16 ms·

A Dyson sphere around a black hole

214 points by SkyMarshal 5y ago

tarey 5y ago
A similar topic on YouTube by Kurzgesagt: https://youtu.be/ulCdoCfw-bY https://youtu.be/ulCdoCfw-bY
3np 5y ago
Anyone got a link to the full-text (or can at least cite the conclusion/discussion)? 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.
- NKosmatos 5y ago
  https://arxiv.org/abs/2106.15181 https://arxiv.org/abs/2106.15181
- raattgift 5y ago
  Thank you for asking for that, it motivated a few minutes of reading several short papers I likely otherwise never would have seen. https://arxiv.org/abs/2106.15181 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 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-collide-black-hole-get-watch-live https://www.pbs.org/newshour/science/massive-gas-cloud-colli... https://phys.org/news/2021-07-milky-supermassive-black-hole.html 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-tidal-disruption-event-relativistic-jet-direct-image https://www.sciencealert.com/arp-299-supermassive-black-hole... [3] Chou 2020 is https://doi.org/10.1016/j.heliyon.2020.e03336 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 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.
saagarjha 5y ago
Better link: https://arxiv.org/abs/2106.15181 https://arxiv.org/abs/2106.15181 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.
- kragen 5y ago
  Oh cool, CC-BY! 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.
  - m_mueller 5y ago
    As far as I understand, small black holes could be used as a super efficient energy storage („Kugelblitz“), but hardly as a source, assuming that primordial black holes are rare. Primordial black holes afaik are the only theorized origin of a sub stellar mass black hole at this stage of the universe‘s timeline.
    - zeristor 5y ago
      If only there was a way to break up the smallest size of black holes (there isn’t). If it was split up into several or thousands of much smaller black holes they, being much smaller would give off energy faster. 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.
      - sbierwagen 5y ago
        Oh, you wouldn't want to be able to split up black holes, since that means they could be destroyed during warfare. 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?
guender 5y ago
Yawn. Way more efficient/ easier to tap energy to build this around a main sequence star. Wouldn’t want to have to deal with a relativistic jet piercing my Dyson sphere or with the hard X-rays from accretion/the jet.
DonHopkins 5y ago
Holy plate of shrimp: I just built a small Dyson Sphere around a black hole, all by myself! Not quite as big and majestic as these though: Black Hole Dyson Sphere TIMELAPSE | Dyson Sphere Program https://www.youtube.com/watch?v=oEpTaXRIhE0&ab_channel=Nilaus 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=Pasukaru 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=OneLastMidnight 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=GameraGame 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=GameraGame 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=PL4o6UvJIdPNpJIYf0NexLTLwk9UosUHMO&ab_channel=KatherineOfSky https://www.youtube.com/watch?v=fScIOw07QRw&list=PL4o6UvJIdP...
- dreamer_soul 5y ago
  Hey! Love watching your videos! Keep up the good work
- andai 5y ago
  >The Dyson Sphere is an "emotional support sphere" for the Black Hole to remind it of its youth when it was surrounded by stars before it consumed them all
- deleted 5y ago
  [deleted]
- Cthulhu_ 5y ago
  I've got Dyson Sphere on my backlog; at the moment I finally budget and bought Satisfactory, it looks really well fleshed out at the moment and it'll take me hours and hours of more work to lay out my factory just to get the second package up the space elevator. Love it.
sydthrowaway 5y ago
Why do we think Dyson spheres exist. It's completely conjecture.
- fullstackchris 5y ago
  Exactly my thinking every time I read about them. It's one idea thought up by one scientist. It's so human of us to think some advanced race would build a giant capture device around a star... did anyone stop to ask that if a civilization could do that, they would likely already have technology that could (to our eyes) generate power out of thin air? Nope, caveman brain build big wall around star to get ALL power!
  - brnt 5y ago
    There are other benefits. 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.
    - Apocryphon 5y ago
      What if by the time you get to that level of technology, you no longer need that much physical space? I don't even mean in a soft sci-fi "evolutionary progress means becoming energy beings", I mean a hard sci-fi "advanced civilizations just digitize themselves into a virtual reality simulations."
      - est31 5y ago
        You won't need physical space but you'd still need energy to run that VR simulation.
Borrible 5y ago
Oh come on, this is meagre. Think bigger! How about a transmission belt around the milky way to propel a hyper space Tesla?
amelius 5y ago
Don't black holes suck-in all kinds of stuff all the time, making it a bad place to be for an energy plant?
- alpaca128 5y ago
  Black holes don't suck anything in. They just have very strong gravity relative to their size. So just like a space probe would drop into the sun if it got too close, things can get into an inescapable orbit around a black hole. If you can build a Dyson sphere around a star you can build one around a black hole of a similar mass.
  - deleted 5y ago
    [deleted]
  - codethief 5y ago
    > So just like a space probe would drop into the sun if it got too close 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).
- ajuc 5y ago
  They only suck in stuff that gets too close relative to their mass. They aren't magical. If you have a planet and turn it into a black hole with the same mass - anybody on orbit around that planet will continue on their orbits like nothing changed. 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.
foxes 5y ago
Taking this one step further, you could build an immensely powerful bomb/superweapon/mega space engine. Cover the inside of the Dyson spheres with mirrors then fire a laser inside. The black hole provides the perfect source to ramp up the energy before you could unleash it in a (un)directed way.
- adrianN 5y ago
  It's pretty hard to make mirrors that work at very high energy densities. We don't know how to make gamma ray mirrors for example, and even for longer wavelengths we don't have perfect mirrors.
  - snet0 5y ago
    Harder than making a Dyson sphere around a black hole?
    - adrianN 5y ago
      Depending on the size of the black hole, yes.
    - kevingadd 5y ago
