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I know that currently, black hole civilizations are only theoretical. But here are my thoughts on why black hole civilizations are impossible.

Black Hole Civilization Issues

1. Gravity

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This is an obvious issue. There is just so much gravity around a black hole. The closest you could get to a black hole without having a weirdly shaped orbit would be 3 times the radius of the black hole or in other words 3 times the size of the event horizon. This changes if the black hole is rotating as pretty much all black holes are but still, that is pretty far. Also, it is very unlikely that a planet would form around a black hole because the gravity is so strong.

But even if you could get closer than 3 times the size of the event horizon, there is another gravitational effect that would get you long before you reach $r_{isco}$. That is tides. The average tidal range on earth is 2 feet or .6 meters. Around a black hole, this would increase by an unimaginable amount. The radius at which tidal forces wouldn't significantly effect you is way bigger than $r_{isco}$. So big in fact, that the planet would most likely have to be a rogue planet just passing by in order to survive near a black hole. That or be made out of something very dense like metal which wouldn't be effected by tidal forces as much as rock would. But a planet made entirely out of metal poses more issues including magnetism which is already an issue with the black hole itself.

2. Magnetic Field

The main problem with the black hole's magnetic field is simply the strength of it. A neutron star can erase your credit card before the neutron star is even visible to your eyes. The magnetic field of a black hole is even stronger, just like the gravity being stronger. Anything magnetic or that relies on magnets would stop working before the planet is even in orbit. If you assume the planet is made entirely out of metal, this black hole magnetic field would cause there to be a strong electric field around the planet which in turn would cause there to be a strong magnetic field. This planetary magnetic field would align with the black hole magnetic field and be yet another reason the planet wouldn't survive. The magnetic force would be so great that it together with the tidal force would spaghettify the planet and anything on it. And even if the force didn't tear the planet to shreds, you would still have the planet move towards the black hole like 1 magnet attracting another magnet.

3. Radiation

Now, I'm not talking about Hawking radiation here. I am talking about the radiation that occurs near a black hole because of fast moving particles. There are jets of radiation at the poles of the black hole. But that isn't the only radiation there is. There is also an almost constant amount of radiation due to particles orbiting the black hole. Any civilization that could survive the gravity and magnetism surely wouldn't survive the radiation. I mean, just think about it. The radiation around a black hole is like a constant amount of gamma rays. And yes, it is gamma rays because the black hole acts like a particle accelerator releasing a lot of very high energy photons. Sure, the gravitational redshift might shift it to X rays but still, a lot of radiation at very high energies. There is no way a civilization could survive black hole level radiation or even neutron star level radiation because even with the planetary magnetic field, it just isn't enough for radiation protection. Total extinction would result, even microbes wouldn't survive, and civilization would be impossible.

But is my reasoning true? Or is it possible for a black hole civilization to survive?

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    $\begingroup$ By "black hole civilization", do you mean a civilization on a planet that orbits a black hole? What does the system in general look like? $\endgroup$ – HDE 226868 Feb 20 at 20:19
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    $\begingroup$ @Caters We are a black hole civilisation, there's a supermassive one that we orbit around at the centre of the milky way. What're you asking that excludes us? There are of course suns, we're all moving in mutual relation according to gravity's attraction and inertia. $\endgroup$ – Chickens are not cows Feb 20 at 20:23
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    $\begingroup$ @Caters It's just that it's unclear that you meant by the term, which I haven't seen used widely anywhere. Maybe editing in some more details about the civilization(s) you have in mind would be helpful. $\endgroup$ – HDE 226868 Feb 20 at 20:25
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    $\begingroup$ Awwe, I clicked on this question because I thought it would be about a civilization inside a black hole (or, inside the event horizon, at least). :( $\endgroup$ – Loduwijk Feb 20 at 20:33
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    $\begingroup$ @Caters The question remains unclear, please edit it to reflect your intentions, so that people don't need to trawl the comments to understand what you are asking. $\endgroup$ – Chickens are not cows Feb 20 at 22:26
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Okay, while your assumption may not be wrong per se, the reasoning you have for it needs some work. Let's start with Gravity.

