A similar topic on YouTube by Kurzgesagt:
https://youtu.be/ulCdoCfw-bY
The publicly available abstract just states "In this study, we discuss whether building a Dyson sphere around a black hole is effective." but no statements as to what they arrive at.
The results are fairly obvious: CMB and Hawking radiation provide almost zero power output, while an accretion disk and relativistic jets can provide a lot of power.
In theory you can get an arbitrary amount of power from Hawking radiation if you have a lot of very small black holes instead of just one big one. I feel like the stability of the negative-feedback control systems for their orbits might be important here, especially if they're orbiting something you care about like your home planet.
Black Hole Dyson Sphere TIMELAPSE | Dyson Sphere Program
https://www.youtube.com/watch?v=oEpTaXRIhE0&ab_channel=Nilau...
[Timelapse] Dyson Sphere around Black Hole (Dyson Sphere Program)
https://www.youtube.com/watch?v=HXwrePn_JJc&ab_channel=Pasuk...
Dyson Sphere Program - FLYING INTO A BLACK HOLE!
https://www.youtube.com/watch?v=nIYooegxSQI&ab_channel=OneLa...
In case you don't know what Dyson Sphere Program is -- think 3D Factorio in Space:
Dyson Sphere Program Trailer
https://www.youtube.com/watch?v=PJ3fDcm5qaQ&ab_channel=Gamer...
Dyson Sphere Program Release Date Announcement Trailer
https://www.youtube.com/watch?v=Eh-voJ3TSt8&ab_channel=Gamer...
Katherine of Sky has some great DPS videos, and she also plays lots of Factorio and Satisfactory too:
Dyson Sphere Program Ep 1 - Factorio + Satisfactory + Astroneer = FUN? - Let's Play, Early Access
https://www.youtube.com/watch?v=fScIOw07QRw&list=PL4o6UvJIdP...
We are essentially searching for civilizations that have no other forms of energy (from a star) and have to rely on Dyson spheres for their energy.
We are excluding all those civilizations that have successfully used a combination of solar, wind, fission & fusion energy to drive their society.
I strongly believe that no matter what civilization, no matter how they look, what they consume or what they are made of, the core tenet of "human psychology", or more correctly "conscious being psychology" would evolve in the same way as is for us.
If, let us say, we solve our energy issues, and we have built ourselves a world that is sustainable. Would we think about building dyson spheres? No, because there is only so much you can do with energy. You need materials, ores, deposits, etc to actually build something new.
I guess the most advanced civilizations would harness the power of the stars, fusion, and build themselves sustainable habitats that can be floated into the eons.
Edit to resolve Poe's law ambiguity: this is a parody. Obviously aliens would have lightning network which would require a smaller overall dyson sphere
If you had the power of a star you could create all of that from base elements via eatting the star, and you'd barely make a dint in it's mass.
Personally I think it will ultimately prove out to not be true in all cases. I might sound like a heretic for saying this and would be berated in the scientific community for saying that, but in the past most laws that establish constraints eventually prove out to be only true under certain controlled environments.
It should also be considered a successful form of fusion energy capture, using the star as a gravity-contained reactor.
A Dyson sphere makes sense when your home planet is not enough by far, and even all the planets in your system are not enough.
Physicists knows the exact reason why energy is conserved and why it might not be conserved. Conserved quantities arise from symmetries in variational systems. Conservation laws are consequence, not unexplained axiomatic law.
Energy is energy wouldn’t a matter black hole and an anti matter black hole just make a black twice the size, minus a bit for gravity waves.
I was thinking given a large enough gravity wave it might be able to stretch a black hole apart. How huge that could be, and how that could be generated is probably beyond the limits of reality.
If you could make a fusion reactor almost infinitely better then it would produce a black hole which would immediately evaporate releasing more energy than you could ever imagine - but no way it can be done.
A black hole is generated when mass-energy density is above a certain threshold. You need to pack about a megatonne of mass-energy in a sphere 3 attometers in diameter to get a black hole approaching a useful lifespan. You could do it with bosonic matter, but photons are easier. Well, "easier":
>In Section V below, we discuss the plausibility of creating SBHs with a very large spherically converging gamma ray laser. A radius of 1 attometer corresponds to the wavelength of a gamma ray with an energy of about 1.24 TeV. Since the wavelength of the Hawking radiation is 8π^2 times the radius of the BH, the Hawking temperature of a BH with this radius is on the order of 16 GeV, within the limit of what we could hope to achieve technologically.
>[...]
>In a previous paper by the first author [9], it was proposed that a SBH could be artificially created by firing a huge number of gamma rays from a spherically converging laser. The idea is to pack so much energy into such a small space that a BH will form. An advantage of using photons is that, since they are bosons, there is no Pauli exclusion principle to worry about. Although a laser powered black hole generator presents huge engineering challenges, the concept appears to be physically sound according to classical general relativity.
The process of aligning the laser array will be interesting, however. From Wikipedia:
>Unlike most objects, a black hole's temperature increases as it radiates away mass. The rate of temperature increase is exponential, with the most likely endpoint being the dissolution of the black hole in a violent burst of gamma rays. A complete description of this dissolution requires a model of quantum gravity, however, as it occurs when the black hole's mass approaches 1 Planck mass, when its radius will also approach two Planck lengths.
See table 2 from the paper. A 0.32 attometer wide black hole has a surface temperature of 98.1GeV and is losing 61.4 kilograms per second-- 5,519 petawatts of ultra-hard gamma radiation! Even at that diameter it has a sizable weight of 108,000 tonnes, but it doesn't have long to live-- a couple weeks. A poorly collimated laser array will produce undersize black holes which will rapidly evaporate. The paper suggests keeping them well away from Earth, on the other side of the Sun if possible. Not hard-- you'd want to put the array well within the orbit of Mercury for better solar power flux.
There are many awful engineering problems of a black hole drive, but they're not so bad as to dismiss out of hand. See https://arxiv.org/abs/0908.1803v1
The internal surface area of a sphere with radius 1AU has so much area that its unlikely any spacefaring civilization would ever spread to that many planets. It would be enough to have billions of versions of your civilization/species with room to spare, AND have them all be able to communicate without light years of transmission delays, never mind time and cost of travel, and thus drift apart. Then, one has the opportunity to otherwise live in very similar circumstances as the planet one evolved on. Looking at our attraction to nature I would guess any species would think of this as a benefit, and would prefer it over terraforming faraway hostile environments or inhabiting cramped space cabins.
So yes, if you have the ability to make a Dyson sphere, I can very well imagine that you would.
Those symmetries are not shown to be universally true, they are just what we have been able to observe, much like Newton's laws are based on observations on the Earth only.
Yet Einstein showed that they are only a special case of a much more general universe.
[1] https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
Depending on what the cosmological constant is, massive black holes could be quite useful in the far future. 10^100 years from now, long after all the stars have gone out, it could be the case that the only life in the universe would be huddled up against the event horizon of supermassive black holes, exploiting the tiny temperature difference between the pseudosurface and the void to generate a trickle of power.
