> Using a moving black hole as a gravitational mirror, kinetic energy from the black hole is transferred to the beam of light as a blueshift and upon return the recycled photons not only accelerate, but also add energy to, the spacecraft.
Okay, so, not something NASA needs to investigate immediately then.
Out of all the theoretical propulsion systems, THIS is pretty much the only one that we know would work without novel (made up) physics like negative energy, etc. In fact, I don't really think we'd need new/novel technology to test this. Biggest hurdle is getting into the right position with respect to a black hole to do this in the first place.
There are much simpler propellant-free ideas for producing relativistic speeds that respect known physics. I particularly like the resonant Photonic Laser Thruster:
> Out of all the theoretical propulsion systems, THIS is pretty much the only one that we know would work without novel (made up) physics like negative energy, etc.
Light sails are totally feasible too, depending on whether you consider them a propulsion system.
I think the best approach to using this mechanism in sci-fi would involve using it for braking rather than sending. The problem with sending is that you want the black hole traveling fast in the direction that you are going and this requires creating a binary black hole system somewhere in the close interstellar neighborhood of the star you want to come from (since if it's 4 light years away you need to have already solved interstellar travel in order to travel between stars with this mechanism...)
The engineering for using one of these to brake against, just requires one to make a smallish black hole that is falling into a gas giant planet and consumes a large chunk of it to form a black hole orbiting your star. Then anyone who wants to visit you can use much more of their fuel to accelerate to a much higher speed with the confidence that they can use your black hole to slow down as they are entering your system.
It looks like it steals energy from an object in motion, in this case a black hole. (Presumably because this is a dense object so you can get closer to it.)
Existing satellite slingshot maneuvers steal energy from the thing they slingshot around -- every satellite that slingshots off earth steals a very, very small amount of energy from the earth. The closer you get to the center of Earth's mass, the more energy you can steal. It's just that in Earth's case there's all this "atmosphere" and "lithosphere" garbage preventing you from snuggling up right close to the center.
With a black hole, all that stuff is compressed, so you can get closer and steal some of its kinetic energy. (If I understand the concept correctly.)
Technically this steals energy from the binary pair, not the black hole itself, but yes it's a potential power source for a sufficiently advanced civilization.
"Event Horizon"[0] was a movie in which a spacecraft had a black hole used for propulsion. Somehow the spacecraft went to Hell and came back infested with demons.
We just need to get close to a moving black hole, then and the Universe is our oyster.
Inversely, if we spot heavy intergalactic traffic of aliens jumping into halo-drive that is going to be a sure sign there is a moving black hole nearby!
A lot of thought has gone into far-future uses of black holes:
- Black hole starships [1]
- Weaponizing black holes [2]
- Colonizing black holes [3][4]
- Black hole computers [5]
Black holes may be the most efficient forms of batteries and power generators yet conceived. If our model of the Universe is mostly correct then the Univese will have an era of stars that will ultimately burn through all the fuel and we'll just be left in the dark with black holes that will evaporate over an extraordinarily long period of time, many orders of magnitude than the stellar era.
For propulsion, this ultimately comes down to photons have momentum. We can use them to move things. This is the idea behind the solar sail.
Black hole propulsion revolves around how you use the momentum from an evaporating black hole into motion for a ship. There are a bunch of factors to balance out here. Too small and the black holes dies too quick (and is too energetic). Too large and it's too hard to move and has a lower output and so on. Turns out there's a sweet spot for all this of about a million tons, which is in the realm of what we call Kugelblitz black holes.
I personally think using stellar output to accelerate and decelerate ships it he most likely outcome for a spacefaring species. If fusion ever becomes viable this may be an alternative for when you're not near a star. Ultimately black holes may replace that but there are significant theoretical and engineering challenges to that like how you'd even make a small (a million tons of small) black hole and how you'd feed it more mass (since it would be the size of an atom). If we could ever find a material to reflect and lase gamma ray (a "graser") then it might be possible but that's far from certain.
> Turns out there's a sweet spot for all this of about a million tons, which is in the realm of what we call Kugelblitz black holes.
"Sweet spot" deserves scare quotes, unless if you have a mirror made out of something that doesn't contain up or down quarks or any known charged leptons.
That mass black hole has Hawking radiation with a temperature of 223 trillion Kelvin, which means most of the photons of light in the Hawking radiation have enough energy to not just cause spontaneous positron-electron pair production, but also proton-antiproton pair production on the surface of whatever you're trying to use as a mirror.
To get down to Hawking radiation photons that "only" photo-ionise normal matter (about 6eV), you need a black body to be "only" about 70,000 K. That in turn means the black hole is now about 1766 trillion tons, and has a power output of 114 microwatts.
Okay, so, not something NASA needs to investigate immediately then.