# How to build a fail-safe system for an engine that can harness energy from a Black Hole?

## Some Context

I'm conceiving a universe set in the far future. Humans have already mastered FTL drives and populated several planets in various star systems.

They have engines based on singularities (i.e small Black Holes).

I'm not looking for a pseudo-sci-fi excuse to allow black holes to be harvested. Humans can do that because I want them to. So startup, containment and storage are fine.

The problem here lies in the fail-safe procedure in case anything goes wrong, because I don't think people would sign up for a joyous space trip where a loose screw could make them say hello to an event horizon.

## The Actual Question

That being said, I want to know:

1. Can Black Holes be safely dissipated/collapsed/destroyed in case of an unpredictable malfunction of the engine?
2. If yes, how would a mechanism do that in an emergency scenario (fast and very reliable)?

## Observations

I am aware of this post. It clarified a lot but it didn't quite give me the excuse I'm looking for to make this mechanism viable.

I've read somewhere that a black hole is the very definition of "collapse" - and this word in my question just means "a way to make the black hole disappear", turn off the engine by brute force (similar to using a fuse to protect an electronic device).

• FWIW, a BH based energy source doesn't make a lot of real physics sense, and as a result it is hard to figure out what it really is, or how it would fail. Aug 20 '18 at 16:45
• I'm curious as to how you are keeping the BH in place? As your space ship accelerates and decelerates your going to need something to stop the BH from hitting a part of your ship and going straight through everything. Maybe you have something like anti matter, that when dropped in the BH will convert the mass into energy, causing it to become too light and explode? Hopefully you eject it before you trigger it. Aug 21 '18 at 2:01
• @ohwilleke On the contrary, a micro-black-hole is the best energy source you can get for hard-SF. The smaller the black hole, the faster it evaporates. The trick is to feed it as fast as it evaporates, and voilà, you have a nearly-100% efficient mass-energy converter. Bonus point: did I tell you how it evaporates faster as it is shrinking? if you think Hollywood reactors going critical are stupid dangerous, hold my beer...
– Eth
Aug 21 '18 at 11:58
• Sounds like a lot of this is based on (very common) ignorance about what black holes are and how they actually work. They're not invulnerable objects that wreak unstoppable destruction on anything nearby and inevitably suck everything in. They're collections of matter so dense light can't escape. So to answer your question, first, what are you trying to guard against in the case of "engine malfunction"? (A loose screw causing the ship to collapse into an event horizon seems even less plausible than the idea of using singularities as energy sources, but maybe you could explain that a bit.) Aug 21 '18 at 16:06

Start with an 0.1% of Everest-mass black hole. Stuff it into a 0.001 meter radius (1 mm) containment device.

At the edge of the containment device, gravity is 0.01 m/s^2 from the black hole. This is microgravity.

It emits 1.358944e+10 watts, or 3 times that of a typical nuclear reactor, and it will last 10^10 years before evaporating (slowly increasing in power output as it does so).

Every time you lower the black hole mass by a factor of 10, power output goes up by a factor of 100 and lifetime down by a factor of 1000.

These micro back holes are dangerous like a hot knife or radioactive core is dangerous; their gravity is extremely local (and if it passes through something the gravity will tear a tiny hole in it). Most of the damage they'll do is from radiating energy at a riduculous rate; actually reaching such a tiny black hole is going to be nearly impossible, as the light pressure will prevent you from falling into it.

The hard part is containing the insane emissions the black hole generates, and making it move along with the ship, moreso than worries about gravity.

A more practical engine might have even smaller black holes and somehow constantly feeding it matter (to grow it and convert to genergy).

The emission rate is a function of size (smaller is more power); such an engine runs into the problem that it explodes if you don't constantly feed it mass: to "park" such an engine you have to feed it a large amount of mass.

In any case, such black holes are even smaller than my 1/1000th of Everest black holes: they have even less gravitational strength. While their gravitational gradient at their surface is insane, it quickly falls to nothing further away.

Black holes are not cosmic vacuum cleaners; they are just extremely compact. You could have a black hole the mass of the sun; but the energy flux is lower than the smaller black holes, the gravity is more annoying to deal with, and thrusting it around is going to be more annoying. And that much matter in a single ship is pretty expensive (there is only so much matter in the galaxy; if each ship requires a solar mass, you'll run out).