Splitting black holes would rob the far future of that power source to be used now. You could do it, but why shorten the useful life of the universe?
Termed the "halo drive", it's a very nice theoretical concept: https://www.youtube.com/watch?v=rFqL9CkNxXw
Hmm. Magic and miracles are not the same. One is breaking some of the core laws of physics that hold true regardless of your "capabilities".
I also don't really like that justification because it can essentially justify any sort of belief.
So the sub-attometer-sized black holes described here seem like they might be a lot more practical than what I was thinking of. But... what does it take to stop TeV gamma rays? (And I guess they might also spew out strange matter and naked truth and the like, but you could probably just toss it back in.)
The billion-tonne gamma-ray laser they're talking about here is absolutely tiny.
For a meteor it makes no difference if gravity is coming from a planet or a black hole of the same mass. The only difference is that once you get very close - the gravity changes much quicker (because black hole is much smaller and denser than a planet) so you will be ripped apart.
By contrast, there's ample reason to believe that black-hole power plants are possible. In fact, the spatial precision required is actually the same order of magnitude as that of LIGO, about an attometer. So we might not even have to wait 350 years for it.
[1] https://en.wikipedia.org/wiki/Penrose_process
[2] https://www.popularmechanics.com/space/a14524087/black-holes...
There is no point to go and harness energy around a star or a black hole, when you can just produce it locally with a lot less resources/waste and materials. The sun itself is actually very inefficient in producing energy.
There is no need to harness the sun million of km away, when you can recreate it in your home planet. The only way to produce a dyson like of sphere, is to tame an over-heated sun, and reflect away un-needed energy. But there is no point to build one to just harness it.
It makes great sci-fi stories, but that's about it. Scientifically, it just doesn't make sense.
I'd speculate that power sources used by such civilizations would be so exotic to us, we would need new physics to fully describe them.
Also consider that the electromagnetic force is on the order of 10^36 times stronger than gravity. The electromagnetic force is what keeps your butt from falling through your chair, and gives all materials their macroscopic and chemical properties. And it sort of sets a soft limit on the sizes of rigid structures, animals, etc. Note that there are on the order of 10^30 atoms in a blue whale or Sequoia tree, and ~10^33 in a cargo ship.
Lets say, you want to manufacture new planets/planetary sized spaceships. You would need very big fusion reactors and you probably want to save the special fusion fuel for those, when they are on their voyage to other worlds. Because then they need fusion.
But why build a fusion reactor, when you already have a big one avaiable?
Its not about need, but about want. If you can have anything, my bet is that a Dyson sphere makes more sense than Cyberpunk tech and eating protein goop. Although there is space enough on the sphere for people that prefer it, of course, which there no doubt will be.
https://arxiv.org/abs/2106.15181 is the preprint.
The authors' model rests on an asymptotically flat incoming Vaidya spacetime[1] equipped with a cosmic microwave background that dilutes away over time (like the real CMB) but without the worries of tracking the cosmological constant sufficiently near the black hole.
They unsurprisingly find that the energy budget from the background light that collides into the black hole in this model is an uninteresting part of the system's extractable energy budget. The real CMB has a low energy-density at all times when our universe has star-filled galaxies in it, and one would need a high boost to get significant power from it, or equivalently, one would have to gravitationally modify the energy-density locally. (That boost could be found by an observer hovering just outside their model non-rotating black hole, or in a different model where (for example) where a black hole's rotation can lead to an observer developing angular speeds approaching the speed of light; however there is no obvious way to transmit harvested power of this sort "up" to a Dyson sphere thanks to gravitational redshifting at least. The authors admit that calculating an energy budget which relies upon black hole rotation is too hard, and move on).
Skipping over the accretion disk section, which I'll return to, the authors also find that in the absence of angular momentum and, an incoming spherically symmetrical uncharged "rain" onto the black hole does generate significant outgoing radiation, which is also unsurprising. This is after all essentially what the CMB is, just with a higher "molecular weight" than light. Of course this particular rain is unphysical: our universe is populated by clouds of gas, and stars, which periodically -- not uniformly in time -- intersect black holes like the one in our galactic centre [2], causing occasional flares. They do not spend much time describing an integral approach around this. For what it's worth most principled uses of the Vaiyda metric consider outgoing radiation as an easy proxy for Hawking radiation (or even more often for modelling the cooling of white dwarfs). A massive incoming Vaiyda "anti-shine" has some interesting consequences or requirements for regions far from a black hole with lifetimes of more than a few million years!
The authors do not properly explain how they equip their model with a relativistic jet (they repeat an empirical luminosity relationship between astronomically observed jets accretion disks, but no causality is found in their model, and their model is different from "real" astrophysics) which is quite striking given their everywhere spherically-symmetric, exactly zero angular momentum, and exactly zero magnetic charge setting for for their CMB and Bondi accretion analyses. Their Chou 2020 reference [3] underpinning their relativistic jets purports to describe a spacetime where the authors break this no-angular-momentum condition. From that breaking they take spherical symmetry into axisymmetry, and with some addition of magnetic interactions, they arrive at an environment that might plausible host relativistic jets. But they also suddenly introduce significant consequences of angular momentum and other nongravitational charges near their model black hole that are not accounted for in their CMB or Hawking analyses. They do not explain how to reconcile their findings from two qualitatively different spacetimes, nor do they attempt an argument that the quantitative differences vanish "below" the altitude of the Dyson sphere. (Do you have to make an axisymmetric Dyson swarm, where the equatorial collectors have a high orbital angular velocity?)
I think that the authors would have written a more interesting (and challenging) paper if they had chosen to explore the components of the energy budget of their Dyson-around-a-black-hole system using a gravitational and matter background that does not change from subsection to subsection.
That they didn't raises two questions I think are large.
Firstly, can they get their §2.6 relativistic jets somehow without introducing both angular momentum and magnetic interactions? (The formation of relativistic jets is a live area of current research).
Alternatively, can they find the heavy calculation lifting in other published work for their energy estimates from CMB-infall and infall-onto-accretion-structure physics? Their Chou 2020 references at least shows that they are competent to do a wide search for papers which purport to do that kind of heavy lifting. Their §2.3 already refers to other work in accretion disc physics that reasonably covers both extremes of angular momentum, but there's a gap between the two which is likely interesting for relativistic jets that an be "mined" by their Dyson sphere, and closer to the spin parameters of known black hole candidates.
- --
[1] §2.4 could be much clearer about this, especially in light of the "Kerr-Vaiyda" metric they found in [4]; they consider a vanishingly small a parameter (sometimes implicitly) whenever they discuss their model black hole, and resile from the consequences of a significant nonzero spin, except briefly for the case of the maximum possible spin. Perhaps they found it easiest to import the maths from [4]'s results accepting them as good especially in the limit where a vanishes, rather than sticking to a more widely used model -- https://en.wikipedia.org/wiki/Vaidya_metric. After a brief glance at the edit history, I have stray thoughts about the timing of the introduction of [4] at the end of that wikipedia page.
