagine two black holes colliding, one constructed of matter and the other constructed of anitmatter. No mass could exist after the collision, only energy which would be contained within the now bigger event horizon. The new black hole would seem to have more mass but in reality has none. It truly only has energy density. If an electron and a proton combined into a neutron within the event horizon, the charge would "neutralize" however the electrostatic energy of each particles' charge would be added to the energy of the black hole. The charge "seen" on a black hole is really the time dilated images of any charges crossing the event horizon.
A charged particle in motion does not have the same charge distribution as one in a rest frame. In fact Gauss's law can not be applied to a moving charge in the same way as a stationary charge. Instead of the spherical shape you see in the rest frame, the field takes on a kind of rotated peanut shape, with the axial dimension along the path of travel becoming smaller and the radial dimension becoming larger. As the charge approches c, the parallel (axial) field approches zero while the perpendicular component (radial) approches infinity. So a charge falling into a black hole would have a diminishing field and one orbiting a black hole would have an increased field. This relativistic change in the electric field also applies to the related magnetic field too.
Magnetism is simply (and only) the relativistic effect of moving charge (just as an object's energy is the relativistic effect of moving mass). I say that with the assumption that the magnectic monopole does not exist in our universe as it would be so massive it should have easily been experimentally observed -- but you never know...
A spinning black hole does not have a spherical event horizon (Kerr black hole) so one would expect a different distribution of rotating charge in its accreation disk but even a nonrotating black hole could have a magnetic field since it comes from the accretion disk and not the black hole itself.