I'm writing a sci-fi story, and there is a giant autonomous computer planet that constantly gathers resources from around the universe using drones. It stores a super weapon at the core of the planet, inside a black hole that is also used as a main power source for the planet, so as to keep the whole device much more compact and unnoticeable. I wasn't sure how the weapon could be pulled back out, and I was thinking perhaps magnets? I haven't really heard about magnetic fields being affected by gravity, and the magnets power could be supplied by the infinite power that the black hole produces. Could this work in any scenario?
I wasn’t sure how the weapon could be pulled back out, and I was thinking perhaps magnets?
Nothing can get out of the event horizon of a black hole, not even light. And light is made of electromagnetic waves.
The only thing you can get out of a black hole is Hawking radiation, but that's completely unrelated to what fell into the black hole.
Could this work in any scenario?
If you are looking for a scientifically sound explanation, even in purely theoretical terms what you're asking just is not possible. As others have stated, nothing can escape black holes.
And even if the black hole were to evaporate via Hawking radiation, there is no possible way to salvage anything that may have entered the black hole previously.
If your weapon is mass/energy then yep you are in the right territory. If it's matter in a particular configuration (a complex piece that took hundreds of thousands of work-hours to make) you may be out of luck.
Black-holes can be used as energy storage, using magnetic fields to spin them up and releasing energy through electromagnetic induction as they spin-down.
Trouble is, as far as I know, the polar discharges of such a device - well, no one's figured out how to aim them whilst preventing the discharge from the pole opposite to the target propelling the people who aim the device fast in the opposite direction. Great for Star-ship propulsion, not so good in a fight - unless the strategy is - "Hit and getaway fast" - it could work then.
First thing to consider is that Black Holes are not holes.
Essentially a Black Hole is a star whose gravity is so strong that even light itself cannot escape.
Black Holes were first discovered when early astronomers noticed that some stars were orbiting around seemingly nothing - a gap (or hole) in their star-charts (whose background was black) was the focal point for the passage of these stars - and so these focal points were named Black Holes.
Since then we have studied these "gaps in the star-charts" and discovered lots about them - however the original name stuck. And this has lead to a lot of confusion - especially in the world of science fiction, where the name is frequently taken literally.
So even if you had magic-tech that could escape the event horizon, you couldn't use a Black Hole as a storage mechanism - just in the same way that you couldn't use a star as a storage mechanism.
You can use a black hole as a storage device while sticking to hard science
But... you won't be getting anything useful out on a short timescale (i.e., in less than the current age of the universe) and you're dealing with a not-universally accepted theory (but also not contradicting any universally accepted theories). Whether those are deal-breakers is up to you.
As others have noted, you can't get anything out of a black hole except Hawking radiation. However, there's some disagreement about the exact nature of Hawking Radiation, for example: does the information contained in the radiation relate to the information that went into the black hole in some--potentially useful--way?
Hawking himself originally thought that the information was destroyed, but this doesn't reconcile easily with other commonly accepted theories that information cannot be destroyed.
Since there's no real scientific consensus on what happens (Hawking himself flipped sides in 2004), you can pick the theory that best suits your story.
Plausible Explanations For "Extracting" stuff stored in a Black Hole
Some (well sourced) methods of reconciling Hawking Radiation with Information Preservation are listed on this wikipedia page. Most relate to getting "information" back, in a sense I don't totally follow.
Some of them deal with what's left after Hawking Radiation, when the black hole ceases to be a black hole, and are concerned with actual matter instead of "information." This neatly sidesteps the problem of "You can't get anything out of a black hole" with the explanation "well, but you can wait for the black hole to not be a black hole anymore and see what's left!"
What Works for Your Story
The best theory for your story is probably "Information is stored in a large remnant" (links available through wikipedia page linked above). The last paper supporting this theory was in 2015 so it's not an obsolete theory, and the first (titled "Black Holes and Massive Remnants") states that the size and mass of the remnant depends on its information content (which you can make arbitrarily large by feeding arbitrary amounts of information into the black hole). Thus you can create arbitrarily large and arbitrarily massive remnants.
Your giant autonomous computer would have to be far more advanced than us in their understanding of how the input matter/energy correlates to what is left, but it doesn't violate any fundamental laws to say that it can feed things into the black hole in such a way that a superweapon remains once the black hole has evaporated.
Until the black hole has evaporated, it would be totally impossible to get any information about what is going on inside, which works perfectly with you desire for compactness and secrecy.
Hawking Radiation takes a really long time. The larger the black hole, the longer it takes. So the more matter you feed in (and the larger you want your superweapon to be), the longer it takes for it to become usable.
From wiki page on Hawking Radiation:
For a black hole of one solar mass (M☉ = 1.98892×1030 kg), we get an evaporation time of 2.098×1067 years—much longer than the current age of the universe at (13.799±0.021)×109 years
This would be a significant hurdle in any story I can think of, but it's up to you to determine if this is important in your case.
If by 'inside a black hole' you mean beyond the event horizon of the black hole, there's no way out at that point. The escape velocity of a black hole is greater than the speed of light. Since nothing can go faster than light, it doesn't really matter what you use to push or pull the thing.
