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Let's assume we have this super advanced civilization that actually goes around making Dyson spheres around stars.

The purpose of this is to capture all the energy of the star and be a hub of the system in shipbuilding, weapon building, administration, and military matters. Think the Citadel from Mass Effect.

All this free energy is being used to power shields and weapons.

If you had a civilization with such technology and such a station, with a near limitless power source, would the station be invincible?

We see in fiction how shields can be overpowered by constant bombardment. But if you have the sun as your power source, would it be possible to ever break the shields?

The station would also have incredibly powerful weapons, given its power source.

I understand that it's not a matter of pure power, but let's assume that the civilization has incredibly sophisticated shields and weapons, and all they need is the power to power them up.

Assume incredible overall technological ability to the point that their shields can be attacked for months without failing and their cannons,an constantly fire. Would that mean their stations would be invincible?

In the context of galactic empires fighting, Dyson Sphere stations seem like they are checkmate. They have a sun and unless you teleport to the system with your own sun you are not breaking through.

I'm no scientist and so the finer points of science would be very handy here. And I'm not looking to hand wave the whole thing by saying: their science is so advanced. At the same time I'm not interested in limiting this advanced universe to 2020 abilities and understanding.

One thing I've considered is that maybe it is simply impossible to translate the raw power of the sun to raw power for the station, or a time delay / technological obstacle preventing instantaneous transition of power to the shields and, to a lesser extent, weapons.

Is that the case? What are other potential problems with these invincible stations?

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    $\begingroup$ Wait for answers but, you said it "ping" is the concept. They may be undefeated when defending, but one thing is using all that power close to their star, and other is to efficiently transport it far away. There is room for strategy and action, far away from their star, they are one more lifeform. Even if they find a way, when attacking, others may effectively defend by sabotage whatever is the way they have to transport power. $\endgroup$ – Hatoru Hansou Aug 31 '20 at 23:41
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    $\begingroup$ Throw a planet at it. $\endgroup$ – Brondahl Sep 1 '20 at 9:36
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    $\begingroup$ Throw a black hole at it. $\endgroup$ – Frank Sep 1 '20 at 12:18
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    $\begingroup$ What if the enemy has two dyson spheres? $\endgroup$ – user253751 Sep 1 '20 at 12:48
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    $\begingroup$ @Brondahl. Throw a bigger sun powered by an even bigger sun at it $\endgroup$ – Mad Physicist Sep 1 '20 at 19:01

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The Problem of Defense:

This is a similar problem to the one envisioned by the French after WW1. They imagined the Maginot line would make France invulnerable to attack, but the Germans simply side-stepped the defense, or, as in the case of the Belgian forts, they simply sabotaged them or used bigger explosives. There is always a struggle between defense and offense. While I agree that the Dyson sphere defense is probably invulnerable to lesser civilizations, any of their peers will simply apply engineering until they come up with a way to overcome it.

While a Dyson sphere is an amazing feat of engineering, it is still a gigantic passive power collector. So you use a Dyson sphere to power your station. Your "Invaders" threatening your "Spherians" arrive with a fleet to attack the thing.

The Invaders have planet-sized ships and antimatter-powered cannons. Your Dyson sphere is putting out a huge total amount of power, but it's still a giant collector for a known and fixed quantity of output - one star. When one ultra-powered race confronts another ultra-powered race, the conflict is using ultra weapons and ultra power sources. The Invaders can have hundreds of planet-sized attack ships and still not use up as much mass as the Spherians did building the sphere. the attackers can keep bringing more firepower to compensate until they exceed the defensive ability of the sphere. The sphere is fixed and cannot move away if the enemy has too many ships.

Further, because of the vast size of the sphere, it makes a HUGE target. The smallest sphere is as big as a star, assuming you're okay with the inside surface not being a habitable area. Shields protecting it need to be correspondingly vast. The Invader ships can be powered by devices that put out more power per unit of size - probably a lot more power, if they don't simply stream vast quantities of antimatter AT the sphere as an attack.