[2] examples https://www.pbs.org/newshour/science/massive-gas-cloud-colli... https://phys.org/news/2021-07-milky-supermassive-black-hole.... and, in a not-too-distant galaxy, https://www.sciencealert.com/arp-299-supermassive-black-hole...
[3] Chou 2020 is https://doi.org/10.1016/j.heliyon.2020.e03336 (pay-for-publication open access by bottom-feeding Elsevier). I believe the author to be https://www.researchgate.net/profile/Yu-Ching-Chou where one finds a link to this reference, and that the publication record of the single-author relativity papers listed there speaks for itself.
The additional benefit is the amount of power. With the power of a star at your disposal, beaming your energy using lasers and microwaves, even if only 1% efficient, would instantly solve the energy needs of any planet in a solar system. If we did it, we could instantly colonize all planets in our solar system because things like "running an AC on Mercury" or "Hovercraft in Jupiter Atmosphere" becomes trivial when you have limitless power.
Heck, you could start projects like moving planets in orbits or collecting astroids to built a planet if you wanted.
Black holes are the stage after that since you can use a spinning black hole to convert a given mass into energy at >10% efficiency (given E=MC^2). That would allow a civilization to power multiple solar systems at the cost of a few planets per year.
Granted, the sphere is difficult to build, but that is what you have dyson swarms for.
I’ve lost my keys, and these guys are asking me to retrace my steps while you’re asking me to consider the machinations of hitherto sleeping whimsical gods. I’ll try your thing next, but first I’ll do the other guys.
In comparison, if you can't get your mirror close enough to perfect, the whole idea doesn't work - the laser will be absorbed or diffused too much by the mirror, or the mirror will be damaged from the energy, etc.
Umm...
The sun is about 2×10^30 kilograms. The rest of the solar system is about 3×10^27 kilograms and we kind of need it to live on and make things with. Put in other words, the entire solar system including the Sun is 1.0014 Solar masses. Why not go with the process we have rather than fusing everything we have to do the same thing?
If you're saying that we don't need that much energy then I think that's pretty meaningless since we have no idea what we could get up to or need in the far future.
Well, hydro energy is just there, requires comparatively little ongoing maintenance, and has less catastrophic failure modes.
I think the same caveats work for Dyson spheres.
Few planets are similarly sized as Earth already btw, although with varying densities.
I wrote: "Physicists knows the exact reason why energy is conserved and why it might not be conserved."
To parse it:
1. exact reason why energy is conserved
2. why it might not be conserved.
After that your efficiency declines until... Well you end up with fusion reactors that look like the sun. You also end up fusing all the mass that we need to live our lives on and make stuff out of.
If you don't think a civilisation could ever need the amount of energy that a Dyson sphere could provide, that's fine, we don't need one.
But if you imagine a civilisation that does need the energy a Dyson sphere can provide, then they make a lot of sense.
And I just assume that a civilisation advanced enough to build a dyson sphere, can handle the non trivial orbital mechanics, too. But maybe still cannot travel FTL. So indeed rather choose to build planets, or planet sized space stations.
That would require lots of energy and maybe the need to harvest the sun directly and control the light going out towards the planets.
If anyone reading this loved building factories in factorio and building huge buildings in Minecraft, try Satisfactory now.
It's unstable without active stabilisation. The stabilisation has to be perfect because otherwise you get massive distorting spherequakes propagating through it, which will make it a very uncomfortable ride for a short time and will soon tear it apart.
A perfect spherical stabilising solution with components and sensors that are light minutes - or possibly light hours - from each other is impossible because of the signalling delays.
Ringworlds have the same problem. Even if the ring is made of infinitely strong unobtanium, it won't stay in a useful orbit for long enough to be built, never mind inhabited.
Swarms have more potential, because you can build them with sparse shells and move things around to handle instabilities. But you still need massive computational effort to keep swarm units from colliding with each other, possibly at very high speed.
None of this matters, because the limiting factor for civs is collective intelligence, not energy.
We already have access to far more energy than we could possibly use on earth for any technology that's actually buildable. But we're not using it because we don't have the species IQ to make the right choices.
That is a lot of mass to extract and transport to the power stations (accelerate, decelerate). So it just makes sense to only needing to build the facilities to collect the power from existing sources (stars, black holes) without the logistics of transporting the fuel.
Moreover, the space around a star or black hole is real estate that would go unused otherwise, while asteroids, moons, ... are more likely usable by such an advanced civilization.
[1] https://www.quora.com/What-would-the-estimated-weight-of-Mou... (I took the highest estimate in the first answer)
This. The entire “moving planets” and “powering asteroids” theme is just a projection of our historical aggressive reproduction, which is not even a requirement for an advanced civilization. Earth alone could serve us a few billions of happy years, if we didn’t have the happy urge to multiply until critical shortage. That’s far from intelligent, imo.
Nobody knows anything about quantum gravity, which is the problem ;)
>order of a nanometer
As db48x says, synthetic black holes aren't going to be much good as power plants, just because it's so darn hard to get them big enough that they can consume normal matter. Let's talk bare minimums. Say the nucleus of an hydrogen atom is about 2.4 femtometers wide. Plugging the numbers into https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati... the mass of a black hole that size is 8.07991 * 10^8 metric tons. That is a lot of laser pulse power to get into a single point just to create the black hole. Wolfram alpha claims it's 2.01718 x 10^25 watthours. The sun radiates 3.828 x 10^26 watts, so we need about a tenth of total solar output for an hour-- assuming we can convert captured solar power into coherent gamma rays with no loss, which there is currently no plausible physical way to do, so we probably need 10 or 100 times as much power. We're beyond future ultratech, and getting into solar engineering.
Assuming the black hole is created, which is hard enough, we have further problems. A 2.4 femtometer black hole is still darn hot: it's radiating a total of 545.6 megawatts of light with a peak of 51.3 MeV-- still ultrahard gamma radiation. Our beam of hydrogen ions need to fight past this headwind and into the singularity-- quickly. It's losing 6.054 micrograms per second, which will be hard to deliver as a single row of hydrogen nuclei.
Making the black hole bigger in the first place will make it easier to feed... but if you can commandeer a tenth of the Sun's output at will, you don't exactly need domestic power from a black hole.
Let's use your parenthetical as an excuse to keep charge vanishingly small, because we can avoid thinking "Which charge? Which charge carrier or carriers? What's the distribution of charges?", and largely ignore the electro- effects of electrovacuum (which answers these, but in surprising ways when you look deeply).
In a chargeless vacuum Schwarzschild or Kerr universe, we have total coordinate freedom because there is nothing there but the mass at an infintesimally small point, p. If one builds a system of coordinates with p always at a single spacelike point (say, the spacelike origin), then the symmetries of this vacuum system let one chop away spatial position (e.g. const.coord.x, const.coord.y, const.coord.z, t -> 0,0,0,t) and consequently the vector-quantity linear momenta of the black hole vanish. Moreover, these vacuum spacetimes are also eternal (the black holes do not grow or shrink), so we can do t = const. too. With suitable coordinates, gives us two free parameters: mass & spin (and in Schwarzschild, just one: mass).