      A perfect mirror is a materials challenge and potentially a fundamental physics challenge. In comparison we already know roughly how to do the math to figure out how far the surface of your sphere would need to be from the center of a sun or black hole in order to harvest energy (via whatever method) without collapsing, based on the strength of the materials at our disposal. Of course the materials we have might not be strong enough, but it's possible to push the sphere out until we can build it without it collapsing (and then the issue is collecting enough raw material). And given that solar panels work fine on earth, a dyson sphere can only improve solar panel efficiency even if its surface has to be very very far away due to materials challenges. 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.
okokwhatever 5y ago
Just image the endurance and structural composition of such Dyson sphere not to collapse around a black hole (I'm not an specialist but it came to my mind)
- snet0 5y ago
  If you're outside the event horizon, your experience is the same as that of being next to any object of equal mass of the black hole, located at its centre. Just don't go in!
  - okokwhatever 5y ago
    Interesting! Thanks for the clarification
- motoboi 5y ago
  We can have a black hole orbiting the sun right now and we wouldn’t know.
eterevsky 5y ago
Why aren't they mentioning extracting energy from black hole rotation (Penrose process [1] or something similar)? I thought it was the most reliable way of extracting energy from a black hole. [1] https://en.wikipedia.org/wiki/Penrose_process https://en.wikipedia.org/wiki/Penrose_process [2] https://www.popularmechanics.com/space/a14524087/black-holes-mass-energy/ https://www.popularmechanics.com/space/a14524087/black-holes...
ardit33 5y ago
The whole concept of a Dyson sphere is kinda idiotic. Any civilization that is capable to build one, it is probably able to work out fusion energy very efficiently. 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.
- hutzlibu 5y ago
  It depends on the amount of energy the civilisation needs. 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?
  - mclightning 5y ago
    Because you can build a more efficient one? You could make it portable, controlled, modular etc etc
    - hutzlibu 5y ago
      Yeah. All needed for interstellar travel for example. But for manufacturing planets?
      - mclightning 5y ago
        Why would you want to manufacture a gravity well? There are plenty of them
        5y ago
lend000 5y ago
A well done data-driven thought experiment, but I do have to ponder why the scientific community has settled on Dyson Spheres, as opposed to more exotic, "alien" ideas. I mean, do we really think a thirty-five million year old civilization is going to settle on an idea some simian came up with in a civilization ~150 years post-harnessing electricity, which amounts to "really big solar panels?" 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.
- renewiltord 5y ago
  No one has settled on anything. It’s just that one approach involves solving a tractable problem to reduce the search space and the other does fuck all. 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.
  - lend000 5y ago
    Nice strawman. Either way, you need new physics (to build a structure that large), so why not have a little more creativity while you're in the process of blatant speculation?
    - renewiltord 5y ago
      > why not have a little more creativity while you're in the process of blatant speculation? 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.
dcolkitt 5y ago
The paper derived results for stellar mass black holes, but why not do the same for super massive black holes at the center of galaxies? 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.
- Stevvo 5y ago
  Don't aim too high. Not because I don't think we would find anyone, but because if we did, who cares? Information reaching our galaxy would be too low resolution to learn much of anything, so the whole thing might end up being a distraction from searching for someone closer to home.
- mdaniel 5y ago
  In my mental model, if we built a structure around any black hole, let alone a supermassive one, I kinda doubt we'd need to go searching for other civilizations to tell them we exist and have The Knowledge
- codethief 5y ago
  The temperature T associated with a black hole's Hawking radiation goes as 1/M, where M is the black hole's mass, i.e. it decreases with M. Meanwhile, its surface area A goes as M². Plugging this into the Stefan-Boltzmann law yields a total power output P of 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.
  - dcolkitt 5y ago
    Cool! Thanks for deriving the equation. A lot about super massive black holes that become unintuitive at that scale. For example, I understand that the black hole at the center of the Milky Way has a lower density than iron.
toss1 5y ago
Reading the abstract, they didn't mention which sources ended up making more power, but I'd guess that (iii) an accretion disk, and (vi) relativistic jets are biggest. 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?
PaulHoule 5y ago
I've wondered if somebody could use a black hole jet to power a civilization. The trouble I see with it is that the power of the jet is highly variable so you need some mechanism to even it out... Maybe some really big dust clouds that re-radiate it at a reasonable temperature.
- guerrilla 5y ago
  Meanwhile, we can barely get scramjets to work on Earth. I'd say this idea (which is a bit vague to me) is pretty far off.
  - PaulHoule 5y ago
    I think it's interesting to work the problem through backwards. Imagine that a population was settled in some situation like https://arxiv.org/abs/2011.07487 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.)
Jeff_Brown 5y ago
Burning planets for energy, if using the energy from stars is indeed a viable alternative, will be a truly sad waste of the beauty and interesting ordered structure of space.
- nwah1 5y ago
  Luckily, there's a lot of planets. And dematerializing one is difficult.
- leonroy 5y ago
  I would hope in any future scenario we'd avoid burning a living world like the Earth and instead harvest resources from a dead planet like Mercury or Venus. 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 https://www.bbc.co.uk/iplayer/episodes/p07922lr/the-planets
  - Jeff_Brown 5y ago
    It certainly would be much worse to burn a living planet, but even a hellhole like Venus is a uniquely beautiful and mysterious scientific wonderland.
    - jjk166 5y ago
      You say that now when your entire civilization is limited to a single planet. But I would imagine the novelty wears off after you've explored billions of venus-like planets. For a galaxy spanning civilization, it would be equivalent to destroying an acre of the Sahara.
      - Jeff_Brown 5y ago
        An appropriately stiff integalactic tax on entropy, enforced only above some enormous threshold like burning a planet, could be optimal. 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.