• +1 for addressing the common myth that black holes just suck in everything around them like some magical vacuum cleaner. Actually, regarding gravity they behave exactly like any other object with the same mass. The problem in case of a failure would be the same with any nuclear reactor: overheating, because energy is still being emitted, while you cannot use it up or transfer it somewhere else due to the failure. So in case of a failure you have to either cool your system down, or eject the hot parts.
– vsz
Aug 21 '18 at 6:21
• +1 for "how are you gonna move the thing" Aug 21 '18 at 13:44
• To the moving it bit, a black hole can be charged. And if it's charged, you could keep it contained and move it via electromagnetic fields. Making it charged should be relatively simple task if you're already capable of making black holes, which you'd almost certainly have to be to get micro black holes of the right size for use in your ship. Aug 21 '18 at 15:21
• @Shufflepants Maintaining a significant charge on something measured in masses of Everest which is emitting nuclear power plant levels of energy is going to be hard; it will attract the opposite charge and want to emit particles that balance the charge. How hard is beyond my knowledge of black hole physics.
– Yakk
Aug 21 '18 at 15:58
• @Yakk the arxiv article I cited in my post covers spinning/charged subatomic black holes (SBHs) and cites another source as saying they will rapidly shed their charge/angular momentum. So you're intuition is correct and the answer would be very hard. Aug 21 '18 at 18:06

# You invented it, you decide

Sorry for coming off as boring but this question is of the sort:

I have invented Big Magic™ for my setting.

Now please tell me how this little aspect of Big Magic™ works.

Well... no-one knows that except you. Especially so since you have taken a figurative hole and somehow made usable energy come out of it, energy that you can harvest, which is something we have no idea how it would happen in real life.

# Common tropes

At it happens you are not the first one to face this problem. And both real life and science fiction have employed different tropes to deal with this:

• You are correct. This BH engine is fully made up and any physical explanation would just be a pseudo-explanation. But BH's aren't made up. The answer I'm looking for is somewhere in the neighborhood of the third point. Is there anything that can be done to a BH to kill or mitigate it's deadly side effects in the event of a malfunction? In any case, you pointed some very interesting stuff I can use if I don't find a satisfying solution. Thanks a lot. Aug 20 '18 at 14:36
• Eject the core is the obvious one, or, more specifically, open a door to the outside that the BH can pass through, move relative to the BH so that it passes through the open door, and if you're nice, leave a drone beacon to position itself in a stable orbit around the BH to warn passers by of the uncharted BH threat for a few centuries at least. Aug 20 '18 at 16:42
• @Magus - MichaelK has a point - you're already doing a ton of things with the black hole that can't be done with a black hole. So why not add a "Black Hole Dissipator" to your existing magic drive and call it a day? Aug 20 '18 at 17:54
• Technically, you can extract energy from a black hole - if it's rotating. A rotating black hole acts sort of like a flywheel that can be sped up to arbitrarily high angular momentum. This energy can be harvested later by relativistic frame-dragging, which behaves sort of like gravitational induction, to use that energy later. Aug 20 '18 at 18:48
• There have been various papers (of mixed quality, admittedly) exploring the possibility of extracting energy from black holes by means of Hawking radiation. As far as I know, Hawking radiation is yet to be conclusively proven, but there is evidence and the theory works out. If true, one could theoretically extract energy from the radiation the black hole gives off, periodically dumping more matter in to keep it from evaporating entirely. Aug 21 '18 at 11:11

Starships should be fitted with two FTL drives powered by small black holes. When one singularity fails, activate the other FTL drive and move away from the failed singularity at superluminal velocity.

On the other hand, basic physics may be the best answer. Assuming the engine malfunctions, then its capacity to extract energy from a small black hole will be reduced to zero. The gravitational field of the black hole will prevent anything from escaping from it.

The problem facing a starship with a failed black hole engine will be the black hole itself. It seems not unreasonable to assume that there will be suitable technology to keep a black hole safe. The real problem will be the mass of the black hole. The ship should jettison the failed engine (preferably with a warning beacon) and thus lighten its load. If they possess FTL communications technology, call for help. Either they should activate any secondary FTL drive or engage a sublight propulsion system in the hope of reaching a safe spaceport of call.

Failed engines powered by black holes may not be a big problem. The physics of the situation may solve this by itself and there are technological solutions that basically amount to move away from the black hole as fast as reasonably possible.

