What Happens When a Magnetar Becomes a Black Hole?

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Discussion Overview

The discussion revolves around the properties of black holes, specifically whether a black hole formed from a magnetar retains an intrinsic magnetic field beyond its event horizon. Participants explore the nature of electromagnetic fields in relation to black holes, addressing questions about the escape of fields and the implications of the no-hair theorem.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants question whether electric and/or magnetic fields can escape a black hole, or if only electromagnetic waves and matter are trapped.
  • One participant hypothesizes that a black hole can induce fields external to its event horizon, similar to its gravitational field.
  • Another participant suggests that a probe could potentially communicate findings from within a black hole by modulating an electric and/or magnetic field, though this is challenged by concerns about time dilation and the nature of electromagnetic waves.
  • Some participants assert that a black hole's electromagnetic field cannot change, as it would imply the existence of waves that could escape, raising questions about the nature of fields and their changes.
  • There is a discussion about the implications of the no-hair theorem, with some participants questioning whether a strong magnetic field could exist without contradicting this theorem.
  • One participant expresses doubt that the magnetic field of a magnetar would survive its collapse into a black hole, while acknowledging the possibility of energy emissions during the collapse phase.
  • Another participant notes that while the no-hair theorem suggests a black hole cannot have a magnetic field, magnetic fields could still arise from matter falling into the black hole.

Areas of Agreement / Disagreement

Participants express a range of views on whether a black hole retains a magnetic field after formation, with no consensus reached. Some agree on aspects of the no-hair theorem, while others propose differing interpretations regarding the behavior of electromagnetic fields in relation to black holes.

Contextual Notes

Participants highlight limitations in understanding the implications of the no-hair theorem and the behavior of electromagnetic fields, particularly in relation to time dilation and the nature of fields before and after the formation of a black hole.

TheOtherDave
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If a magnetar gets big enough to become a black hole, would said black hole have an intrinsic magnetic field beyond outside of its event horizon?

Perhaps a better phrasing would be: can electric and/or magnetic fields escape a black hole, or is it just electromagnetic waves (and matter) that are trapped?
 
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A black hole obviously induces a gravitational field external to its event horizon, so I believe it safe to hypothesize it can induce other fields external to its event horizon.
 
Chronos said:
A black hole obviously induces a gravitational field external to its event horizon, so I believe it safe to hypothesize it can induce other fields external to its event horizon.

Yeah, I guess so.

Oooh! That gives me a follow-up question!
Could we construct a probe that'd go into a black hole and communicate its findings back out by modulating a electric and/or magnetic field that it creates?
 
TheOtherDave said:
Yeah, I guess so.

Oooh! That gives me a follow-up question!
Could we construct a probe that'd go into a black hole and communicate its findings back out by modulating a electric and/or magnetic field that it creates?

I believe that would be an EM wave that would not get through. Plus, this ignores a great many issues, such as time dilation causing an infalling object to appear to never cross the event horizon to an outside observer. Put simply, we would never see this probe enter the black hole.
 
TheOtherDave said:
can electric and/or magnetic fields escape a black hole, or is it just electromagnetic waves (and matter) that are trapped?
The field doesn't have to escape the black hole: it's already outside! And inside too. The part inside can't influence the part that's outside, though.
 
Sorry, I'm about to fall asleep so my brain's not quite all there (here?)...

Are you saying that a black hole's EM field can't change because that would make it a wave? Why can the field escape but not the change in the field?

And what if the probe "falls in" because the black hole grows, rather than by moving relative to the center of the black hole? Does that have the same time dilation problem?
 
TheOtherDave said:
Sorry, I'm about to fall asleep so my brain's not quite all there (here?)...

Are you saying that a black hole's EM field can't change because that would make it a wave? Why can the field escape but not the change in the field?

And what if the probe "falls in" because the black hole grows, rather than by moving relative to the center of the black hole? Does that have the same time dilation problem?
The field didn't escape: it's always been everywhere, and it always will be. The magnetic field was strong around the magnetar. And without anything to cancel out that strength, it will remain strong when a black hole forms.


And what if the probe "falls in" because the black hole grows, rather than by moving relative to the center of the black hole?
Assuming your ideas are well-defined, are you sure they are actually different?
 
Hurkyl said:
The field didn't escape: it's always been everywhere, and it always will be. The magnetic field was strong around the magnetar. And without anything to cancel out that strength, it will remain strong when a black hole forms.
Right, my mistake (and that's what I get for PWT - Posting While Tired).
I'm still unclear as to what would happen outside the event horizon if a field from within was changed (and this lack of clarity may just be from RWC - Reading Without Coffee). Wouldn't changing a field constitue a wave that can't escape? Yet what sustains the field if the thing that generates it has ceased to do so?

Hurkyl said:
Assuming your ideas are well-defined, are you sure they are actually different?
I'm not sure on either question. I think they're well-formed... But this could quite easily be a case of me not knowing what I don't know and/or incorrectly interpreting what I only thought I did know.

Regarding whether or not an object falling in vs the event horizon moving past it is the same thing, the difference is that the object experiences different accelerations (I think), and I seem to recall that getting acceleration correct was the key to figuring out General Relativity (but college was a long time ago... I'm sure I've forgotten parts of that class).

Sorry if my questions seem odd... I tend to learn how things work by poking around in the corner cases, to borrow a programming term.
 
Hurkyl said:
The field didn't escape: it's always been everywhere, and it always will be. The magnetic field was strong around the magnetar. And without anything to cancel out that strength, it will remain strong when a black hole forms.

According to Wikipedia:

The no-hair theorem postulates that all black hole solutions of the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum. All other information (for which "hair" is a metaphor) about the matter which formed a black hole or is falling into it, "disappears" behind the black-hole event horizon and is therefore permanently inaccessible to external observers.

There is still no rigorous mathematical proof of the no-hair theorem, and mathematicians refer to it as the no-hair conjecture.
​

So, wouldn't a strong magnetic field contradict that? It certainly would be information about the matter that formed the black hole.
 
  • #10
A black hole is believed to have 3 intrinsic properties - mass, charge and spin. I believe you would need matter in the vicinity of a black hole to generate magnetism.
 
  • #11
Chronos said:
A black hole is believed to have 3 intrinsic properties - mass, charge and spin. I believe you would need matter in the vicinity of a black hole to generate magnetism.


Right. But the OP is talking about a magnetic field that formed before the matter collapsed into a black hole, not EM generated afterward. It seems to me that to avoid contradicting the no-hair "theorem" the EM field must collapse too.
 
  • #12
I doubt the magnetic field of a magnetar would survive its collapse into a black hole, but, concede the possibility of error on this point. It may, however, emit a sizeable burst of energy during the collapse phase.
 
  • #13
ImaLooser said:
It seems to me that to avoid contradicting the no-hair "theorem" the EM field must collapse too.
That's what I thought too -- at least, the magnetic component -- but then I also believe charge + angular momentum = magnetostatic field, so I am confused on this point and didn't want to say anything.
 
  • #14
The no-hair theorem says that the black hole itself can't have a magnetic field, but what will happen is the matter just falling into the black hole will appear to "freeze" from the point of view of a distant observation, and you can get perfectly good magnetic field from the matter that is about to fall into the black hole.
 

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