214 points by SkyMarshal 5y ago

A similar topic on YouTube by Kurzgesagt: https://youtu.be/ulCdoCfw-bY https://youtu.be/ulCdoCfw-bY

3np 5y ago

Anyone got a link to the full-text (or can at least cite the conclusion/discussion)? 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.

NKosmatos 5y ago
https://arxiv.org/abs/2106.15181 https://arxiv.org/abs/2106.15181
raattgift 5y ago
Thank you for asking for that, it motivated a few minutes of reading several short papers I likely otherwise never would have seen. https://arxiv.org/abs/2106.15181 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 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-collide-black-hole-get-watch-live https://www.pbs.org/newshour/science/massive-gas-cloud-colli... https://phys.org/news/2021-07-milky-supermassive-black-hole.html 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-tidal-disruption-event-relativistic-jet-direct-image https://www.sciencealert.com/arp-299-supermassive-black-hole... [3] Chou 2020 is https://doi.org/10.1016/j.heliyon.2020.e03336 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 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.

saagarjha 5y ago

Better link: https://arxiv.org/abs/2106.15181 https://arxiv.org/abs/2106.15181 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.

kragen 5y ago
Oh cool, CC-BY! 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.
- m_mueller 5y ago
  As far as I understand, small black holes could be used as a super efficient energy storage („Kugelblitz“), but hardly as a source, assuming that primordial black holes are rare. Primordial black holes afaik are the only theorized origin of a sub stellar mass black hole at this stage of the universe‘s timeline.
  - zeristor 5y ago
    If only there was a way to break up the smallest size of black holes (there isn’t). If it was split up into several or thousands of much smaller black holes they, being much smaller would give off energy faster. 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.
    - sbierwagen 5y ago
      Oh, you wouldn't want to be able to split up black holes, since that means they could be destroyed during warfare. 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?

guender 5y ago

Yawn. Way more efficient/ easier to tap energy to build this around a main sequence star. Wouldn’t want to have to deal with a relativistic jet piercing my Dyson sphere or with the hard X-rays from accretion/the jet.