If you have some sort of FTL drive, sure. You've entered the science fantasy realm, so you shouldn't worry too much about following the laws of physics. Come up with some reasonable limitations to make it feel plausible, toss out some technobabble and move on.
If the thing you are trying to smash down is orbiting the black hole outside of the event horizon, you are using a black hole as a glorified trash compactor I guess? This is beyond my physics knowledge, but my engineering sense is that you'd have to come up with a really good excuse to use such a dangerous object to smash stuff down, when other options exist!
Many other people have pointed out the information paradox of black holes, which, based on our understanding of black holes today says that you could not input information into black holes and obtain meaningful results from any type of output (Hawking radiation) in this case.
But, we're talking about a black hole that sits inside a planet, so I would wager that whoever built this probably has a better understanding of black holes than we do. If you want to stay within the realms of our current understanding, I would put forward two possible scenarios (both from 30,000 ft so as not to get bogged down in the muck and mire of all the unknowns surrounding black holes):
Decoding Hawking Radiation
The black hole was created (and permanently exists within) a larger planet aka. a closed system, unlike any black hole we know of. The circumstances surrounding the construction and implementation of this theoretical information-storing black hole are known. It is possible that under these circumstances all variables can be accounted for - all matter and energy that has ever entered the black hole is known - and an algorithm was developed based on this in order to obtain meaningful information.
This depends on the nature of your weapon though. If it is a physical device, you won't be able to put it into and out of a black hole like it's a box. If it's some sort of cyber-weapon consisting of data alone, it's doable.
Black Hole is the Weapon
If you don't want to interpret the radiation that is coming from the black hole, perhaps the black hole itself is the weapon. When ready for use, the black hole could be expelled from the planet on a trajectory that sends it to the target destination - wiping out everything along the way - and eventually destroying the planet or solar system it is targeting. In the meantime, the planet is at work gathering resources to construct another black hole.
Going to destroy the weapon would be a suicide mission, as disabling the planet would cause it to implode and anything in the vicinity would be destroyed as well.
This is also the ultimate dead-hand weapon. Containing a black hole would require substantial energy and matter. Assuming you could contain a continuously growing black hole indefinitely, it would require an indefinite amount of resources - depleting the surrounding space in turn. If the weapon is never used, it sits until the heat-death of the universe or close to it, where it will be used regardless, ensuring that it wipes out
I mean, don't get me wrong - you won't be able to fight the gravity of the black hole. That's a losing battle. But you can fight the location of it, and by using some lateral thinking, you can 'pull' your secret weapon from out of the black hole.
One example: You use the classical 'out of phase with the universe' technobabble to effectively blink it out of existence. It's still there, just not interacting with our universe. Afterwards, you maneuver the black hole to it's location - stopping anyone else from 'rephasing' it. It's not until you move the black hole out of the way that you can safely 'pull' the weapon back into existence.
Basically, figure out a way to make the gravity portion of the picture not matter. Make it out-of-phase, put in a subdimensional pocket, de-massify it, whatever avenue appeals to you. Make the location of the black hole the relevant factor, which is something you can change.
(...) inside a black hole (...)
Depending on what you mean by inside, this might be possible.
If something is inside the event horizon, it's lost forever. Other answers have already elaborated on it.
However, if something is outside the event horizon, but inside the ergosphere, then it is trapped in the black hole until it suffers a momentum change that knocks it out. This is part of the Penrose process, which can be used to extract energy from black holes as well.
The prerequisite is that the black hole must be rotating. Only rotating black holes have ergospheres. They drag spacetime around themselves, causing spacetime to rotate as well.
The math behind this is beyond me, but this is not something unheard of in media. This is how...
...the protagonists escaped Gargantua...
... In Nolan's Interstellar.
As for what could add enough momentum to knock an object out of the ergosphere... A ballistic projectile hitting it from behind (laysman wording for "causing the target to accelerate prograde") will do the trick though, again, I don't have the math to calculate the trajectories nor the energy amounts involved.
- Make your black hole as large as possible. The larger the black hole, the weaker the gravitational tidal forces.
- Discover a way to travel faster-than-light. (We're assuming it's possible here.)
- Place your weapon inside a vehicle that is capable of withstanding the tidal forces of your black hole, and that is also capable of faster-than-light travel.
- Place your vehicle in the black hole. While being stored in the black hole, the vehicle will have to be constantly traveling outward at light-speed (or possibly slightly faster than light-speed) to remain stationary and avoid hitting the center and (presumably) being destroyed.
- When you want to retrieve the object, send a simple radio signal to the vehicle. The signal will have no problem entering the black hole. When the vehicle receives the signal, it will flip its FTL drive to maximum speed and travel out through the "event horizon" (which is no longer aptly named since FTL travel is possible).
White holes are the hypothetical opposite of a black hole. That is, it is impossible to enter one, but things may still exit. As far as I am aware, they have never been explicitly disproven, and there has been at least one ‘potential’ observation.