These solutions do not superpose additively. By the Raychaudhuri focusing theorem, if we add a point mass to the Schwarzschild black hole universe, the two masses will eventually collide. We have broken the spherical symmetry of the Schwarzschild solution, and when we solve the geodesic equations, we find caustics, where our two infinitesimal masses can be in the same infinitesimal space. We have also broken time symmetry: at past time the two masses are spacelike separated. In the future they are not, as they will merge into one black hole. We also have the problem that the black holes move with respect to one another, so we either adapt or system of coordinates to be comoving with the black holes, or we have one or both of them move against the spatial-coordinate part of our system of coordinates.
When we take this further by breaking other symmetries than the time one, e.g. by adding angular momentum to the system, we have to consider the evolution of orbital angular momentum of the pair, and possibly the spin angular momentum of each. Either of our previous-paragraph choices with respect to encoding the coordinate evolution of the spatial distance between the pair of black holes complicates the calculation of the related vector quantities.
We're still in the land of a small number of parameters, but have gone from the three time-independent [mass, spin, charge] to eleven time-dependent [mass, spin, charge, 4-location, 4-linear momentum].
We can explode the number of parameters though by returning to "what is charge?", and equipping these universes with fields of matter. At this point one runs right into the question of: "does the no-hair conjecture hold in a physically plausible universe surrounding a theoretical black hole?" or almost equivalently "when do theoretical black holes fail to approximate astrophysical black holes?", and Ligo/Virgo are good laboratories for studying whether merging black holes go completely bald.
(The hair that is supposed to bald away may be soft and indirect: the circulation of gas and dust at a distance may reveal that a given black hole was previously more than one black hole. After merger, none of that should make a difference to anything (including a 3rd black hole) falling into the balded merged black hole that the merged black hole was previously two black holes. More critically, in the enormously distant future, the evaporation of the merged black hole should not reveal the number or types of objects that fell into it during its history, whether those are black holes or some neutral mix of standard model particles. The Hawking radiation spectrum at any moment should depend only on the eleven parameters in the previous paragraph. But maybe that still-outside dust and gas has memory that remains relevant arbitrarily far into the future. Or maybe classical general relativity is wrong and rather than being crushed into a memoryless ultramicroscopic point, the ingested gas, dust and other black holes retain or at least reveal their individual identities even during evaporation).
Questions like these make black holes extremely interesting, I think.
With that, being happy does not seem to be a universal quality of life; some would even posit that the opposite is true (ask Gautama B.)
I don't see humans completely overcome their biological nature any time soon, if ever.
The central theoretical problem is that taking Hawking seriously, the stuff inside stays inside, but inside goes away. What happens to the stuff? There are more theoretical answers to that written down than there are actual theorists, and presently no astronomical or laboratory observations which let us throw practically any of them away.
(Also of course, inside might not go away after all -- not at all or not completely -- with large numbers of explanations of how that might work, and nothing concretely observed that lets us discount such possibilities in favour of total evaporation.)
Humans are exceptional in this regard as that can expand beyond the limits, and at the expense of, all other known species. Humans are also exceptional in that they could extinguish themselves, either by choice on their own folly.
It's the inevitable end-result of Bitcoin.
Not intentionally, but we've managed to create societies where reproduction is extremely costly in every conceivable way and are even working to convince people that reproduction is outright tyrannical, sexist, and environmentally genocidal.
The instinct will certainly remain, but population decline seems inevitable the way things are going. Even a few decades from now, the human breadbaskets of the developing world adopt the anti-natalist systems we have in the developed world.
Why couldn't a Dyson sphere have uncoordinated movement joints distributed all around to absorb those spherequakes? Or even active locally coordinated ones?
I'm sure a Dyson-sphere-building civilization would figure this out.
https://en.wikipedia.org/wiki/Projections_of_population_grow...
Yes, this would be constrained to ultra-advanced type III civilizations. But it would let us search for ET civilizations across the entire universe, not just our own galaxy.
Given incidence of dam bursts like the 1975 Banqiao dam failure[1], with an estimate death toll of 26,000 to 240,000 people and flooding of over 12,000 square kilometers, I'm inclined to disagree with this assessment.
I don't see any requirement why an intelligent civilisation would need to grow its population forever. It seems far more intelligent to attack the cause of the energy need instead of building crazy structures like Dyson spheres around black holes.
Repeat at 90 degrees, 45 degrees, etc until you have a sphere.
You're right that speed-of-light delays make negative feedback control difficult, but it might be possible to have rockets that sense local photon flux on the inner surface of the ring/sphere and fire autonomously when it increases.
https://en.wikipedia.org/wiki/Star_lifting
Even without considering what to do with the resultant mass star lifting is just good stellar husbandry for a civilization looking to last.
Of course. But they reach a limit. Only one species is able to exploit resources, and the biosphere, to its own ends. Agriculture, mining, and technological advancement are peculiar to humans. All of these permit us to live longer, with fewer setbacks, and in environments no other animals can survive (space, and soon other planets). Are these not exceptional traits?
We can also engineer a virus, or deploy strategic weapons that could wipe out our entire species (and a bunch of others). Is that not exceptional?
So what if it's inefficient? It's happening anyway and it's not like you can siphon the hydrogen off and do anything else with it.
What kind of purism is it to not tap the largest energy sources in existence because you bristle at their inefficiency.
But some animals have agriculture (ants?) and some plants exploit metals in the soil (phytomining)! Still, Sapiens are the champs.
That's just what species are selected for. If you have two competing populations and one multiplies and expands just slightly better than the other, everything else equal, then that one will eventually outcompete the slower one.
But it does not have to be that way. Humans are the top predator, so no competition here. We could make do with limiting expansion if we chose to. It's still life, but without this "quality" that you seem to consider a necessity.
The technology required to build a tier 1 swarm is not advanced. All you need is the ability to build a thin reflector (aluminium foil would work) with radio and a basic onboard computer. You do not even need onboard propellant because a reflector can generate the required thrust for minor corrections with solar pressure alone. These reflectors can then concentrate that light from close in to the star out to collectors further out in the system. This essentially makes your habitable zone as large as you want since you can also light entire planets this way and only utilize less than a percent of a percent of your total energy budget.
The real challenge is having good enough automation to construct and send these stations out without requiring too much oversight, and getting good enough at construction off earth (since you really don't want to deal with that gravity well). We're definitely close to having the required automation technology and we should be performing the first manufacturing off earth before the end of the next decade.
Artificial fusion can be made far more efficient than the fusion in the core of our sun, yes. However, the sun outputs enough energy to light 2 billion earths worth of surface. Moreover, it essentially contains all of both the hydrogen and rocky material in the solar system with the other bodies being essentially a rounding error in comparison, so even in a controlled fusion based economy, you would still create a Dyson sphere like construct since you would be using that star as your main fuel and mass source.