DonHopkins 5y ago

Holy plate of shrimp: I just built a small Dyson Sphere around a black hole, all by myself! Not quite as big and majestic as these though: Black Hole Dyson Sphere TIMELAPSE | Dyson Sphere Program https://www.youtube.com/watch?v=oEpTaXRIhE0&ab_channel=Nilaus 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=Pasukaru 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=OneLastMidnight 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=GameraGame 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=GameraGame 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=PL4o6UvJIdPNpJIYf0NexLTLwk9UosUHMO&ab_channel=KatherineOfSky https://www.youtube.com/watch?v=fScIOw07QRw&list=PL4o6UvJIdP...

dreamer_soul 5y ago
Hey! Love watching your videos! Keep up the good work
andai 5y ago
>The Dyson Sphere is an "emotional support sphere" for the Black Hole to remind it of its youth when it was surrounded by stars before it consumed them all
deleted 5y ago
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Cthulhu_ 5y ago
I've got Dyson Sphere on my backlog; at the moment I finally budget and bought Satisfactory, it looks really well fleshed out at the moment and it'll take me hours and hours of more work to lay out my factory just to get the second package up the space elevator. Love it.

sydthrowaway 5y ago

Why do we think Dyson spheres exist. It's completely conjecture.

fullstackchris 5y ago
Exactly my thinking every time I read about them. It's one idea thought up by one scientist. It's so human of us to think some advanced race would build a giant capture device around a star... did anyone stop to ask that if a civilization could do that, they would likely already have technology that could (to our eyes) generate power out of thin air? Nope, caveman brain build big wall around star to get ALL power!
- brnt 5y ago
  There are other benefits. 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.
  - Apocryphon 5y ago
    What if by the time you get to that level of technology, you no longer need that much physical space? I don't even mean in a soft sci-fi "evolutionary progress means becoming energy beings", I mean a hard sci-fi "advanced civilizations just digitize themselves into a virtual reality simulations."
    - est31 5y ago
      You won't need physical space but you'd still need energy to run that VR simulation.

Borrible 5y ago

Oh come on, this is meagre. Think bigger! How about a transmission belt around the milky way to propel a hyper space Tesla?

amelius 5y ago

Don't black holes suck-in all kinds of stuff all the time, making it a bad place to be for an energy plant?

alpaca128 5y ago
Black holes don't suck anything in. They just have very strong gravity relative to their size. So just like a space probe would drop into the sun if it got too close, things can get into an inescapable orbit around a black hole. If you can build a Dyson sphere around a star you can build one around a black hole of a similar mass.
- deleted 5y ago
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- codethief 5y ago
  > So just like a space probe would drop into the sun if it got too close 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).
ajuc 5y ago
They only suck in stuff that gets too close relative to their mass. They aren't magical. If you have a planet and turn it into a black hole with the same mass - anybody on orbit around that planet will continue on their orbits like nothing changed. 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.

foxes 5y ago

Taking this one step further, you could build an immensely powerful bomb/superweapon/mega space engine. Cover the inside of the Dyson spheres with mirrors then fire a laser inside. The black hole provides the perfect source to ramp up the energy before you could unleash it in a (un)directed way.

adrianN 5y ago
It's pretty hard to make mirrors that work at very high energy densities. We don't know how to make gamma ray mirrors for example, and even for longer wavelengths we don't have perfect mirrors.
- snet0 5y ago
  Harder than making a Dyson sphere around a black hole?
  - adrianN 5y ago
    Depending on the size of the black hole, yes.
  - kevingadd 5y ago
    A perfect mirror is a materials challenge and potentially a fundamental physics challenge. In comparison we already know roughly how to do the math to figure out how far the surface of your sphere would need to be from the center of a sun or black hole in order to harvest energy (via whatever method) without collapsing, based on the strength of the materials at our disposal. Of course the materials we have might not be strong enough, but it's possible to push the sphere out until we can build it without it collapsing (and then the issue is collecting enough raw material). And given that solar panels work fine on earth, a dyson sphere can only improve solar panel efficiency even if its surface has to be very very far away due to materials challenges. 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.

okokwhatever 5y ago

Just image the endurance and structural composition of such Dyson sphere not to collapse around a black hole (I'm not an specialist but it came to my mind)

snet0 5y ago
If you're outside the event horizon, your experience is the same as that of being next to any object of equal mass of the black hole, located at its centre. Just don't go in!
- okokwhatever 5y ago
  Interesting! Thanks for the clarification
motoboi 5y ago
We can have a black hole orbiting the sun right now and we wouldn’t know.