Your Dyson sphere is more a city or a factory and less a battleship. Cities can have walls, but no walled city is invulnerable. There's always a bigger cannon.

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    $\begingroup$ You and I were thinking the same thing at the same time... +1. $\endgroup$ – JBH Sep 1 '20 at 1:02
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    $\begingroup$ If you have a star, it's huge. Yes, it's putting out a lot of energy, but it is spontaneous fusion spread out over the size of a star. A spacecraft from a high-end civilization will have a reactor using something like antimatter that produces vast amounts of power. It's kind of like the difference between a diesel engine and powering a power plant by burning coal. The power plant puts out more energy, but the diesel engine is more space-efficient. $\endgroup$ – DWKraus Sep 1 '20 at 4:14
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    $\begingroup$ @Seallussus A star has about the same power output per volume as a compost pile. They just have so much power because they are so big. A simple steam engine already has a higher power density. $\endgroup$ – Christian Sep 1 '20 at 9:14
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    $\begingroup$ @Seallussus The typical weakness of smaller engines is that they’re less efficient - to run an antimatter engine you need to make the fuel, transport it, contain it, safely burn it, and then utilise the energy. The advantage is that you can put out a lot more energy in a smaller package when it matters - like in a battle $\endgroup$ – Pingcode Sep 2 '20 at 0:34
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    $\begingroup$ Perhaps a better use of the Dyson sphere as defense is just as a nigh inexhaustible refueling source for a massive fleet of ships patrolling a good distance from it. If anti-matter power is a thing, then the Dyson sphere power can be used to manufacture vast quantities of anti-matter to power ships which intercept threats before they can make it to the sphere. $\endgroup$ – Shufflepants Sep 2 '20 at 8:49
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Invincible?

We honestly have no idea how closely a dysonsphere/swarm/blah could be built to a star. Future tech would suggest that it could be built as close as wanting... but that doesn't change a couple of issues.

That's a LOT of surface area to protect. As intoned by the great George S. Patton: "Fixed fortifications are monuments to man’s stupidity. If mountain ranges and oceans can be overcome, anything made by man can be overcome." You might have a near limitless amount of energy to work with compared to Ming the Merciless' Amassed Armada, but while you're filling the solar system with the future equivalent of hot lead hoping to hit those little itty-bitty ships, he's dropping a thunderous number of bombs on that massive shell.

To be fair, I'm having trouble finding a reference to the sun's total output and not just the amount of output experienced by the Earth. For example, this reference suggests 3.846×1026 watts... but goes on to ask something akin to "how can the Earth survive?" which leads me to wonder if that's all the energy released from the surface of the entire sphere, or just the portion we experience. But, to be honest, if that portion that reaches Earth is only equivalent of 696 TW over ground the size of Texas... maybe we can fudge it and assume that said number is the best you're going to get anywhere on your dysonsphere surface.

So that's the maximum you have to work with (estimatedly speaking). And the entire Texas-sized chunk of surface must be defended with it. And machinery run with it, etc.

Now, to be fair, unless Ming's fleet is gi-honking-normous, then an awful lot of energy batteries can be brought to bear. But that would be foolish. Energy weapons have effective ranges because there are limits to how well the optics (even if run with Clarkean magic-driven gravimetric fields) can focus and how far the energy can go before it begins to dissipate. So what would Ming do?

He drags mines, or the inexpensive equivalent. Nobody it going to attack a dysonsphere unless they think they have the technical capacity (or the foolish arrogance) to actually punch a hole in it. So Ming has the same gravity-based mass-collection system used by the Sphere builders to capture and haul mass to the construction site — only he's using it to drag asteroids. Bazillions of asteroids.

And he doesn't really have to punch a hole in your sphere. He just needs to shift it. And your defenses can't just break up the asteroids, they have to atomize them! Otherwise the mass will still hit your gravity-heavy sphere (which works in Ming's favor). Push it off center just a little bit, and you're hosed because you suddenly have to dump tons of that energy you were using on defense to push your sphere back on center. Stars are just a bit uncaring, dontchaknow!