Black holes though? They make even aneutronic fusion look like a fire cracker in comparison. You're getting less than a percent mass energy conversion with even the best theoretical fusion models there are, whereas with black holes, you can get anywhere from 16% to 40% depending on the technique and mass range of the black hole. Stellar black holes have so much energy inside that they would dwarf even our star (consider that their progenitors were stars with far more mass than our own and with an even bigger disparity in power output). They also last for periods of time that make even red dwarfs look like still births. If you could make artificial ones in the gigaton and megaton ranges, then they can also make for amazing batteries. In this case, you are able to tap the hawking radiation emitted for power and this promises potentially even greater efficiencies than the stellar mass ones (albeit, you would not be gaining a net power gain here, this is strictly for power storage and potentially gravity generation purposes).
Below energy means any nonzero of any component of the stress-energy tensor T.
In vacuum flat spacetime, Tmunu def. 0, so the energy at every point is 0, the total energy is 0, and the average energy density at every point is 0.
We can take this logic to several families of vacuum spacetime.
In the sort of steady-state universe discussed in the early 20th century there is some distribution of stress-energy, varying the energy at any given point, but total energy = const., average energy density = const.
The problem is when we start adjusting the number of points without altering the stress-energy. Interiors of (e.g. Lemaître-Tolman-Bondi) black holes develop lots of new spacetime points, but most of those points have zero stress-energy. Futures of expanding universes have lots of new spacetime points, and most of them have zero stress-energy. Put the two of them together in a "swiss cheese" spacetime, (we can cast a clumping \Lambda-CDM cosmos as one of those, with galaxy clusters forming and ultimately collapsing into ginormous black holes, and extragalactic space evolving into exceptionally hard vacuum "cheese" surrounding these holes), and you have a recipe for a growing number of points in space which with no stress-energy in them. For a sufficiently long-lived expanding swiss-cheese universe, what's the average value in the components of the stress-energy tensor? Zero. Approaching future timelike infinity, good luck finding any non-zeros at all.
Weiss & Baez have a nice but brief expansion upon the \nabla_mu \cdot T^munu = 0 point that Carroll at your link declined to explain :) under the "Divergence and Integration" heading at https://math.ucr.edu/home//baez/physics/Relativity/GR/energy...
Like them I would not go into Noether's (first) theorem in any detail in explaining this even to an audience with more familiarity with physical cosmology than Carroll had in mind, for the following reason.
Note that while Weiss & Baez write that certain cases meet most of the conditions required for Noether's theorem to hold, they do not say with specificity that in general dynamical spacetimes do not possess the conditions required by Noether's theorem, as noted in her commentary about the non-independence of the Euler-Lagrange equations in her second theorem paper E. Noether, Nachr. d. Konig. Gesellsch. d. Wiss. zu Gottingen, Math-phys. Klasse pp. 235–257 (1918). In modern language the current j^mu_beta vanishes iff the divergence of \sqrt(-det g_munu)\cdot T^mu_beta = 0. I can dig up some refs for this if you're realllllllllly keen, although I'd probably have to start with parts of Katherine Brading & Harvey Brown's 2000s-era "really, which symmetries?" work.
I'd rather just say, hey, in reasonably realistic spacetimes we break all sorts of symmetries compared to popular classics, and even those broke one or more symmetries compared to flat spacetime, and among the consequences are that things you (in flat spacetime) think are scalars (e.g. energy) may demand to be treated as tensor contractions, and vectors (e.g. momentum) are often even worse. These quantities should take on different names to avoid confusion. And those are what you are conserving, approximately conserving, or not conserving.
Or, to the point, any practical definition of energy is spacetime-dependent and the spacetime of special relativity is just one of an infinite number of possible spacetimes, and the only one that in 3+1 dimensions is uncurved.
The bright side is that in Lorentzian (3+1) spacetimes, you always have at least an infinitesimal patch of flat spacetime around any point, and often an even larger patch of effectively flat spacetime, so we have still have the local conservation laws ancestors discovered.
In most parts of our universe, and everywhere on Earth, the radius of (Riemann tensor) curvature is large. So one can talk with virtually perfect accuracy to many digits of precision about special-relativistic energies or momenta of particle colliding experiments (including natural "experiments" carried out in post-supernova/post-white dwarf deflagration nebula), and use conservation laws to decide there must be some species of particle carrying away some seemingly-missing energy or momentum, even though one can use atomic clocks or Pound-Rebka devices to measure the actual nonflatness of the patch of spacetime the collider building is in, or observe with spectroscopy and interferometry manifestly general-relativistic effects around the stellar remnants. All of that is relevant if one is hunting axions, for example, where one will benefit from local Lorentz invariance and the conservation laws that flow from that.
This is a little important in the face of people misunderstanding "energy is not conserved in general relativity" as permitting EmDrive-type nonsense.
And finally, recalling my first line, in classical General Relativity we can define cosmologies for curved spacetimes with arbitrary integer numbers of dimensions greater than 1 such that non-conservative extra functions (encoding a "friction" dissipative coupling between a metric and stress-energy, for instance; people have actually done this in bi-metric gravity for inflation reasons, and so have "neo-(quasi-)steady-state-universe" people) must accompany the Lagrangian. This accompaniment is incompatible with Noether's (first) theorem, even locally, except where (and if) the dissipative force has decayed away.
However, my first thought is wouldn't the Dyson sphere itself get in the way of the accretion disk?
Stars are generating energy by fusing previously accreted mass. But black holes need new mass to accrete and accelerate to add energy. It seems a hapless incoming star about to get spread into accretion disk material might do some serious damage to the Dyson sphere?
And it's such a banal obvious claim I'm making, and yet it's being downvoted to oblivion. I can't imagine cane toads doing anything I mentioned.. or even ants having some sort of hegemonic control over the fate of all other species.
We have no predators but we do have a lot of existential threats. The more we spread out the more chances we have of surviving. If we sit on our hands and stay put we’ll wind up like the dinosaurs one way or another.
> it just makes sense to only needing to build the facilities to collect the power from existing sources (stars, black holes) without the logistics of transporting the fuel.
Yeah OK, but then you have to transport your entire reactor to the fuel. For that matter, you also have to transport the energy where it's needed once you've harnessed it.
With D + D fusion (easy to believe compared to CNO) an interstellar civilization could be essentially independent of stars.
I'm sure part of the resolution of the Fermi paradox is that dry inside-the-frost-line planets like Mercury, Venus, Earth and Mars are not the generic places you find life.
If things were a bit different I could see a would-be Galileo on an less cooked version of Io or a more cooked version of Europa who looks at the Earth with a spectroscope and sees oxygen and has a huge amount of trouble with his local "church" that thinks it is impossible to have life on a planet with less tidal activity, less radiation, inside-the-frost-line dryness, etc.
Even if creatures like us became interstellar we might have millions of kilometers of condominiums and shopping malls in the oort cloud and could care less about inner solar system planets.
If you haven't played DSP since they introduced blueprints, I'd encourage you to give it a revisit. Although ironically the mass deconstruction in Factorio is equally handy but is still missing in DSP, which can make any rework still be a grind
Seems like it would require, at minimum, several planets worth of raw material.
Yet there are actual dust rings and discs in the sky surrounding stars for long periods without falling into the star, being blown out of the gravitational influence of the star, or condensing into planets on life-evolving timescales.