eterevsky 5y ago

Why aren't they mentioning extracting energy from black hole rotation (Penrose process [1] or something similar)? I thought it was the most reliable way of extracting energy from a black hole. [1] https://en.wikipedia.org/wiki/Penrose_process https://en.wikipedia.org/wiki/Penrose_process [2] https://www.popularmechanics.com/space/a14524087/black-holes-mass-energy/ https://www.popularmechanics.com/space/a14524087/black-holes...

ardit33 5y ago

The whole concept of a Dyson sphere is kinda idiotic. Any civilization that is capable to build one, it is probably able to work out fusion energy very efficiently. 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.

hutzlibu 5y ago
It depends on the amount of energy the civilisation needs. 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?
- mclightning 5y ago
  Because you can build a more efficient one? You could make it portable, controlled, modular etc etc
  - hutzlibu 5y ago
    Yeah. All needed for interstellar travel for example. But for manufacturing planets?
    - mclightning 5y ago
      Why would you want to manufacture a gravity well? There are plenty of them
      - 5y ago

lend000 5y ago

A well done data-driven thought experiment, but I do have to ponder why the scientific community has settled on Dyson Spheres, as opposed to more exotic, "alien" ideas. I mean, do we really think a thirty-five million year old civilization is going to settle on an idea some simian came up with in a civilization ~150 years post-harnessing electricity, which amounts to "really big solar panels?" 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.

renewiltord 5y ago
No one has settled on anything. It’s just that one approach involves solving a tractable problem to reduce the search space and the other does fuck all. 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.
- lend000 5y ago
  Nice strawman. Either way, you need new physics (to build a structure that large), so why not have a little more creativity while you're in the process of blatant speculation?
  - renewiltord 5y ago
    > why not have a little more creativity while you're in the process of blatant speculation? 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.

dcolkitt 5y ago

The paper derived results for stellar mass black holes, but why not do the same for super massive black holes at the center of galaxies? 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.

Stevvo 5y ago
Don't aim too high. Not because I don't think we would find anyone, but because if we did, who cares? Information reaching our galaxy would be too low resolution to learn much of anything, so the whole thing might end up being a distraction from searching for someone closer to home.
mdaniel 5y ago
In my mental model, if we built a structure around any black hole, let alone a supermassive one, I kinda doubt we'd need to go searching for other civilizations to tell them we exist and have The Knowledge
codethief 5y ago
The temperature T associated with a black hole's Hawking radiation goes as 1/M, where M is the black hole's mass, i.e. it decreases with M. Meanwhile, its surface area A goes as M². Plugging this into the Stefan-Boltzmann law yields a total power output P of 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.
- dcolkitt 5y ago
  Cool! Thanks for deriving the equation. A lot about super massive black holes that become unintuitive at that scale. For example, I understand that the black hole at the center of the Milky Way has a lower density than iron.

toss1 5y ago

Reading the abstract, they didn't mention which sources ended up making more power, but I'd guess that (iii) an accretion disk, and (vi) relativistic jets are biggest. 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?

PaulHoule 5y ago

I've wondered if somebody could use a black hole jet to power a civilization. The trouble I see with it is that the power of the jet is highly variable so you need some mechanism to even it out... Maybe some really big dust clouds that re-radiate it at a reasonable temperature.

guerrilla 5y ago
Meanwhile, we can barely get scramjets to work on Earth. I'd say this idea (which is a bit vague to me) is pretty far off.
- PaulHoule 5y ago
  I think it's interesting to work the problem through backwards. Imagine that a population was settled in some situation like https://arxiv.org/abs/2011.07487 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.)

Jeff_Brown 5y ago

Burning planets for energy, if using the energy from stars is indeed a viable alternative, will be a truly sad waste of the beauty and interesting ordered structure of space.

nwah1 5y ago
Luckily, there's a lot of planets. And dematerializing one is difficult.
leonroy 5y ago
I would hope in any future scenario we'd avoid burning a living world like the Earth and instead harvest resources from a dead planet like Mercury or Venus. 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 https://www.bbc.co.uk/iplayer/episodes/p07922lr/the-planets
- Jeff_Brown 5y ago
  It certainly would be much worse to burn a living planet, but even a hellhole like Venus is a uniquely beautiful and mysterious scientific wonderland.
  - jjk166 5y ago
    You say that now when your entire civilization is limited to a single planet. But I would imagine the novelty wears off after you've explored billions of venus-like planets. For a galaxy spanning civilization, it would be equivalent to destroying an acre of the Sahara.
    - Jeff_Brown 5y ago
      An appropriately stiff integalactic tax on entropy, enforced only above some enormous threshold like burning a planet, could be optimal. 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.