So, frankly, I think that "invincible" really only applies where there's a serious technological difference between attacker and defender. If they're on par, it's just like any other siege — desperate.

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    $\begingroup$ This is an excellent answer. Thanks. I find it interesting that even in the far future sieges won't change much. My favorite idea about sieges is like Sun Zu advises. avoid them. $\endgroup$ – Seallussus Sep 1 '20 at 4:06
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    $\begingroup$ I'm picturing a series of large asteroids heading towards the sphere in a single-file line. You have to destroy the lead asteroid before you can start working on the one hidden behind it. If the sphere can't destroy them fast enough, its doomed. $\endgroup$ – bta Sep 1 '20 at 23:00
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    $\begingroup$ Playing billiards with asteroids and Dyson spheres sound like good time $\endgroup$ – val is still with Monica Sep 2 '20 at 4:50
  • $\begingroup$ Two observations: is not about the max TW you can get per instant from your star, it's about you having it granted all the time, day and night, and relatively free compared to what we have now. That's a weapon in itself. Also, if the "sphere" is made of some cell-like structure saving certain distance with the next ones, that distance is a defense again possible incoming asteroids, natural or not. Killing one cell doesn't disable the whole sphere. If the enemy has the technical capability and resources to counter that design, they deserve to win. $\endgroup$ – Hatoru Hansou Sep 2 '20 at 12:12
  • $\begingroup$ @HatoruHansou Those are both excellent observations! Thanks! $\endgroup$ – JBH Sep 2 '20 at 15:06
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Assuming I am a war-hungry civilisation, of equal or slightly greater tech your Dyson Sphere loving society, how would I destroy you in a frontal attack?

  1. Find or create a small black hole.
  2. Blackholes move towards mass, and behave as any other mass for orbital calculations, so have big, massive, ship nearby, both will be drawn towards each other. The black-hole will be very mobile, cause it's small. You want about 100 kg total mass. Too light and it'll come to you for hugs immediately upon creation. Too heavy and it'll be too hard to move.
  3. Thrust the ship away from the blackhole, towards the enemy Dyson Sphere. The blackhole will follow (as it's attracted to mass)
  4. When you start getting close to weapons range to the Dyson Sphere, accelerate something heavy back into the hole away from the Dyson Sphere, floor it and then GTFO.
  5. Black hole consumes the mass you released, grows (making it much harder to change direction now), absorbs the momentum (so it doesn't overshoot the system), homes in on the Dyson Sphere mass, and collides with the Dyson Sphere.

Now this is borderline ridiculous, I think it'll work if you really want to do it, but there are ways to bring down a society without firing a shot, and a realistic military strategy wont attack an enemy at their strongest point.

Attack their transport / trade ships going between, set up barriers between spheres to stop them communicating, or hack their communications. Create rumors about the leadership. Spawn a religion that undermines their values. Give them a virus (computer or Covid).

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    $\begingroup$ Note: "Blackholes move towards mass" is called gravity. Your proposal amounts to: drop a black hole on the sphere. Forget the ship thrusting thing. Just drop the black hole while falling towards the sphere (not orbiting it) and gravity does the rest. $\endgroup$ – user253751 Sep 1 '20 at 12:50
  • $\begingroup$ And the micro black hole passes through the entire system at its interstellar speed, possibly hitting one satellite when entering the system and another when exiting, probably messing with some orbits as it does so, but doing little actual damage. $\endgroup$ – Michael Richardson Sep 1 '20 at 14:26
  • $\begingroup$ @MichaelRichardson - You would aim it at the Dyson Sphere. If you miss, I guess go grab another one. Although you are right, I should mention "Decelerate" in there. $\endgroup$ – Ash Sep 1 '20 at 14:33
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    $\begingroup$ A black hole of the size you are talking about is very tiny. It would do about as much damage as a rock of similar mass (ie negligible). You would get much more bang for your buck by simply accelerating sand grains to relativistic speeds and flinging them into the system on intercept courses with the individual satellites in the sphere. $\endgroup$ – Michael Richardson Sep 1 '20 at 16:34
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    $\begingroup$ Black hole weighing 100kg will evaporate in 8.41072E-11 second (read instantly) releasing 30GJ of energy in that time. $\endgroup$ – Tomáš Zato - Reinstate Monica Sep 1 '20 at 23:06
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Subterfuge