Some of them are pretty occlusive and get hot (~ 1000 degC) and are consequentially observational targets for MATISSE at the European Southern Observatory. It's not at all my field, but I gather https://theskylive.com/sky/stars/kappa-tucanae-star is a principal hot dust belt target.
Retreating towards my islands of comfort, there are plenty of small rocky bodies ( flavour of "dust" in the sense of the previous paragraphs) in the solar system that are in no immediate danger of colliding, plunging inwards, or escaping to infinity in spite of ablation from solar radiation, differential radiation pressure and other effects that lead to the term "active asteroid". Most of them will be there, and have been there, for billions of years despite evil Jupiter trying to mess them up. Speculating a little away from my weak-gravity island, I would be surprised if there were no ongoing organic chemistry reactions in the more icy carbonaceous active asteroids in our solar system. It's not a huge leap from that to an asteroid-based power station that slowly produces some alcohol(s) at much denser concentrations than the wood alcohol molecular clouds like in W3(OH) astrophysical alcohol megamaser region. [1]
Icy-carbonaceous bodies in closer orbit and with sufficient spin angular momentum (perhaps supported by the Yarkovsky effect) for cheap thermal management could in principle host terrestrial photoautotrophs suitable for processing into biodiesel. Impractical at small scale, but maybe useful if one has many billions of such "power plants" around a single star. And of course, one could use the photoautotrophs to feed microbes more suitable for the production of more complicated molecules like, oh I dunno, oxycodone (for fun and profit).
(A sufficiently dense dust belt in the goldilocks zone could even make use of ideas from the panspermia hypothesis, so that one only has to populate an initial fraction of the bodies with economically useful microbes, and the little bugs will reproduce and spread throughout.)
One can easily imagine greater efficiencies from engineering non-organism power collectors and storage systems instead of adapting blue-green algae to output biological compounds, and one would probably want to do this for a dust that is in bulk much hotter than the vast majority of our solar system's asteroids tend to get, or if practically the entirety of the dust is manufactured rather than already in place ready for populating with mats or colonies of economically useful microbes.
Returning to the paper at the top, I think the only advantage a black hole gives is that a sufficiently large one might have a much larger localised "goldilocks" volume which one might seed with small icy-carbonaceous body light-powered power plants, compared to having many such belts around many many stars. However density of production could be an important advantage, even if one has to move a swarm of objects to it rather than adapt a swarm of substrate objects already in place.
Super-concisely: Earth has a lot of oil, inefficiently, seemingly by accident, after a long delay from initial "activation" of the biosphere, and with a fairly high collection cost because it below so much atmosphere (and water and rock) through which it would have to be lifted for off-world use. Sufficiently advanced aliens (and perhaps future humans), if they wanted oil, undoubtedly could do better than Earth.
- --
[1] http://www.star.bris.ac.uk/mark/w3oh.html but don't inhale it, the W is for Wood alcohol (hydroxymethane).
If you really want to extract mass from a star, just wait long enough, or find one ready to release a suitable amount. There are plenty of options from CMAs to flares to UV Ceti style flare stars, to deflagrating white dwarfs, novas, and supernovas. Let the star's nuclear energy do the "heavy lifting", eliminating all the problems catalogued before the "... plasma jets hundreds or thousands of astronomical units long ... [t]he details of extracting useful materials from [that has] not been extensively explored".
(The linked article at the top discusses relativistic jets driven by million-solar-mass and larger black holes. The jets are likely to be more constant and/or more predictable than natural mass outflows from stars, but extracting and storing energy from that is even less "extensively explored". An option in between is to use the natural mass flow from a companion star onto a compact star (white dwarf, neutron star, black hole) by sticking the moral equivalent of a water wheel into the stream. It's just a (compared to water hot & sparse) fluid flowing downhill, and there are plenty of examples in our galaxy.)
Why do people keep insisting on the Dyson concept as though it was real, even though it's just the fanciest thing we can imagine with our current understanding of physics?
The same way in the 19th century nuclear power couldn't be envisioned, there's definitiely a bunch of technology we don't know about now that an alien civilization could be using; we could even be observing this right now and we would be none the wiser. Talking about Dyson shperes, or swarms, or rings, and then concluding that there's now alien civilzations because we can't see them is just anthropocentrism, failing to consider other possiblities
One solution is for the expansion joints to have infinite range— a disconnected swarm of platforms which is exactly what a Dyson swarm is.
If the argument is that somehow the former is more environmentally friendly, then I don’t understand why we should accept dystopia for an uncaring god.
A.) Keep it in place and not hit the star/black hole.
B.) Avoid it being demolished by intercepting every piece of space trash, comet, asteroid in a solar system?
Hypothetical scenarios can be constructed for both methods of power generation (What if Braidwood plant melts down, somehow killing the 5 million people living within a 50 mile radius? What if the Three Gorges Dam busts, inundating an area inhabited by 600 million people?[1]) Either way, it is far from obvious to me that hydro has the superior safety profile, especially when their fatality rates are on the same order of magnitude even when the Banqiao incident is removed[2]*.
[1] https://web.archive.org/web/20210620174812/https://www.japan... p/opinion/2020/09/01/commentary/world-commentary/big-china-disaster/
[2] https://ourworldindata.org/safest-sources-of-energy
* Sovacool et al. (2016), the source of the data in [2] includes a hydro fatality rate per tWh 2.4x larger than nuclear when Banqiao is included.
When in the past, unanswered, mystery questions like these were finally answered (for example "how come light seems to appear in discrete packets?"), the answer ended up being a door to a completely new understanding of reality, and not a little footnote question at the End of Physics
So yeah, when there are no more unanswered physics questions, I will consider that the laws of physics have been exhausted. Right now that's not the case.
But on the off-chance you're not - and for everyone else who's intrigued by this comment's concepts - I can recommend watching it.
The thing with very small black holes is that they're virtually impossible to feed stuff into - not only are they an extremely small target, but the flood of energy escaping will push away any matter that comes nearby.
That's an interesting take. It does seem that we're more limited by our poor choices than by underlying physical limits. We invest ridiculous amounts of money into weapons by comparison to things like nuclear and fusion power.
EDIT: Inb4 the downvotes.
I bought it about a week after it became available on Steam and played it all the way through (Unlocked the last item in the tech tree) and wanted to try again with some different gameplay options, but not until blueprints were implemented. I must have missed the announcement.
So you think an alien civilization advanced enough to build a dyson sphere would not know how to leverage it for the fundamental goal of implementing blockchain?
I guess it's true what they say: there's a sucker born every minute.
This is precisely the appeal of Larry Niven's Ringworld concept. You instead take a slice of that sphere and spin it around the star and generate gravity through centrifugal force. They can take a number of configurations but the default one would give you the equivalent of 2 million earth's worth of living space inside one continual surface. This is obviously hugely mass intensive and having a ring of that kind of radius spinning at the tangential velocities required for the equivalent of one g of simulated gravity (around 1200 km/s) requires materials so magically strong that they make graphene look like soggy tissues. The only material that could do this even in the theoretical realm is magmatter, which would be materials constructed out of magnetic monopoles, something that has been predicted to exist by some but has yet to be observed.
