Magnetic field from a spinning black hole? (2001 Oct 29)

[sailj29]

Hi All from a newbie but long-time lurker, amateur astronomer, and armchair cosmologist.

While perusing the archives I came across an APOD from 10/29/01 (http://antwrp.gsfc.nasa.gov/apod/ap011029.html) proposing that a spinning black hole could be causing surrounding gas to shine brightly by transferring the black hole's rotational energy via a strong magnetic field. Fascinating concept, but I'm a bit confused. Given that (a) the carrier particle for the electromagnetic force is the photon and (b) no photons from within a black hole can travel beyond its event horizon, how exactly does a black hole's magnetic field extend beyond its event horizon?

[dduggan47]

Hi All from a newbie but long-time lurker, amateur astronomer, and armchair cosmologist.

While perusing the archives I came across an APOD from 10/29/01 (http://antwrp.gsfc.nasa.gov/apod/ap011029.html) proposing that a spinning black hole could be causing surrounding gas to shine brightly by transferring the black hole's rotational energy via a strong magnetic field. Fascinating concept, but I'm a bit confused. Given that (a) the carrier particle for the electromagnetic force is the photon and (b) no photons from within a black hole can travel beyond its event horizon, how exactly does a black hole's magnetic field extend beyond its event horizon?

Great question and, being that I don't even qualify as an amateur scientist, I don't know the answer. I'm going to take a shot though that it has something to do with quantum uncertainty. I.e. there's a chance it's within the event horizon and a chance that it's not. That would mean photons really can escape.

Hey, it's just a guess!

- Dick

[Qev]

I believe the most common view is that the magnetic field is being generated not by the black hole itself, but rather by the moving plasma of the accretion disk around the black hole. I think the rotational energy transfer is occurring through the effects of the black hole's ergosphere, but I'm not 100% certain on that bit.

[Chris Peterson]

While perusing the archives I came across an APOD from 10/29/01 (http://antwrp.gsfc.nasa.gov/apod/ap011029.html) proposing that a spinning black hole could be causing surrounding gas to shine brightly by transferring the black hole's rotational energy via a strong magnetic field. Fascinating concept, but I'm a bit confused. Given that (a) the carrier particle for the electromagnetic force is the photon and (b) no photons from within a black hole can travel beyond its event horizon, how exactly does a black hole's magnetic field extend beyond its event horizon?

It is not known if black holes can carry a magnetic charge- the most common theory is that they can't. But they can carry an electric charge, and the spinning charge induces magnetic fields in surrounding material. That is the presumed source of energy that drives relativistic jets. I don't know if that's the source of the magnetic field suggested in the APOD you link.

Gauss's Law describes the mechanism by which the electric field inside an enclosed conductor influences the region outside the conductive surface- even if no photons can escape. Basically, the electric field is always felt at a distance, whether or not internal photons can pass through the surface. Maxwell's equation describe this rigorously, both for electric fields and for magnetic fields as well.

[bongman]

http://en.wikipedia.org/wiki/Schwarzschild_radius

I think that is what you are talking about

[Qev]

It is not known if black holes can carry a magnetic charge- the most common theory is that they can't. But they can carry an electric charge, and the spinning charge induces magnetic fields in surrounding material. That is the presumed source of energy that drives relativistic jets. I don't know if that's the source of the magnetic field suggested in the APOD you link.

Wouldn't it be unusual for a black hole to maintain a significant electric charge, though? Don't charge imbalances like that rapidly neutralize, in general?

[Chris Peterson]

Wouldn't it be unusual for a black hole to maintain a significant electric charge, though? Don't charge imbalances like that rapidly neutralize, in general?

I think black holes hold charges indefinitely. Objects only neutralize when there's some place for the charge to go.

[Doum]

That may help,

http://www.space.com/scienceastronomy/b ... 21111.html

[Qev]

Wouldn't it be unusual for a black hole to maintain a significant electric charge, though? Don't charge imbalances like that rapidly neutralize, in general?

I think black holes hold charges indefinitely. Objects only neutralize when there's some place for the charge to go.

Wouldn't a black hole that has a net electric charge simply repel like charges and attract opposites, until its charge was neutralized, though?

[Chris Peterson]

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.

[Qev]

I don't mean spontaneously, exactly. I was of the understanding that large persistent electric charges just don't happen in nature due to the way that any charge will repel other like charges, while strongly attracting opposite charges (and vice-versa), neutralizing themselves. I'm not saying that a charged black hole couldn't exist, it just seems unlikely to me that it could occur, or if it did, persist like that over any large timescale.

[Moonshadow]

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.

[Chris Peterson]

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.

The electric charge of a black hole is not generally considered to arise from outside the event horizon, but exists solely because of the charge inside- a consequence of Gauss's Law that is not affected by the presence of an event horizon. An intrinsic magnetic field could exist in an analogous way, although whether black holes actually have intrinsic magnetic fields (they certainly have external ones) is a more open question than whether they have intrinsic electrical charge- the wide belief being that they do.

[Moonshadow]

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.

The electric charge of a black hole is not generally considered to arise from outside the event horizon, but exists solely because of the charge inside- a consequence of Gauss's Law that is not affected by the presence of an event horizon. An intrinsic magnetic field could exist in an analogous way, although whether black holes actually have intrinsic magnetic fields (they certainly have external ones) is a more open question than whether they have intrinsic electrical charge- the wide belief being that they do.

That really doesn't make sense relativistically speaking. A magnetic field arises (wrt an observer) only because there is a relative velocity between a charged particle and the observer. Within the event horizon there can be no relative velocity because time does not exist inside the EH the same way it does for the observer. Time is in effect "stopped" relative to the outside world.

So any charge inside the event horizon would appear stationary (if it could be seen) and consequently could produce no magnetic field relative to the outside world.