Let's assume you have the power of a star and can harness it with near perfection. In nearly all cases it's not possible to have a higher energy density within a solar system. The energy output is so high, that not even the whole astroid fields in our solar system have the energy to compare. Depending on how your cannons and shields work, they can vaporise or deflect anything short of a planet before it even became a threat. So you would basically need to hurl a planet, which isn't an insignificant feat, to even have enough energy to move the Dyson sphere a fraction. But any engineer would add something so it would be able to correct it self. So possibly the sphere would be invincible.

except that it's not*

The invincible object has been done often in fiction. Let's look at a famous example, the 2nd Death Star. I'm taking the second one, as I'm assuming not even small fighters can come close to a Dyson Sphere civilization. Computers would make calculating trajectories and blowing stuff up a piece of cake. What you need is subterfuge.

This can happen in many ways. Hacking the structure would be a great start. Although you can assume the computers are compartmentalised so you can't take over the whole structure easily, even taking over one part can be catastrophic. Redirecting the energy can lead to structural failure or possibly authorising a force to get onto the structure. You can have stealth to approach the structure and blow a hole in it. As soon as you can make a hole, the whole structure mightvalready collapse as sensitive areas might melt or fry, making a bigger hole etc.

But it might get even simpler. Possibly you can buy off lesser people in the civilization or place your own people. Like hacking, they might destroy, deactivate, authorise, remove authorisation or even give away control of large parts of the structure.

If all else fails, grab a gas giant or two. At a certain moment you will overwhelm the firepower and shields, or possibly cause the sphere to use so much energy in areas that the structure breaks on it's own. But throwing planets generally requires a Dyson sphere to pull off...

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  • $\begingroup$ You'd probably have to bring your own planet, because chances are that building all those power satellites and habitats will have used up all or most of the mass around the star. Which means it will be difficult to sneak in on them. $\endgroup$ – NomadMaker Sep 1 '20 at 9:38
  • $\begingroup$ @Trioxidane, I appreciate the idea. It does follow the art of war. But in my particular case subterfuge is not an option. Those stations are like the most important military installation in the system. In fact certain systems are just a station and it's fleet with layers of security. The core systems are unhackable as they are not connect to anything external. Human access is extremely limited and only few high ranking humans can direct the fleet and decide strategy. The AI does not allow any order that compromises the station. And even other measures are in place. So only attacks $\endgroup$ – Seallussus Sep 1 '20 at 22:21
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The practical Dyson sphere builder will construct not a solid shell, as the material cost would be absurd and the physical stresses positively insane, but an orbiting swarm of layers upon layers of satellites. Indeed, these could be fortified impressively against all-round attack, as the defender could amass trillions of weaponized stations good for defending against smallish things that stations could feasibly fight against. The weakness here, though, is the fact that energy can't be repeatedly bounced around and re-concentrated without power transmission losses; each station generally makes do with what its solar panels provide and radiates the rest into space (perhaps for the next layer out to absorb for their own processes; Matrioshka brain, anyone?) So you can't exactly concentrate the full star's worth of energy onto the shields or weapons systems of any particular station at once.

Such a swarm's key vulnerability would be to defeat in detail. As the swarm is spread out over quintillions of cubic kilometers of space, an attacker would only have to amass small, compact forces (admittedly, they may need a lot of them) capable of tanking the firepower of the few satellites they'll be in range of at any given time ("giant armored space battleships" suddenly don't seem like a bad idea here!), and then just sweep their ultra-fleets through the swarm, picking off everything around them and clearing out swathes as they go. Targeting the giant, fragile solar panels would be the obvious choice for an attacker to weaken the enemy's ability to fight back.