It is also possible that white holes and black holes are connected, in such a way that matter entering a black hole will eventually exit a white hole.
Now, with a healthy dose of sci-fi hand waving, this black-white hole pair could be used as a matter transporter, or (if there was some way to determine or control how long an object takes to reappear) be used as a storage device.
Of course, due to the size of both these objects, the reappearance point would be some distance away from where you left your items. But if you’re using a black hole as storage in the first place, you probably have a way around this.
On the topic of magnetic fields; they would not be able to force an object out of a black hole. And I would assume that the magnetic field would not be able to enter a white hole either.
Black holes are as impenetrable as they are mysterious. If you try to put something into it, it will be ripped apart and squished. Whatever it was before, it is nothing but subatomic matter and energy now. The protagonist may suspect that the AI has figured out how to hide his doomsday weapon inside the hole, away from prying eyes but the truth is that the the black void is not a veil draped over some gargantuan laser cannon it is the the destroyer. While experimenting with the power storage properties of the black hole, the AI figured out that the immense gravity can be used to slingshot particles with mass to near the speed of light and into far away targets. There would be no reason for anyone to know the difference between a small object flying using black holes for a gravity assist, and a particle coming out of the black hole, especially when anyone capable of revealing that secret is instantly vaporized due to the kinetic impact.
What if you didn't have to get it out?
Presumably, a planet-sized AI which has existed for long enough to design and construct its own super-weapons and super-weapon storage facilities would likely have an extremely comprehensive understanding of physics compared to ourselves. Humans have figured out how to use electricity to store, compress, and reconstitute information on demand. It is plausible to me that an autonomous planetary AI, which grasps the true relationships between matter, energy, spacetime, gravity, etc. could have figured out how leverage these to store, compress, and reconstitute matter on demand. Now it could store the weapon as a compressed (optionally even encrypted!) "matter blueprint", and rebuild it whenever it is needed. When it is no longer needed, just dispose of it in the black hole. Since the black hole is already being used as a power source, this process essentially (almost) pays for itself.
Black holes bend "the fabric" of spacetime around them in such a way that the relationship between black holes and (more theoretical) wormholes is unclear.
Thinking back to good old Edwin Abbott, the people of "Flatland" (a 2D world) were unaware of the magnitude of their projections in 3rd dimension (and Edwin well-demonstrated that all perceivable objects must have a projection in all higher spatial dimensions). The people of Flatland might thus bear much weight pulling them down in "Spaceland" (the realm of 3D objects). If Flatland were folded around the 3rd dimension like a piece of paper, the people on it wouldn't notice because they live on the paper. But then, if many of them banded together in one spot, their weight might pull them down toward the other part of flatland (across the fold). Naturally, it would take quite a bit of flat-mass to perform that act.
Then, extrapolating, if Spaceland were folded around the 4th dimension, a similar thing may happen. We are necessarily unaware of the magnitude of our 4th dimensional component, but it may be small or large. And if we had some 4D gravity pulling us in some 4th-dimensional direction, we wouldn't notice, but black holes could bend 3D spacetime "4-downward" toward the other side of the fold.
Now, bends in spacetime might not be all that common, and it might be that your fictional planet exists right at the tip of the fold, so that it has a wormhole with entrance and exit nearby one another. If your autonomous computer is aware of this and maintains the mass of the black hole such that it is just barely too small, so its bend in spacetime doesn't reach the other side of the fold, then all it would have to do is drop some additional mass into the black hole and it would open into a wormhole, exposing the contents of the black hole on the other side. Then, removing anything from those contents would cause it to shrink back into a black hole and close the wormhole.
The problem is that the wave function of whatever you put into the black hole will collapse, and it will change its state of matter into whatever fundamental "mass" makes up the core of a black hole. You'd lose the weapon, but you could theoretically extract some of its mass by adding an equivalent mass nearby.
You might be better off defining the weapon as one which folds space so that your black hole opens into a wormhole at a predictable location in the 3rd dimension. In this way, you could instantly cause large quantities of mass to appear anywhere you wanted, but it would only work as long as you maintained very specific masses in the black hole, etc.. It would be a complex operation. Hiding the weapon wouldn't be totally necessary, because the weapon would be useless as long as the black hole wasn't tuned for the weapon's current intended use. Whoever wants to fire the weapon must also have possession of the black hole itself.
What you need to ask yourself is "What are black holes in my universe?". We don't know much about black holes. Even if they exist (the famous 'picture' of a black hole may not be a black hole, but something else). In the electric universe theory black holes do not exist. They forms of high current plasma ejecting positive particles in one direction and negative particles in the other direction. Storing a weapon in hot plasma may not me such a good idea, but maybe you can make a force-field around the weapon. You may use the plasma as energy source to power engines to get the weapon out.
Signals sent into the black hole can function as a selector for which of its internal energies leave through Hawking radiation. It's just a matter of finding the right signals and sending them at a strength high enough to affect the Hawking radiation measurably. Just throw the OT and a pallet of towels in there.
Next you'd want to find a way to optimize your bandwidth.