Assume two massive space empires begin to war with each other. Now, they come to a standstill because they are so alike that they are unable to outnumber, outwit, or outmaneuver each-other and come to a bloody standstill (think WWI). One side devises an evil plan to destroy the other. They want to weaponize a black hole. My question is simple:
How would one move a black hole approximately 2.85 Solar Masses?
Could one create a black hole to avoid the hassle of moving one at FTL speeds?
This really depends on the technology level the space empires have reached. The best way to move a giant gravity well is ultimately gravity.
As Samuel pointed out, the mass you list isn't large enough to become a black hole on it's own...but there's possibly methods of forcing a collapse at some tech level. Maybe?
The first to note is this really isn't a quick assault. Even if you are capable of accelerating a blackhole to the 10% the speed of light, the travel time between the closest of star systems is still in the 40-50 year travel range. So this attack is an extending of the conflict really...it's not going to be quick, the opposing empire is going to have years of time to attempt to respond and they'll likely see it coming. If they are both on the same tech level, odds are that if one side was capable of moving the black hole, the other will be capable of slowing it down or deflecting it. If the blackhole can get up to 50% the speed of light, it's still a 6-8 year travel time between extremely near stars. Empire A launches the blackhole, Empire A manages to overwhelm Empire B's system, and several years later Empire A's newly acquired territory is about to be destroyed by an attack they launched years ago themselves ;)
The best way I can see a black hole being moved is by a civilization capable of manipulating gravity and energy. Energy is used to create a mass near the black hole. The black hole begins moving towards this mass like a ball rolling downhill. When the two are close enough, the mass is disassembled into it's energy state and relocated a bit further away and moving in the direction the black hole has started to. This process is repeated (potentially adnasuem) and slowly the blackhole is accelerated.
Of course this is using our knowledge of physics. A potential alternative would be something similar to an Alcubierre drive where you are warping the structure of space around the blackhole...though in this case the blackhole isn't really moving quickly. Is that a potential that one of your empires has access to this?
If the Empire that launched the attack is capable of converting matter to energy...is there anything stopping the Empire being attacked from having the same technology to convert mass to energy and can simply convert the blackholes mass over to energy and basically use it as a giant energy battery?
As a conclusion...unless there is a fabulous future technology that these space empires possess that is far beyond what we know now, this really isn't a feasible attack. The time frames involved and the energies require just aren't that realistic and a race that is capable of moving it is just as likely capable of converting the mass of the blackhole into energy and using it as a power source. There are better things for these empires to invest their time and energy in...firing a stream of neutrons accelerated to 99.9999% the speed of light for example.
With a mass of 2.5 solar masses you'll more likely have a neutron star, it won't have collapsed below its Schwarzschild radius.
Moving a black hole
You move a black hole in the same way you move a star. That is, you don't. There isn't a way to push the thing. The best option would be to pull it with an even-more-massive-black-hole. How to move the even-more-massive-black-hole? Well, it's turtles all the way down from there.
If there is some super-tech, like easily converting energy to mass and back, then that can be used to pull the black hole along. However, this is like trying to roll a boulder onto someone by shooting it with your rocket launcher. Just turn their home planet into energy and cut out the massive middle man.
Creating a black hole
You can create a black hole by adding sufficient mass for one to form. Estimates vary, but it seems that 3.2 solar masses will do the trick. So, if you gather together about three of our solar systems and move all the mass to the center, viola, a black hole will be born. This, of course, probably destroyed the enemy because you threw two extra solar systems at them.
Forget throwing it at them. Use it right where it is.
The best way to harm someone with a black hole would be to weaponize the huge potential energy between it and anything that is not in the hole. Gravity based weapons such as a (substantially modified) space faring trebuchet could theoretically launch a projectile with more total force than whatever your space empires are using for power.
The potential energy could also be harvested with a mechanism not unlike a the weights on a cookoo clock, only with planetary bodies instead of cast iron pinecones. The difference that much "free" energy could have on an interplanetary stalemate would be profound.
In theory you can move a black hole. If you pump charged particles into the black hole, it keeps the net charge. This would make the hole respond to an electric field and in theory movable.
Of course, if you have the technology to create immense charged beams and huge fields that can move a multi-solar-mass item about like a billiard ball, you might as well just use this stuff on the target right off, and skip the messing about with a black hole.
How about create a relativistic jet with one?
An astrophysical jet is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. It is usually caused by the dynamic interactions within an accretion disc . When matter is emitted at speeds approaching the speed of light, these astrophysical jets are called relativistic jets.
So you find a spinning black hole, aim one of its poles at your enemy, feed it a bunch of matter -- I'm assuming the type of matter, pattern, etc, can make the process more efficient as a weapon -- and POW, you've got a pretty powerful rifle that's maybe spewing radiation as well as matter out of the pole. The downside of this is getting all of the matter to feed it and the collateral damage to anything that the other pole is facing. (I could imagine it's something in your territory.)
