How do accretion disks and black holes interact with electromagnetic forces?

[Holesarecool]

First thread for me.

Quick question. If photons mediate electromagnetic forces, and all photons are trapped inside a BH, how does the magnetic field get generated by the black hole itself?

Observers never see matter fall over the EH, so does the net charge of the accretion matter generate the permanent field?

 

[Chronos]

It is generated outside the event horizon.

 

[Jonathan Scott]

First thread for me.

Quick question. If photons mediate electromagnetic forces, and all photons are trapped inside a BH, how does the magnetic field get generated by the black hole itself?

Observers never see matter fall over the EH, so does the net charge of the accretion matter generate the permanent field?

If the "no hair" conjecture is correct, then a BH cannot have a significant intrinsic magnetic field, because its magnetic field is determined purely by its angular momentum and its overall charge (which is expected to be very small, otherwise it would neutralize itself by selectively attracting particles of the opposite charge). The field is effectively created when the black hole collapses and remains in existence afterwards.

In contrast, a neutron star or similar can have a magnetic field many orders of magnitude larger, effectively created by circulating currents and intrinsic magnetic moments within neutral matter, but no such currents can flow within a black hole.

There are theories that if a neutron star collapses to a black hole, then the "fossilized" remains of the previous magnetic field could remain trapped near the event horizon. This might provide an explanation for observations which apparently suggest a strong intrinsic magnetic moment in the central body of a quasar, in particular Q0957+561. A variation on this idea is the controversial "MECO" (Magnetospheric Eternally Collapsing Object) theory, which suggests that the object does not actually collapse because of radiation pressure, so it is not subject to the "no hair" rule.

 

[zhermes]

Welcome to physicsforums Holesarecool---and great first question!

You've already gotten a couple of good answers, but I wanted to add something:

If photons mediate electromagnetic forces, and all photons are trapped inside a BH, how does the magnetic field get generated by the black hole itself?

For astrophysical purposes (i.e. real black-holes), their 'intrinsic' magnetic fields (those which remain 'frozen' after their collapse) are entirely negligible. The (sometimes) large magnetic fields which currently seem to characterize the systems are due entirely to the material around the black-hole, namely the accreting gaseous plasma. When the black-hole accretes material, the material can 'drag' the normal interstellar magnetic field down to the horizon with it. Also, if there is an accretion disk, we think there may be processes which can generate and amplify the fields significantly.

P.S. Here's a cool picture:

 

[Holesarecool]

Thanks for the great responses. Greatly appreciated. There haven't been many theoretical threads on the makeup or basis for relativistic jets that I've seen. Could the jets be formed by the recombination of the magnetic fields produced by the accretion discs? I know there isn't any concrete model for them, but I'd like to hear some opinions, as they are likely going to be my thesis topic in grad school.

 

[zhermes]

Its actually pretty amazing how little we know about jets. We don't even know if they're 'Poynting' (i.e. magnetic field) or 'Baryon' (i.e. electron-proton or electron-positron plasma) dominated. In either case you need lots of magnetic field. Recombination of field lines might help in driving general outflows, or possibly the blob-like transient jets; but unlikely (I think) for steady jets. My understanding is that the best recombination scenario is that the field lines get completely twisted by a high-spin kerr black-hole, and kind of snap-unwind along with a violent discharge.

One of the other primary ideas is enhancing the poloidal magnetic field through dynamo processes in the accretion disk; the strong poloidal field, combined with the accretion flow and winding from rotation might be enough to collimate a strong jet...

 

[Holesarecool]

Do we know if there is any kind of periodicity to the jets? A dynamo process within the jets' radial charge distribution due to the charge and spin of the BH could lead to recombination processes on both sides of the black hole.

Or is it more likely that the field of the black hole is more directly responsible?

 

[zhermes]

Sorry, I don't really follow what you're saying. But, to my knowledge there is no known periodicity of any jets---besides a few cases where there are a few blobs of roughly comparable size. There does seem to be a jet cycle for microquasars, in which the system shifts between hard and soft states: a la

 

[abledoc]

Guys,
have a look at the paper by Jimenez in Arxiv
arxiv.org/abs/1112.1106
They have made a re-look at the equations of electromagnetism and shown how removing the Lorenz gauge condition (which is valid only in flat space time which is not valid at the time of big bang or may be in a collapsing and highly warped environs of a black hole).
According to their theory even empty space can have magnetic field.
One of the effects of this is that dark energy and dark matter could be just the residual electromagnetic fields waves from big bang (longitudinal and temporal waves)

 

[twofish-quant]

According to their theory even empty space can have magnetic field.
One of the effects of this is that dark energy and dark matter could be just the residual electromagnetic fields waves from big bang (longitudinal and temporal waves)

It's a very good and interesting paper, but I don't think it's relevant to black holes.

One thing that this paper and other dark matter/dark energy papers do is that the set up the physics so that you get exactly the same physics at short distances. This is intentional. If the physics were the same at short distances (i.e. anything less that the solar system), we would have seen something weird. This paper creates an EM field that's different at large (i.e. galactic distances), but is the same as ordinary EM at ordinary distances.

The other thing is that this paper (like most other papers) assumes nothing strange happens with the early universe. One important thing about dark energy is that it's not a "big bang" effect. The big bang looks fine without dark energy. You only see dark energy in the last billion or so years. So one thing that you have to explain with dark energy theories is why they *don't* appear in the big bang.

I like the paper because:

1) unlike a lot of theorist papers, they at least think about observation. My first reaction to reading any weird theorist paper is how does this impact the CMB observations, and they mention those.
2) it's original. I've seen dozens of papers that try to explain dark energy with "weird gravity" but this is the first paper I've seen try to explain it with "weird EM."

Something that makes accretion disks and collapsed objects more interesting to me is that you can't make up anything you want. In the early universe, you can easily invent a new field or new particle to explain things, with accretion disks you can't.

Something else that surprises people are that accretion disks are quite a bit more complicated than black holes. Single black holes are pretty simple objects. What's the color of a black hole? It's black. What happens when I toss something into a black hole? It gets sucked in and doesn't come out. What's in the inside of a black hole? Well, since I never see the inside, it doesn't matter, so I just say it's jellybeans. That answer is as good as any other.

What's the color of the accretion disk? Uhhhh... What happens when I toss something into an accretion disk? Errrrrrr... What's on the inside of an accretion disk? Ehhhhh...