Black holes are not points of infinite mass, they're points of infinite density a black hole with the mass of our sun weighs (and exerts the same gravitational force) as our sun does. That is to say, if our sun turned into a black hole overnight, the earth would continue to orbit it with no change in the orbit attributable to a change in mass. (This is a simplification but functionally correct.)

Now; you mention radiation, or more correctly, charged particles, which are not the same thing, even though I'm as guilty as everyone of describing them as the same for brevity. The point is, if your planet is orbiting inside the magnetic field of the black hole or has its own strong magnetic field, then you're probably not too badly off. Even our sun emits a solar wind of highly energetic charged particles that would do us a lot of harm if it wasn't for our own magnetosphere on earth.

So in point of fact, to some degree at least, the magnetic field you mention and the charged particle wind (this time rushing in rather than rushing out) cancel each other out to some degree.

That said, you do have many problems with the concept of a civilisation in this model. This implies that the planet has existed for a few billions of years with life on it, slowly evolving, in an environment that is relatively stable. We've already had a handful of extinction events on the Earth and it's possible intelligent life would have evolved much earlier without them. But, your planet not only has to deal with charged particles, but any other debris surrounding the black hole that is getting sucked in. This could cause a much higher rate of extinction events for your planet, making intelligent life much less likely. There is also the energy level issue; is the fluorescing gas getting sucked in giving your planet enough ambient energy for endothermic reactions of any kind, like photosynthesis, to form? If your life has some form of geothermic-synthesis, that would at least explain why it has such a strong magnetic field, but such life would have a very different ecosystem to what we would normally imagine.

Your point about the competing magnetic fields does mean that if you get intelligent life to form, and if they reach some form of technological proficiency, their technology will look very different to ours because (mathematically speaking), from our perspective the EM unification is the easiest and most useful fundamental force unification we have. It would be useless to your planet because of the interference, meaning that their technology would probably go down a very different path.

In short, your intelligent life is probably unlikely from my point of view, but not for the reasons you state. In point of fact, 1 is a misconception, and 2 & 3 more or less cancel each other out.

BUT, your black hole still introduces some measure of energy shortages by comparison to a conventional star, and potentially more orbital instability that will cause an increased liklihood of extinction events. So, intelligent life and by extension civilisation will find your environment a very hard place to survive.

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Living too near a black hole is definitely perilous and, with the universe as it currently stands, it's not really advantageous to do so. A star system provides plenty of energy. Where things get more interesting is as the universe ages and the stars slowly die out. Late-universe life that wants to continue surviving would need to come up with a strategy for dealing with the death of their stars and black hole angular momentum is an appealing long-term energy reservoir once convenient self-sustaining fusion is no longer available. Of course, by this point, you're dealing with such far-future tech that it's hard to speculate what life might be capable of, technologically.

I'm imagining that a late-universe black hole civilization would probably either build their habitats a comfortable distance away from the black hole itself or would just harness former star systems that have gone cold and use those to contain the habitats. Energy would be gathered closer to the black hole and then somehow transported to the habitats. Perhaps long-range electromagnetic transmission into vast distributed-collector capture antennae. A light-house stationed around a cosmic whirlpool transforming the slowly decaying currents into the last light for a dying civilization.

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  1. Gravity

Not such an obvious issue after all. If we're talking about stellar-mass black holes, the ISCO is measured in mere tens of kilometers; of course a planet won't form there, because there's not enough room! At normal distance for planetary orbits, gravity is no different from what is it around a normal star.

Planets could form around a black hole in some of the same ways that they form around neutron stars; e.g., a black hole formed in a binary star system gets fortuitously shot through the heart of its binary companion by an asymmetrical supernova, immediately gaining an enormous disk of material whose "outer" reaches can form into a protoplanetary disk.

If we're talking about supermassive black holes, then the ISCO may indeed be a respectable distance out--so, you end up with a large, spread out planetary system. No big deal. At that distance, tidal effects will be negligible (at least from a "tearing the planet apart" perspective).

  1. Magnetic Field

Also not a huge problem. Black holes don't have frozen magnetic fields like neutron stars do. A black hole's magnetic field comes exclusively from it's spin combined with its electric charge. And astrophysical black holes will be pretty darn close to electrically neutral. And even magnetars only have strong magnetic fields near their surfaces. The only reason a neutron star can erase your credit card before it's even visible is because neutron stars are tiny and not very bright in the visible spectrum. At typical distances for planetary orbits, their fields are just not that strong--comparable to those of regular stars at similar distances.

