Yes, it’s another black hole question…
Let’s imagine a bar magnet falling toward a black hole. The bar magnet has your standard magnetic field around it; the kind you could visualize using a piece of paper and some iron filings. Let’s also assume a uncharged, nonspinning black hole with no other matter falling into it and no magnetic ‘charge’ either (no monopoles).
As the magnet approaches the event horizon, what happens to the shape of its magnetic field? Do the ‘lines of force’ reflect in some way? Are they compressed? Stretched toward the horizon? Is the magnetic field even aware of the event horizon?
As I understand it, as the magnet passes through the event horizon its magnetic field is snuffed out. From the black hole’s perspective, is there a difference between absorbing a bar magnet as opposed to a plain iron bar? Some infinitesimal difference in mass, perhaps?
Good question-- I must confess that I don’t know the answer to this one. I’ll be thinking about it, and if I can’t come up with something by Monday, I’ll ask some of the professors.
Thanks - this was inspired by my reading a little bit about the magnetic field being conveyed by the exchange of virtual photons. Since I know nothing about the characteristics of virtual photons, it occurred to me to wonder how they behave around an event horizon, and what the effect on an external magnetic field would be. It would be interesting if the lines of force were broken when they crossed the horizon, but I suspect that they either are compressed against it like a balloon against a brick wall, and/or skitter off and take some longer path arbitrarily close to the event horizon as though it had infinite magnetic permeability.
Also, are these magnetic virtual photons the same sort of thing that gives rise to Hawking radiation, or some other class of virtual particle?
This has all been worked out, but I don’t know the details. There is a sense in which the event horizon can be thought of as a perfectly conducting surface, which exhibits the Meissner effect, so the field of a bar magnet near the horizon is given by the sum of the unperturbed field and the field of a mirror image of the magnet reflected by the horizon. (or something like that, I don’t really know for sure.) The details are a straightforward calculation, first done by Hanni and Ruffini in the early 1970’s.