The only way a found black hole might be useful as a weapon is if the space empire in question is capable of generating wormholes that the black hole can pass through. If artificial gravity is not available, by charging the black hole electromagnetism can be used to move it, though given its mass, you'd have more success using that power to move something else.
On the other hand, generating a black hole is "simply" a process of compressing matter enough that its radius falls below its Schwartzchild radius. If you can compress matter this much, it doesn't really matter how much you have, you don't even need solar masses of it. As an added bonus, if you make a black hole small enough that it will evaporate quickly enough to serve a military purpose, you in effect have a total mass to energy conversion bomb. Using the calculations on the former Wikipedia page, collapsing 228,000 kg of matter to a black hole would evaporate in 1 second releasing energy equivalent to a 5 teratonne of TNT explosion.
You dont need to use the blackhole as a direct weapon but as a part of a weapons system. Eg:
You could use the black hole as a gravity lense. This could be used to focus energies across really big ranges. So a laser (or any other beam weapon at interstellar/stellar ranges)
Gravity well generator, the black hole can generate your weapons system awfully lot of kinetic energy. Suppose your ftl capability does transfer kinetic energy at other end but forbids ftl collisions for some technical reason (such as wormhole travel). then accelerate mution at center of black hole and open the gate....
on same topic you could open the gate to the black hole... etc etc.
Weapons is one thing, but any ftl capable civilisation is probably quite good at destruction. What if the gravity lense of the black hole was used to look onto some critical thing the opponent did in the past.
Suppose theres some critical tech the other side has that keeps the stalemate up. By looking into the manufacturing of these things. Or even some other civilisation is known to have counter tech but does not share.
Then theres a diplomatic dimension, lets say a atrocity that can be shown/staged in past so that galactic big brother can be gotten to your side. Again infinite possibilities.
This could even work in a cold war like cloak and dagger situations.
So as others have said, if you are able to move a black hole, you already have enough technology to do other things that can be just as destructive with less work. Plus the other guy will see it coming.
So making a black hole may be a better route. People were already concerned that the LHC might make a black hole and destroy the earth. The only problem is that any black hole the LHC made would be so tiny it would evaporate within a second, be traveling so fast it would leave the solar system pretty quickly, and if it did survive long enough to get enough matter to survive and stayed within the earths gravity field it would be consuming so little matter the sun would go nova before we'd be in any real danger.
But this is an advanced technological race, so let's build a bomb!
Two super advanced linear accelerators that are small enough to be mobile but more powerful than the LHC, smashing atoms together in such a way that all momentum is canceled, meaning our black hole won't headed out at close to the speed of light.
Second, you have to feed it something so it won't evaporate instantly. A normal black hole that small would pass through most matter without touching many atoms along the way, so the accelerators are focused on something really dense, like neutronium.
Once you feed it enough that it's big enough to keep itself fed, it's still going to take a while to consume a whole planet, meaning the enemy will have some time to evacuate, or strike back...
Peter F. Hamilton used a weapon like this in The Dreaming Void called a Hawking M-Sink, which is some kind of black hole weapon with a greatly expanded event horizon, and it made the planet uninhabitable in just a couple days and ate it completely in about a week. He didn't explain how it worked, and it didn't matter because it worked for the story.
Well, technically speaking you could make any object with sufficient mass into a black hole. You just have to make it dense enough. Now, by "dense enough" and "massive enough", it's worth noting that you could make the Earth into a black hole if you compressed it down to the size of a marble.
So fantastic, you now have a black hole with the right size to be worthwhile for firing at your enemy. Not that it's scary enough that you just happened to turn a whole planet into a black hole or anything, nevermind that small bit where exerting enough force on your shiny new black hole to move it in a direction you want would also be enough energy to blow a whole planet to dust. Well, it would be a pretty effective weapon I'd say, because while I'm also sure your enemy would be all "hey, where'd the Earth go?" when the black hole eventually hits them, it will easily devour their ships whole.
Not that you couldn't have used the astronomically gargantuan amount of energy[1] directly against your enemy and make them into a black hole. Or blow them to bits.
[1] And when Astronomers use that term, they really, truly mean it. We're talking about having a whole lotta zeroes after the largest unit of energy you can conjure up. A billion billion times the energy the sun produces in a year might, almost be enough. Or probably half as much as you'd need.
Since 2.85 solar mass black holes would not form naturally this would be an artificial black hole.
One explanation of how the black hole moves could be that the black hole is created in such a manner that the hawking radiation from it is asymnetric. - This would violate the no hair conjecture though.
To steer just point the least radiating side towards the enemy.
Others here have already talked at length about the logistical problems of moving a black hole. But long before any mass-moving or space-folding technology gets powerful enough to move a black hole to any appreciable degree, it will become powerful enough to move almost anything else, and this presents another possibility: if you cannot bring the black hole to your targets, bring them to the black hole instead.