However, you could construct one using known materials by creating a superstructure around the ring that is much more massive and orbits the other direction at normal orbital speeds for a body about an AU out from the sun with the ringworld electromagnetically suspended from the superstructure, normalising their collective momentum to zero. That obviously greatly increase the mass requirements but this could mostly take the form of hydrogen and helium which is far more abundant than heavier elements and would be usable as fuel for fusion reactors. That superstructure can contain many smaller rotating habitats and the whole thing could be used as a vast habitat for microgravity living as well - something I envision as many humans doing as living on worlds with gravity by the time we would have constructed one of these.
You also want to have a superstructure because at such a high tangential velocity, if a human being of 80 kg struck the ring from the outside, it would blow with the kinetic energy of a thermonuclear bomb. Point defence is absolutely necessary and the superstucture offers both an excellent shield covering the non sun lit side of the ring and a great platform for hosting those point defence systems. Ringworlds are also not in stable orbital configurations over astronomical timescales so you need to maintain their orbit through corrective thrust. This could be done through light sails which would also be appealing for it's mechanical simplicity.
Ringworlds are very mass intensive especially if your civilisation still has no access to unobtanium building materials and I don't think this would be something we would ever build a lot of in our particular solar system but even with that mass cost, you could easily gather enough rocky material from the sun to build a few and still have the vast majority of people living in other far smaller structures (but only relative to a star encompassing ring) in the swarm.
For a realistic visualization of what one would see when falling in a black hole see https://jila.colorado.edu/~ajsh/insidebh/schw.html
That said, seems the Earth was once inhospitable around the time Venus was 'alive'. Good documentary which lends weight to what you said: https://www.bbc.co.uk/iplayer/episodes/p07922lr/the-planets
P ~ A · T⁴ ~ 1/M²
So while a larger total amount of energy can be extracted from a supermassive black hole (because it's more massive – duh), it also takes much (much!) longer to extract a given amount of energy: A supermassive black hole's mass is in the order of 10⁶ to 10⁹ solar masses, i.e. 10⁶ to 10⁹ times the mass of a stellar-mass black hole, meaning that its power is just 10^-12 to 10^-18 × the power of a stellar mass black hole.
The probability for this to happen is actually quite low, ie. it's suprisingly hard to "hit" the sun. The reason is that unless you put the probe on a completely straight path to the sun (which is hard because the sun is so small in terms of its solid angle on the sky), it will have some angular momentum and probably spin around the sun and come right back (more or less).
Imagine that a population was settled in some situation like
https://arxiv.org/abs/2011.07487
and they were interested in visiting the Earth after 1000 years of not doing so. Alternately imagine some species with an interstellar lifestyle based on oort cloud objects and D + D fusion wanting to do the same.
They could have highly developed technology in many respects but they might find the process of developing a "space shuttle" that can enter the Earth's atmosphere with a full load of fuel, then SSTO back to space to be difficult.
Even an interstellar-capable civilization might take as long to develop an excursion vehicle for the Earth as it took us to develop a vehicle capable of leaving the Earth.
Notably the Earth is a lot harder to get on and get off of than other worlds; the 2nd stage of the SpaceX Starship can SSTO any world in our solar system which could plausibly be landed on except for the Earth.
Interstellar voyagers who had spent 10,000 years en route would inplausibly have one ready to go, the best they would have is 10,000 year old instructions to make one with a 10,000 year old 3-d printer. (Which got optimized to make fusion reactor parts and might not quite make reentry body parts exactly the same as it did the last time it made them.)
You could mine it from asteroids, which have plenty of base metals. There's already a speculative market for investors wishing to mine them. The only thing is; the technology for transporting it around doesn't exist yet. (We can land probes on them however for recon, but that's about it so far!).
I've heard Dyson spheres referred to as a K2 Civ's wonder or vanity project.
Science advisor:
Eternal leader, those pesky Alpha Centaurans just completed work on a galactic wonder, the Dyson Sphere! [+1000 energy, +100 habitat]
Shall we redirect our own Dyson Sphere efforts to a new project?
Eternal leader:
Damnit! Yeah fine whatever, let's switch to building a Kemplerer Rosette.Is this really the limiting factor? I've always assumed the limiting factor was distance. I can envision a species that is more intelligent or more altruistic. But what I can't envision is how collective action is coordinated over interstellar distances. It would seem to me that the extreme communication delay would cause any interstellar civilization to inevitably break apart into a bunch of separate "local" civilizations.
I've poured about 60 hours into this game and I'm done. Back to Satisfactory for me.
Bayes’ Theorem and the fact that probabilities exist smoothly from zero to one on hypotheses.
For the same reason that you consider checking your kitchen table for keys before you contemplate their disappearance into the quantum froth.
Alternatively, “show me the code”. Let’s see what you’re talking about in specific instead of hiding in generalities.
If tomorrow all ants disappear then most land-oriented ecosystems would just collapse.
We humans are having so egoistically anthropocentrical world views that it's not even funny anymore.
Solving world hunger does not need expansion. Stopping climate change does not need expansion. Preventing the upcoming antibiotics crisis does not need expansion. To the contrary, expansion makes all those things worse or has even started them in the first place.
- a civilization which has discovered a means to efficiently transform atomic mass into energy with arbitrary elements not considered good candidates for fission or fusion.
- a civilization which has a means to extract work from zero point energy.
- a civilization which is power efficient enough to never need an entire star's worth of energy localized in a single solar system. Note that John Von Neumann's brain consumed about 20 Watts of power.
Obviously, there is no right answer, because we aren't there yet. But the point of the original comment is that the ratio of media about Dyson Spheres to other speculation is really unbalanced for how impractical/uncreative (in this day and age, not when they were conceived) of an idea they are. No need to get combative about it.
Actually this is another megastructure process known as Star Lifting. And it's not just hydrogen you can siphon off - the sun, by virtue of its immense size, contains the overwhelming majority of every element in the solar system.
Unfortunately because they burn up so quickly, the hydrogen isotopes we would use in fusion reactors are actually incredibly rare in the sun. The sun is estimated to have about the same amount of deuterium as Earth's oceans. On the other hand, Helium-3, another potential fusion fuel, is very abundant - about 10-20 Earth Masses are present. Still though, burning all this helium 3 at the same energy production rate of the Sun would only last us 870,000 years - a drop in the bucket compared to the lifetime of the sun.
Because you're right -- at some scale, for some payoff, burning a Venus is the right decision. But ceteris peribus, harvesting from the entropy already present is better than adding to the universe's entropy.
DSP's is https://store.steampowered.com/news/app/1366540 and the update in question is https://store.steampowered.com/news/app/1366540/view/3008952...
They also added some kind of global scoreboard system (Milky Way) if one is into that kind of thing and want a bigger challenge that just "local universe automation nivrana": https://store.steampowered.com/news/app/1366540/view/3044975...