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Surface area is the bottleneck.

Let's assume you have some voidshields protecting you. This has to be subdivided into sections with each section having a bunch of shield emitters. Let's assume for a moment that the shields can absorb energy on a 1:1 ratio.

Now imagine if the enemy starts bombarding A section of shield of say 1km. A miniscule area compared to the total surface area of the Dyson Sphere. They use an array of railguns 1 lightday away from the target. The railguns accelerate their projectiles to near the speed of light, making it easy for the railguns to relocate from any retaliation while the DysonSphere can't really move out of the way or detect the shots in time to shoot them down.

Now imagine that the attackers fire enough projectiles that will hit simultaneously to equal the power output of the sun for 1 second.

During the impact, the shield emitters on a 1km surface area have to run the entire sun's energy output through them. You can imagine that something will burn through with this much energy coursing through the entire structure. And let's not forget: The energy is distributed across the surface of the sphere. Energy farther than halfway up the sphere would require 7 minutes or more to reach the 1km surface area, with 14 minutes or more for energy located at the opposite end assuming it could travel in a straight line at lightspeed. So it is likely that a concentrated barrage timed to have each shot land at the same time, a trivial matter for this techlevel, would simply cause the shields to fail at the point of impact. You can then do interesting things with timing another barrage to arrive a fraction of a second later from a lot of angles, passing the shield and impacting on the surface kilometers away from the shield opening if the angle is steep enough.

And that is ignoring the question: What happens with the energy of the impact? Is it absorbed? Deflected? Does the energy basically "stop" in place? With the exception of deflection you could exploit it an force a local overload to breach the sphere.

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  • $\begingroup$ Really insightful. "the attackers fire enough projectiles that will hit simultaneously to equal the power output of the sun for 1 second." Is that even possible? I mean those guys have a literal frigging sun. So you think it would be possible to amass enough warships with enough guns to achieve that? "What happens with the energy of the impact?" Excellent point. For maximum epicness is it adsorbed and reused by the station. That is possible, right? $\endgroup$ – Seallussus Sep 1 '20 at 22:38
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    $\begingroup$ @Seallussus I can carry 10 kilograms for 1 hour (3600 seconds). However I cannot carry 36.000 kilograms for 1 second. To match the suns output I dont need to match it continuously, I just need to throw enough energy for one moment at that region to overwhelm it. As for absorbing the energy, ignoring how this is done you have to do this quickly before the energy dissipates into space. That means that first the shields have to match the output, streaming a suns worth of energy through it in a fraction of a second, then all that energy (2 suns output!!!) Has to flow back through it. $\endgroup$ – Demigan Sep 2 '20 at 6:20
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A few random responses:

We don't know enough to build a Dyson Sphere with shields. By the time this is learned, anyone who may still properly be called your enemy may have techniques that circumvent a 3D sphere (things we might call today warp drive, wormholes, etc.)

If your enemy is part of your general culture (say, US vs. USSR) then tech might be on a relatively even footing. But if it's just some random race in space, one or the other will be millions or billions of years ahead. If you're ahead of them you wouldn't even notice them. If they're ahead of you... they would in turn sweep you aside without noticing you. For instance their attack could focus multiple suns' power at you.

Can the shields you mention be pointed towards any threat point, or are they simply fixed in all directions evenly? If the later, then an attack with many orders of magnitude less power, concentrated on one spot, could surely overwhelm those shields. Say we're standing outside your sphere, at 1AU (the earth's orbit). Even if your solar cell and shield generator were 100% efficient, the shield generated from 1m2 of solar cell couldn't be more powerful than the sun's rays on the 1m2, which isn't a lot of power. I think a few astronauts would be able to carry explosives or projectiles that have more power than that. Bigger "space battleships" would have greater cross section but more power. It seems like an astronaut, a ship, or a fleet could make a commensurately-sized hole in the sphere to get through.