Portals or jump gates of some kind are one (relatively) practical solution, because they leave the target without much time to react and escape: one moment they're in some nice safe space, and the next moment they're perilously close to an event horizon. Depending on the mechanics of your portals, this can also create weaknesses that the other side can learn how to exploit: for example, maybe the machines that create the portal have to be very close to the target.
So I know you already chose an answer, but there are other things out there a lot scarier than black holes that could be weaponized far easier that might work for your story.
Stranglet Bomb
If you had a chunk of negatively charged strange matter, and you dropped it on the planet, it would convert the entire planet into a hot lump of strange matter.
Grey Goo
Why bother destroying the planet? Planets have resources, and sucking it down a black hole or turning it into strange matter is a huge waste of those resources. Why not convert the planet (and plants, and animals, and people) into raw, usable mineral resources? Just hit it with some nanobots designed to self replicate exponentially and break everything down into individual molecules to be used to make more nanobots. When the planet has been eaten down to its core, you swoop in, tell the nanobots to disassemble each other back into raw material sorted by type, and you have some easy mining to a better economy!
Iron Sunrise
Your civilization is angry beyond all reason. They don't want to grey goo their enemies into molecular dust, and a strange matter lump is just a memorial for any off planet survivors to rally around... So lets wipe that planet, along with every other planet in their system out of existence, by bombing their sun. In the book Iron Sunrise a causality weapon is used to transform the core of a star into a lump of iron which causes the star to go nova. In Judas Unchained a nova bomb is used to do the same thing in a different way.
Stars are pretty big, but that means they are also hard to defend. With the right technology you can mess up the star, and even if it doesn't go nova, the radiation from solar flairs and dimming of solar output can render a planet uninhabitable for a very very long time, if ever.
I forget the author of the short story in which a microscopic black hole is accidentally released into a planet. The black hole sinks to the center of the planet and begins to swallow everything. It will take some time before the effects are noticed, maybe enough time to escape...
I think the easiest and probably most realistic way as well as the most feasible way would be to have a black hole already in existence and find a way to focus some of the more elements inherent to the black hole at your enemy's homeworld, and simply pour major resources into protecting that setup. It enables you to take a more secure posture of defense, forces the enemy to expend their forces in order to destroy your operation. With any luck, they'll over-extend, and you'll find a weakness to exploit. If not, well, you've all the time in the universe to sit there and wait while the dangerous focused elements are busy making life on your enemy's homeworld slowly, but steadily less habitable.
If it takes long enough, the people may even revolt and replace your enemy with a newer more cooperative leader, maybe a Democrat who can be easily fooled into peace negotiations while you prepare to launch an assault on their most important strategic assets.
Of course, to be honest, if they're so evenly-matched, they're probably better doing a peace treaty and opening up trade between their Empires.
I'd suggest creating a kind of slingshot where multiple massive objects (perhaps all black holes (are orbiting a common potential well. Such orbits are chaotic and unstable. That means that there times when a small perturbation can result in a large change in the orbits and it's possible that some objects can become ejected out of the potential well. If one calculated the orbits sufficiently well, and could create a large enough perturbation at exactly the right points, one could eject one black hole and shoot it in sling-shot fashion at some external object such as a planet.
The trouble with black holes as weapon is that they are small and need to be aimed and guided very accurately to actually hit a planet.
I think there are other astrophysical objects that could be better weaponized. For example sending a near critical star and feeding it with additional matter at just the right time to make it go supernova. Use the same slingshot effect but the star could do much more widespread damage like a bomb.
A weakness of using any large object like a star is that the time scales involved in order to decide to target someone and actually hit them would be very long. If one were to to keep a large number of very small black holes in close proximity in tight orbits, the time scales could be reduced to be almost practical. Of course that requires creating many small black holes but I'll assume they've already solved that problem.
So long as moving the black hole is out, I can consider a few reasonable methods of weaponizing it:
Gamma Ray Bursts:http://en.wikipedia.org/wiki/Gamma-ray_burst
I would propose flipping the question around. Instead of moving the black hole to the target, how about moving the target to the black hole?
If one of the Empires has the ability to create wormholes on demand and to precise locations, then perhaps they would be able to create a sort of Wormhole Missile that, upon impact, opens a temporary wormhole into the center of the black hole. Even if the wormhole was active for only a fraction of a second, the gravitational effects from the black hole center would be intense, as a gravitational singularity exists inside a black hole (See: gravitational singularity). The destruction this temporary wormhole could do to the target would be immense.
I wanted to expand on my original answer. The wormhole missile would do most of its damage by temporarily subjected the target to the infinite gravitational field of the black hole's singularity. This would cause partial spaghettification of the target, and severely damage if not outright destroy the target. The amount of spaghettification damage would depend on how long the wormhole remained before collapsing.
You could nudge the black hole into a region where you've canceled mass, (spoiler alert!)
accelerating the black hole to the speed of light.
