Extracting energy from a black hole with a laser.

[Aelfyre]

Ok, so I was thinking about the concept of a gravitational slingshot. Like what they did to accelerate the Voyager probes an it got me to wondering.

Given that a black hole produces such a profound curvature in space time, if you could locate one alone by itself in space, without an accretion disc that is, would it possible to aim a laser near it so as to return back to you with more energy than it had when it left?

Obviously it can't accelerate per se but any energy gain would be seen in the form of a blue shift, and if it *is* returning back to you with more energy, then that energy had to come from somewhere.

In this case the black hole.. I mean I would think it would take a freaky long time to drain all the energy out of the black hole but that basically means it would be a cosmically gigantic source of energy just sitting there for the taking. logistical nightmares aside of course, just a thought.

Is this possible even in theory?

[Tchebu]

I might be wrong on this, but my impression is that the gravitational slingshot relies on the fact that the slingshotting planet is orbiting the Sun and you pass behind it and have it pull you forwards at the expense of its own orbital angular momentum. This means that if you have a stationary black hole, it's no better for slingshotting than the Sun is...

That being said, if you have a rotating black hole, which is the case for just about every astronomical black hole, there's a region that is outside the event horizon but where spacetime gets dragged faster the speed of light in the direction of rotation, called the ergosphere. Since it's outside the event horizon there's no problem in entering and exiting this region, and if you're clever about how you do it, you can actually leave with more energy than you came in. This is called the Penrose process.

[Diadem]

Basically what Tchebu said, on both counts.

Additionally, matter that falls into a black hole tends to give off lots of radiation. Up to 40% of its rest mass, if you throw it in under the right angle. That's a huge amount of energy (the active black holes in the centre of young galaxies give of so much radiation they are visible from the other side of the universe). If you are clever in how you throw it in, you can tab some of that radiation as usable energy.

You're not really extracting energy from the black hole in this case, but you're using the black hole to extract energy from normal matter. The fun part is that it doesn't matter what kind of matter you throw in. So you can safely dispose of your toxic waste and gain huge amounts of energy in the process.

[Technical Ben]

You might be able to steal some energy from it's spin, by slowing that down. Not sure if you could do much with that energy though. Or if you could do it without using another large mass in the process.

[Birk]

You might be able to steal some energy from it's spin, by slowing that down. Not sure if you could do much with that energy though. Or if you could do it without using another large mass in the process.

There are mechanisms that allow the extraction of spin energy, for instance the Blandford-Znajek Process. Then you just put a big vat of water along the jet and power your shit with space steam, clearly.

[Technical Ben]

Nice. I'm bookmarking that one. Now just to find a small enough black hole...