Usually it is said that black holes cannot have electric or magnetic dipole, only electric charge and angular momentum are allowed quantities besides mass

So, it would seem that black holes behave as perfect diamagnets that don't allow field lines to enter into the event horizon?

If I place a magnetic dipole pointing near an event horizon, what is the boundary condition of the magnetic field on the horizon? does the field lines flow around the event horizon perfectly?


1 Answer 1


Several solutions for the magnetic field around a black hole are examined in (Komissarov & McKinney, 2007). When applying a uniform external magnetic field along the axis of rotation of a maximally rotating black hole, in an ideal vacuum the field lines are indeed expelled from the event horizon, akin to the Meissner effect.

Two figures for the magnetic field around a black hole: in vacuum the field lines are expelled from the hole's event horizon; but when surrounded by conductive plasma, field lines pinch toward the black hole equator, and several pass through the shell of plasma around the surface.

But if the space around a black hole has a small amount of conductive plasma (the "conductive case"), then magnetic field lines can be pulled very tight against the event horizon. This is the typical case for real astrophysical black holes, which we're just beginning to be able to observe.

I was unable to locate work on a dipole magnet approaching a black hole, which might have astrophysical relevance when a star or stellar remnant with a strong magnetic field approaches a black hole.


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