Do black holes have magnetic fields?

[Feynstein100]

I searched for this in the forum but apparently no one has asked this yet. So my question is, do black holes have magnetic fields and if so, does the magnetic field extend beyond the event horizon?

I have a thought experiment in mind. Consider a neutron star, which has a very powerful magnetic field that extends to some distance in space. Say we were to add some mass to the neutron star, enough to collapse it into a black hole. Its event horizon would necessarily have to be smaller than the size of the neutron star. But what would happen to its magnetic field? Would it suddenly disappear? That doesn't seem plausible to me.

And if the magnetic field were to remain unaffected, then it would still maintain its extents way beyond the event horizon. So what would actually happen?

This actually raises an interesting question. For charged bodies, the magnetic field is pretty straightforward. But for electrically neutral bodies that still have magnetic fields, for e.g. the earth/the sun, how would that work? I mean, from what I know, this magnetic field is only affected by two things: the rotation rate of the object and its internal temperature (read Mars lost its magnetic field because its interior cooled down). So if we were to compress the earth to let's say half its size, it would spin faster and its inside would get hotter. Would that mean its magnetic field would now be much stronger than before? And if that's true, would that also apply for our previous example of the neutron star that changed into a black hole?

 

[anuttarasammyak]

Charged BH in rotation generate magnetic field as well as electric field. Ref. https://en.m.wikipedia.org/wiki/Kerr–Newman_metric

 

[Feynstein100]

Charged BH in rotation generate magnetic field as well as electric field. Ref. https://en.m.wikipedia.org/wiki/Kerr–Newman_metric

No, I know that. Which is why I specifically mentioned the non-charged neutron stars and subsequent black holes. Would those have magnetic fields too?

 

[Vanadium 50]

So what exactly is your question? Its not the title. Does it really take 5 paragraphs to ask it?

 

[anuttarasammyak]

No-hair theorem or conjecture gives an negative view to your question of neutral BH with magnetic field. But it is not proved/disproved yet. Ref. https://en.m.wikipedia.org/wiki/No-hair_theorem

 

[PeterDonis]

Say we were to add some mass to the neutron star, enough to collapse it into a black hole. Its event horizon would necessarily have to be smaller than the size of the neutron star. But what would happen to its magnetic field? Would it suddenly disappear?

No. It would be radiated away as EM radiation. This is an example of the general result known as Price's Theorem, which says that when some object that has a bunch of properties collapses to a black hole, any of those properties that cannot be possessed by a stationary black hole (which means anything except mass, electric charge, and angular momentum) will be radiated away.

 

[Feynstein100]

No. It would be radiated away as EM radiation. This is an example of the general result known as Price's Theorem, which says that when some object that has a bunch of properties collapses to a black hole, any of those properties that cannot be possessed by a stationary black hole (which means anything except mass, electric charge, and angular momentum) will be radiated away.

That's the no hair theorem, right? And I have heard of that. Black holes cannot have any property except the 3 you mentioned. However, magnetic field lies in this kind of grey area since it's related to charge. I mean, a charged black hole would have a magnetic field by virtue of its charge. So that implies a black hole can have a magnetic field. I was just wondering if that extends to non-charged black holes as well. I guess that's a no then?
But what about the second part with the earth? Like if we could compress it to half its size (not turning it into a black hole), how would that affect its magnetic field?

 

[Feynstein100]

So what exactly is your question? Its not the title. Does it really take 5 paragraphs to ask it?

Yes, it is the title. Does an electrically neutral rotating black hole have a magnetic field?

 

[Feynstein100]

No-hair theorem or conjecture gives an negative view to your question of neutral BH with magnetic field. But it is not proved/disproved yet. Ref. https://en.m.wikipedia.org/wiki/No-hair_theorem

That doesn't really answer my question but thanks

 

[Ibix]

Does an electrically neutral rotating black hole have a magnetic field?

That's just a Kerr black hole, so no.

 

[Feynstein100]

That's just a Kerr black hole, so no.

Thank you. So in that case, the question with the neutron star. If it goes from having a magnetic field to not having a magnetic field, how does that work? Does the magnetic field get radiated away as PeterDonis mentioned? That sounds interesting. I've never heard that before. How does that actually work?

 

[phinds]

Yes, it is the title. Does an electrically neutral rotating black hole have a magnetic field?

No, it is NOT in the title. The title is "Do black holes have magnetic fields?" It does NOT say "electrically neutral rotating".

This is like asking if fruit is yellow and then insisting that what you REALLY asked was "are bananas yellow?"