A new paper from Luca Comisso and Felipe A. Asenjo in the Physical Review titled "Magnetic reconnection as a mechanism for energy extraction from rotating black holes" explains, well, how to use magnets to retrieve something from the gravitational collapse of a black hole.
So the good news is: it might be possible!
The bad news is: it ain't easy. The scientists write:
When a black hole is immersed in an externally supplied magnetic field, reconnection of magnetic field lines within the ergosphere can generate negative energy (relative to infinity) particles that fall into the black hole event horizon while the other accelerated particles escape stealing energy from the black hole. We show analytically that energy extraction via magnetic reconnection is possible when the black hole spin is high (dimensionless spin a∼1) and the plasma is strongly magnetized (plasma magnetization σ0>1/3).
Which already sounds pretty complex to my peon brain.
Phil Plait tried to translate the process in more layman's terms over on SyFy.com:
Very hot gas can be so energetic the electrons are stripped away from their atoms, and we call it a plasma. As the charged particles in plasma move around they create a magnetic field that can get very intense. The magnetic field lines (like you see in bar magnet diagrams) can be very strong, and that gives them tension — think of it like bending a piano wire; you have to apply a lot of force to keep it bent. If you let go, SNAP!
Particles are created from energy and blasted away in pairs. If things are aligned just right, one of those particles screams away in the direction the black hole is spinning, and the other in the opposite way. If this happens inside the ergosphere, the one moving in the same direction as the spin will get out, escaping into space. The other, going against the spin, will fall in. But, because it was moving in the opposite direction of the spin, this in effect slows the black hole down.
The black hole slowing means it releases energy, which is taken away by the particle that escapes. From your point of view, standing (well) outside the black hole, you just see particles come hauling out at near the speed of light and the black hole slowing a bit.
In other words, energy has been extracted from the black hole.
It still sounds a little tricky, and I doubt that DIY Black Hole Retrieval will be catching on any time soon. But if it ever does, it'll be pretty incredible—the process is expected to have an energy efficiency of 150 percent, meaning that, if you land it right, you can use all of that particle momentum to extract energy from the Black Hole itself.
Magnetic reconnection as a mechanism for energy extraction from rotating black holes [Luca Comisso and Felipe A. Asenjo / Physical Review]
Turns out, you can get something out a black hole…but it's not easy [Phil Plait / SyFy]
Image: Public Domain via European Southern Observatory