• I'm really banging my head against a wall for not thinking of ejection. It's so simple haha. It is a really good answer but it would really make the crew drift and depend on external help. Ideally, this failsafe should kill the BH and turn off the engine so the engineering crew could access damage and try to fix it - only when this diagnostic reveals that the engine is truly dead should they ask for help. Anyway, thanks for the great input @a4android. This failsafe is sure going in. Aug 20 '18 at 14:21
• @Magus I would situate the black holes away from the inhabitable space as well. Like the star trek enterprise, except instead of the propulsion units there are black hole generators. That way even in a catastrophic collapse that sucks up the whole generator the crew will still be safe and the ship will even remain functional. Aug 20 '18 at 22:40
• @Magus Refer to paragraphs one & two of this answer. That would allow two ways to fix the engine. You should specified fixing the engine in your question, to get better answers. Aug 21 '18 at 4:46

There is an arxiv article on black hole ships which addresses this pretty well:

[A] BH with a life expectancy of about 3.5 years has a radius of about 0.9 attometers [and] a mass of about 606,000 tonnes and a power output of about 160 petawatts. Over a period of only 20 days a 160 petawatt power source emits enough energy to accelerate 606,000 tonnes up to about 10% the speed of light.

It even addresses (to some degree) what to do if the black hole gets too small, because black hole engines produce more energy the smaller they get:

Conceivably, unfed SBHs of radii less than 0.9 attometers, having less than 3.5 year life expectancies, could be used to rapidly accelerate interstellar robotic probes to relativistic speeds. Robotic probes do not necessarily need to “stop” and could tolerate much larger accelerations than humans. The problem of navigating such objects could be difficult however. The SBH would have to be ejected (or otherwise escaped from) before it explodes.

So the danger is less of being sucked in and more of being overwhelmed by the energy emitted, and the solution, whether because it's at its end of life and is too energetic or someone sabotaged it and its energy can no longer be contained, is an emergency core ejection.

It's also interesting because it changes the spin up/down dynamics: generally relativistic flight is shown as symmetric: it takes a long time to slow down as it did to speed up, but for this type of drive that wouldn't be the case, as the singularity would be producing significantly more energy after a long trip (according to the table in that article, a singularity with less than 2 months of life remaining would produce ten times as much energy and weigh 1/3rd as much, giving nearly 30 times the stopping power)

I just looked at this article, and it says that the formula for a black hole is mass is greater than its angular momentum squared plus charge squared.

The article suggests that to destroy the event horizon, you need change the BH's angular momentum or the charge. Because of the squared terms in the equation, that means you need to bring them closer to 0 (if it's positive, subtract, if it's negative, add). You can try to change the momentum by moving it or throwing something into it, or you can take the relatively simpler option of changing its charge.

You could get electricity out of another black hole and feed electrons into it if its charge is positive (electrons are negative), or take electrons out if the charge is negative.

Then, once the right side of the formula is greater than the mass, the event horizon will disappear and something will happen. No one is sure what will happen then, but you could claim that after testing, scientists found that it neutralizes the BH. You already said that you had a way to get energy from black holes, so you can probably fit this into your story.

Maybe black holes have to be "charged" with electrons before a trip so that they don't collapse. That would make for some more interesting situations. What if the BH's power runs out and it needs a jumpstart? Why not have a BH with a positive charge so you can always draw power from it but need to give it a ton of e- to deactivate it because you have taken so much out?

Hope this helps!

• Putting a charge on a black hole makes it possible to contain with an electric field of the right shape. Aug 20 '18 at 23:10
• @MackTuesday That might make it safer if the ship gets damage. Assuming the ship has power (black hole or other), that would be an added bonus. Aug 21 '18 at 0:57
• I really liked this. Thanks a lot for your input, john. Aug 21 '18 at 1:35

# The Black Hole Bomb

A black hole bomb is the name given to a physical effect utilizing how a bosonic field impinging on a rotating black hole can be amplified through superradiant scattering.

I understand that's hard to read, and pretty meaningless. Can I recommend you check out this video by Kurzgesagt

In short, what it describes is a system by which we can harvest energy from a black hole, and if we never release that energy it will eventually explode in one of the largest explosions any civilisation could hope to create.

I see no reason you couldn't apply the same principle for your engine, if you're drawing energy from a black hole, all you have to do is close the box and let it overwhelm itself.

• I believe we have very different definitions of the words "Fail-Safe". Aug 21 '18 at 14:38
• I stand by it :) Aug 21 '18 at 17:49

The Twin Singularity Engine

Adding to John Locke's answer. Conceive that your engine is powered by the difference between to oppositely charged singularities. The failsafe is to bring (crash) them together thus diffusing both. Great care should be taken to keep their charges in sync so that a failsafe action would leave nothing behind from one or the other.