Black hole accretion disks can generate powerful magnetic fields, but if your black hole still has an active accretion disk, well, nothing is living there anyway.

  1. Radiation

Jets are only a problem if you are in line with them, while a typical planetary system formed from a protoplanetary disk around the black hole would be perpendicular to the polar jets; they would be a complete non-issue for anyone living on such a planet. And on top of that, you only gets jets if you also have an active accretion disk--and you won't have an active accretion disk.

And it is not true that a black hole acts like a particle accelerator shooting out gamma rays from charged particles orbiting it. Neutral particles certainly aren't an issue; they will gain speed falling towards the black hole, sure, but they'll lose it on the way back out, so orbiting a black hole is in that regard no different from drifting alone through deep interstellar space. Charged particles will emit radiation as they accelerate towards and away from the black hole... but not much. Any charged particles that end up close enough to the black hole to be moving fast enough to emit significant synchrotron radiation won't stay in orbit very long--they'll be well inside the ISCO, and losing energy to that very radiation. And the rate of replenishment, even in the middle of a dense nebula, just isn't very high.

Now, if the black hole has an active accretion disk, then yeah, the radiation environment would be nasty... so just don't choose a black hole with an active accretion disk! And if the planet(s) formed in place as I suggested above, then there isn't an active accretion disk anymore by the time they're done forming. Those things pretty much go together.

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In the very far distant future, 10^127 years from now, much of the mass of the Universe will be contained in black holes. It may in fact be possible for civilizations to continue to exist that far in the future by using the Black Holes something like a hard drive:

https://www.scientificamerican.com/article/black-hole-computers-2007-04/

A one-kilogram hole has a radius of about 10-27 meter, or one xennometer. (For comparison, a proton has a radius of 10-15 meter.) Shrinking the computer does not change its energy content, so it can perform 1051 operations per second, just as before. What does change is the memory capacity. When gravity is insignificant, the total storage capacity is proportional to the number of particles and thus to the volume. But when gravity dominates, it interconnects the particles, so collectively they are capable of storing less information. The total storage capacity of a black hole is proportional to its surface area. In the 1970s Hawking and Jacob D. Bekenstein of the Hebrew University of Jerusalem calculated that a one-kilogram black hole can register about 1016 bits—much less than the same computer before it was compressed.

In compensation, the black hole is a much faster processor. In fact, the amount of time it takes to flip a bit, 10-35 second, is equal to the amount of time it takes light to move from one side of the computer to the other. Thus, in contrast to the ultimate laptop, which is highly parallel, the black hole is a serial computer. It acts as a single unit.

How would a black hole computer work in practice? Input is not problematic: just encode the data in the form of matter or energy and throw them down the hole. By properly preparing the material that falls in, a hacker should be able to program the hole to perform any desired computation. Once the material enters a hole, it is gone for good; the so-called event horizon demarcates the point of no return. The plummeting particles interact with one another, performing computation for a finite time before reaching the center of the hole—the singularity—and ceasing to exist. What happens to matter as it gets squished together at the singularity depends on the details of quantum gravity, which are as yet unknown.

The output takes the form of Hawking radiation. A one-kilogram hole gives off Hawking radiation and, to conserve energy, decreases in mass, disappearing altogether in a mere 10-21 second. The peak wavelength of the radiation equals the radius of the hole; for a one-kilogram hole, it corresponds to extremely intense gamma rays. A particle detector can capture this radiation and decode it for human consumption.

So even with the very limited amount we currently know about black holes, we can conceptualize using them as computers. Given the limitations described, civilizations at the end of time will either use supermassive black holes at the center of galaxies as their repository, or perhaps constellations of black holes orbiting each other to provide the equivalent of a RAID storage unit or supercomputer Beowulf cluster.

Since black holes already exist, civilizations in the galaxy can begin experimenting now so the ultimate supercomputers will be well worked out and ready 10^127 years from now when finally needed...

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Of course black hole civilizations exist and of course our only current comprehensional understanding may be enlightened as artisional .

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    $\begingroup$ The question has a science-based tag. Could you add some support for your answer? $\endgroup$ – puppetsock yesterday

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