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  • $\begingroup$ A Dyson sphere is simple to build. Simple does not mean easy, however. It would be enormously expensive, but would not require any new science to do so. $\endgroup$ – Michael Richardson Sep 1 '20 at 14:28
  • $\begingroup$ @MichaelRichardson Citation needed. $\endgroup$ – Daniel B Sep 2 '20 at 5:14
  • $\begingroup$ @MichaelRichardson we may be able to take elements occuring on earth and fashion them into a piece of what, if we could build a lot of it, might be a candidate for a sphere. Actually building the sphere requires getting basically the mass of the gas giants out of their current gravity wells, transmuting them into the elements we need, and then building the sphere IN SPACE, and it must be utterly robotized as the entire earth's population would suffice if you needed as much as one man per BILLION tons of sphere. And an actual sphere needs attitude control, meteor defense, etc. we can't do. $\endgroup$ – Swiss Frank Sep 2 '20 at 7:03
  • $\begingroup$ By most expectations (including those of Freeman Dyson, himself), this "sphere" is not a contiguous structure, but is instead a swarm of satellites so thick as to be able to absorb the entire output of the surrounded star. Yes it would be monstrously expensive to gather the materials for assembly into these objects and then to position them. But that is a problem of cost, not complexity. For mining and construction; autonomous mining is currently in use and is getting better all the time, and we've had robotic assembly lines for decades. All of this is a problem of scale, not complexity. $\endgroup$ – Michael Richardson Sep 3 '20 at 21:37
  • $\begingroup$ we've had robotic assembly lines for decades not that can produce a billion tons of goods per man-hour, we haven't. This is a problem of scale as you say, but a scale of many orders of magnitude difference, that is insurmountable with our current technology. With the tech we've got, and efficiency equal to the least-human-interaction-per-kilogram automated production we have, could 100% of mankind working 24/7 produce a dyson sphere in a million years? I don't think so. And we've yet to raise a single atom of mass from a gas giant gravity well. $\endgroup$ – Swiss Frank Sep 4 '20 at 3:54
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Dyson spheres are colossal by galactic standards

English lacks words grandiose enough to convey just how large a Dyson Sphere is. The language peaks out somewhere between "astronomical" and "galactic". A Dyson Sphere is a megastructure larger than any naturally occuring astronomical object, including the stars they enclose. "Galactic" doesn't really fit, and a militarily-significant DS dwarfs "astronomical":

Also if assuming a radius of 1 AU, there may not be sufficient building material in the Solar System to construct a Dyson shell. Anders Sandberg estimates that there is 1.82×1026 kg of easily usable building material in the Solar System, enough for a 1 AU shell with a mass of 600 kg/m2—about 8–20 cm thick on average, depending on the density of the material.

A shell about 8-20 centimeters thick will be slightly more resilient than tissue paper, at least to any weapon being used by any sort of starship. You are going to need more material. A lot more. Either large parts of your DS are undefended, and thus easy to rip to shreds - or the whole thing will need to be thick enough to not crumble under the force of guns firing off from defense batteries or ships taking off.

So let's be somewhat conservative. Use the high end of the cited guesstimate (20 cm thick for 1 solar system's worth of building material). And assume a uniform density of 1 meter will be enough to make the Sphere militarily defensible.

Even massaging the numbers by making two very optimistic assumptions, fifty star systems are still required to produce one sphere. In reality, significantly more will probably be required.

It doesn't scale

This simply can't be done en masse. A major power controlling hundreds of star systems would struggle to pull this off for even a small handful of Spheres protecting absolutely critically vital points. They'd more likely bankrupt themselves trying to transport dozens of star systems' worth of material to a single point.