• Okay you could do this if you had a black hole and a antimatter black hole, but if you smashed two normal black holes together you just get a bigger black hole. Aug 20 '18 at 23:44
• @ClayDeitas Smashing together matter and antimatter black holes could have one of two results. Either a bigger black hole or the mass-energy turns into radiation and can escape what was a black hole. In the latter case, this will be a radiation catastrophe. Aug 21 '18 at 4:57
• @a4android By definition, radiation cannot escape a black hole, but I wonder if a black hole collision would create dangerous gravitational waves.
– Eth
Aug 21 '18 at 13:15
• I would expect two large black holes smacking into one another would indeed produce gravitational waves. They've been observed relatively recently being caused by a binary system of neutron stars collapsing, so two black holes ought to do it too. Depends very much on how big your black holes are. Aug 21 '18 at 14:37
• @a4android Antimatter and matter colliding only has one result, which is a violent reaction, or so to say, an explosion. It could make a radiation exsplosion, or it could turn into photons or something. Two black holes with the same alignment (matter or antimatter) could combine, but they would probably release a flood or radiation during the process. Aug 21 '18 at 14:52

Strictly speaking singularities probably don't exist. They are a mathematical anomaly. In reality Hawking radiation dissipates the mass of the black hole before the final collapse into a singularity can complete (from outside the event horizon the process is slow, trillions and trillions of years, from inside the process is nearly instantaneous).

However a small black hole (1 million tons and smaller than a grain of sand) would be a wonderful source of energy. Although it would probably be easier to manufacture them rather than harvest them simply because the universe would have to be really really really really old for small black holes to exist naturally.

A black hole small enough to be portable and also useful enough for energy production would emit so much outward radiation pressure that you wouldn't have to worry about a tipsy passenger falling into the event horizon.

Black holes are incredibly stable, except at the very end of their lives where their radiation production increases explosively. You could stabilize your black hole by feeding it mass (assuming you had a way of forcing mass through the radiation pressure). This would be a slow process at best (if you could feed it fast enough at all) and would require hundreds of thousands of tons of disposable mass.

Ejecting the black hole is the second best option. If you could eject it at a significant fraction of c you could buy yourself plenty of time before the boom.

The best option is to not drag around this heavy and caustic black hole in the first place. Use an energy ansible between your ship and the black hole. The black hole is safely isolated and you have the option of shutting down the umbilical in the event of a catastrophe.

There are three main ways you can extract energy from black holes: accretion disks, superradiant instability, and Hawking radiation. In the first case you drop matter into the disk and gain energy from the radiation: the energy gained is proportional to the mass of the black hole so it favours large black holes. This is unlikely to be very effective for spacecraft, but I do recommend it for running large static installations. Superradiant instability basically extracts energy from the rotation of the black hole (it is somewhat related to the Penrose process); this can give a lot of energy fast, but the black hole needs to be near-extremal - presumably they are created spun-up when inserted in the ship and used black holes spun up at local starbases. Finally, there is Hawking radiation which scales as $\propto 1/M^2$ - the smaller the black hole the more power you get (there are theoretical ways of boosting this). So this favours very small black holes (good for spacecraft) but unfortunately over time they grow hotter and harder to feed matter to keep cool. So this is a plausible failure mode for a black hole generator.

Fortunately there exists a very simple solution: throw a bigger black hole on the failing black hole. This "douses" the Hawking radiation by increasing the mass of the merged black hole, and it can now be disposed of responsibly. So I suggest that there will be an extinguisher black hole stored on the ship, used to get rid of engine failures. It is too large to have much Hawking radiation and doesn't rotate so there is no superradiance - indeed, it brings down the spin a bit.

The main problem may be to move the extinguisher black hole fast enough in a crisis. Superradiant instabilities occur over a few light-crossing times in the tank you are keeping the black hole, so detection of a fault and dousing needs to happen near lightspeed. A slowly ramping up Hawking-radiator gives plenty of time to act.

The gravitational waves of the in-spiral are likely to be disruptive only for about 100 times the Schwarzschild radius, so for small holes this will not damage the ship.

What if you used science B.S. to somehow increase virtual particle emergence around the horizon to evaporate the black hole far faster than normal and vent the hawking radiation out? Maybe as a side-effect the vessel gets a giant boost of speed from the blast that you could work into the idea.

Also what if the black hole in the engine requires maximal entanglement with a second black hole stored safely elsewhere, and through more science B.S. you can safely dissipate the one by doing something with the other? What if this process isn't foolproof and sometimes it results in some or all of the vessel ending up at the location of the other black hole?