They alleged to have released some portion of the source code (https://store.steampowered.com/news/app/1366540/view/5694225...) in order to make mods easier, but there is some massive confusion about where, exactly, one can find such a thing and I'm evidently too old to find my way around Discord
It's for sure a game-changer(!) for "welcome to this new planet with some iron and copper next to each other: let me cook some interesting resource that I need (e-mag are my go-to because it seems one can never have enough of those)
As a QoL improvement, it's also handy for "oh, look: titanium -- let the drones lay out 15 smelters with their power poles to exfiltrate that resource"
Some of the screenshots folks have done about power production are insaneo -- to the point their GPU starts to drag from the game ticks :-)
This problem is twofold. First is that much food is produced far from many poorly nourished people. This problem is totally solvable by modern transport. There are enough charity donations to prevent hunger everywhere.
Second, and most importantly, it's the social structure what prevents ending the hunger where the hunger still is. People prevent production and distribution of food in order to keep the social structures where they are on top, or are fighting to be on top. They themselves are not hungry, and don't care about the rest much.
> Humans are also exceptional in that they could extinguish themselves, either by choice on their own folly.
Could you imagine ants finding a method to annihilate themselves? I don't claim that animals are unintelligent, but we are unique in our ability to create technology to achieve almost any goal, no matter goal's 'sapience'.
> If tomorrow all ants disappear then most land-oriented ecosystems would just collapse.
I don't dispute this, but it's completely orthogonal to my claim. Humans have disrupted the biosphere in a manner that is unprecedented since when meteor strikes and seismic events were still shaping earth
However, the trouble with larger black holes like the 2.4-femtometer example is not primarily that they require a lot of mass-energy to produce, but that they last a long time. According to the calculator linked above, it is indeed radiating 545.6 megawatts, which sounds like a lot. But compared to the 808 million tonnes you put into it, it's not very much; it will take you 4.2 trillion years to get back the energy you put in, about 250 times the current age of the universe. (The lifetime calculation on that page is shorter, only 777 billion years, because it's assuming you're allowing the black hole to evaporate rather than feeding it to keep it the same size.)
(BTW, actually that was all for a 2.4-femtometer-radius black hole, not a 2.4-femtometer-diameter one.)
This is really shitty energy-efficiency from a time-discounted perspective. If you use a conservative 3% yearly discount rate, the time-discounted earnings from your power plant over those 777 billion or 4.2 trillion years are equal to their non-time-discounted earnings over only the next 33.3 years. So from an economic point of view your efficiency is not 100%; it's 7.9e-10%, 0.000000000786% efficient. And that's not even taking into account the costs of building the giant gamma-ray laser.
So, for reasonable economic efficiency, you really need to build a black hole with a lifetime of 100 years or less. That means 400,000 tonnes or less, radius of 6.1e-10 nanometers (0.61 attometers) or less, 300 trillion kelvin or more, 102 GeV or more, 2.1 petawatts or more. In nuclear-bomb terms, that's 0.5 megatons per second (still very small compared to the sun's 390 yottawatts or 85 petatons per second), except that it's coming out in 102 GeV photons. That's about the mass of a silver atom, though still very small compared to the Oh-My-God Particle.
As skykooler points out, this could complicate the task of feeding the monster. Maybe you could set it to orbiting at a few kilometers per second inside a solid object such as Ceres, so that it occasionally gulps a proton on its way through the body, leaving a trail of rapidly cooling subterranean plasma in its wake.
Of course, if all the energy that's coming out has to go in through a nuclear gamma-ray laser, it's not really a power source, just a battery that's conveniently portable and has a built-in rocket engine.
A Dyson sphere is foolish because one can simply build a small local fusion reactor instead of using the giant free one nearby?
Imagine a sphere of reflective particles around a star. Planets within would never experience nightfall. Free energy would be everywhere. Sounds useful.
Moreover, such a solar system would be difficult to detect. If it is mobile, and uses a low-signature form of propulsion such as UFOs exhibit, then it would be extremely elusive.
This solves the problem of someone launching stealth asteroids at one's base at relativistic speeds.
Moving a solar system presumably requires tremendous power. Using a star as the fusion reactor presumably helps.
On Earth, none of those are available in the quantities needed to provide sufficient energy for us (close to a planetary civilisation in energy needs) on the scale of millions of years.
If you want your civilisation to not deplete available energy in the next million or so years and to allow for growth of energy demands in that time, you're going to have to make use of star power.
A planet only gets so much starlight that it becomes a limiting factor in energy budget. The easiest next step is to make more use of the abundant energy source next door: your star.
There are other options, e.g. using black holes. But those are orders of magnitude more challenging: black holes are extreme stars (extremely far from any developing civilisation).
On Earth, we've seen that every leap-frog in civilisation (fire, agriculture, industrialisation, ...) increased the available energy budget by orders of magnitude. There's not that many orders of magnitude left on this planet to grow further; if we want more, we'll need to get it from elsewhere.
Alien civilisations that remain bounds to their planet's energy budget similarly must remain stuck at a certain level of energy use. They can make advances in efficiency, but their total energy budget is capped.
The age of star formation is mostly over yes, but not completely yet - and I believe it's figured that no red dwarf can have reached even a tiny fraction of its lifespan so far?
That doesn't sound like it could be true. Black holes are densest possible objects in the universe. For instance, take a piece of iron of mass M = 1kg. Compress it down till it's diameter is smaller than twice the Schwarzschild radius associated with this mass (which is 2GM/c² = 1.4· 10^(-27) meters, where G is the gravitational constant and c is the speed of light[0]). You now have a black hole – one which has the same mass as your original piece of iron but which occupies a much smaller space than that piece of iron did.
For a supermassive black hole like Sagittarius A* (4.3·10⁶ solar masses, radius 11.25·10⁹ m) one obtains a density of volume / mass ~ 4.5·10⁶ kg/m³ – clearly this is much higher than the density of any iron that you can find on the surface of Earth.
Side note: Terms like "density" (= mass divided by volume) and "volume" don't really make sense in the context of black holes – the region "behind" the event horizon (the "inside" of the black hole) is not of "space" type (in physics lingo: it's not a 3-dimensional spacelike hypersurface). Instead, it's a 4-dimensional region of spacetime, meaning that it includes a time direction. It makes little sense to talk about the volume of such a thing (let alone of the density of mass "spread out" in such a spacetime).
But even more crucially, the region of spacetime behind the event horizon contains spacelike hypersurfaces of infinite volume. So it makes even less sense to assign a volume to it.
What people usually mean when they refer to the volume (and, by extension, the density) is the following: It's (roughly speaking) the volume you have to compress a mass down to for it to become a black hole. More precisely, for an observer at infinity, at any given instant of time a (static, spherically symmetric) black hole's event horizon appears to be a sphere of surface area 4π·(2GM/c²)², i.e. radius r = 2GM/c² [1]. Now take that radius and plug it into the usual formula for the volume of a ball of radius r. You have now defined the "volume" for a (static, spherically symmetric) black hole.
[0] https://www.wolframalpha.com/input/?i=schwarzschild+radius+1...
[1] This is really the definition of the radius. This is because there is no center of the black hole on the "inside", so in particular it doesn't make sense to talk about the radius (= distance of the black hole's spherical surface to its "center").