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  • $\begingroup$ Dyson Spheres are not solid objects, but a huge number of orbiting power satellites that are enough to cut off and covert to electricity the entire output of a star. $\endgroup$ – NomadMaker Sep 1 '20 at 9:40
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    $\begingroup$ Noone said that the dyson sphere itself needs to be habitable. Dropping this requirement allows us to make the sphere much smaller. The sun's radius is just 0.5% AU. Shrinking the radius 100 times would allow us to make the sphere 10 000 times thicker, so if your math is right, 1 kilometer thick dyson sphere with radius twice asi big as the star may be feasible without transporting building material interstellar distances. $\endgroup$ – Viki Sep 1 '20 at 13:37
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    $\begingroup$ @NomadMaker Those are calles dyson Swarms, specifically for this distinction. $\endgroup$ – lidar Sep 1 '20 at 16:47
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The Worldbuilding

The main problem with the question is conflating things that are theoretically possible, and things that are absolutely not. From a story building perspective, if you decide to bend reality to your world to a certain degree, trying to apply realistic physics or theory creates such conundrums.

You mention "Void shields" and the energy they require, provided by a star. But truly the energy they need is inconsequential since there is no void shield physics to quote, void shields are not a thing. They could just as well need two AA batteries to run.

Thus, to make your world balanced, you may instead put a reasonable limit on void shields, such as their size, need to reset or something of the sort. No need to touch Dyson sphere physics at all.

The physics

Anyway so far I think that perhaps it is simply impossible, like with any tech, to translate the raw power of the sun to such raw power of the station.

You're onto something here. The thing is, energy isn't something you just consume. Energy really is made of things being on one spot and not the other. When you restrict a flow, like a dam on the river or Dyson sphere around a star, there still must be an outflow on the other side. Otherwise, your dam or Dyson sphere will burst eventually.

Thus, trying to consume all the raw energy from the star means you need to radiate it away on the outside of your sphere, otherwise it will get hotter and hotter inside. Even if you magically just consumed light, remember that machines powered by solar panels still heat up as they work.

Thus, a Dyson sphere can still only consume a fraction of stars energy. It cannot be a truly enclosed sphere, even with the best technology and a bit of magic, this is still absolutely unimaginable.

Truly, the strongest attack your Dyson sphere faces is from the inside.

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Invincible - against what?

If a sunstation is the ultimate form of combat vessel, then the obvious choice when you want to take one down is to bring a larger one into the battle. Or an array of other ones.

Push come to shove, though, you can use strange matter. A single strangelet can destroy a whole star system, it's just a matter of time.

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    $\begingroup$ Sub-atomic cancer... +1. $\endgroup$ – JBH Sep 1 '20 at 0:57
  • $\begingroup$ Neat and strange, but not proven yet. I'd go so far as to say that if this hypothesis is correct, then we would have encountered it in our studies of the galaxy. $\endgroup$ – NomadMaker Sep 1 '20 at 9:42
  • $\begingroup$ Strange matter itself is not proven to exist, and the concept of it spreading additionally requires the current hypothesis of strange matter to be wrong in the expected properties of strange matter. $\endgroup$ – Tomáš Zato - Reinstate Monica Sep 1 '20 at 23:00
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If you try to build a Dyson Sphere at Earth orbit, you will need too much material. Ton Day's answer has the math and the numbers are just too big.

So, you build it much smaller and closer to the star. You mention "force shields" which probably means the sphere doesn't have to be much bigger than the star itself. It will mostly be a powerful force shield and its generators.

And compared to defending against the star itself, defending against a battle fleet is easy. So the sun station itself is invincible.

BUT, the important part of the star system isn't the star, it is the planets. That is where people live. If a foreign invader manages to invade and conquer a planet, the sun station has a problem. Sure, they could fry the whole planet without problems, but most of the people there will still be their own countrymen.

On the other hand, the invaders also have a problem, the sun station will probably deny power to the planet making it costly to hold.

In short, it will be a hostage situation, which have all sorts of potential for good stories.

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It may come down to a matter of power levels, that is if we ignore other types of possible solutions to successfully attacking one.

Let's assume a fraction of the Dyson Sphere's power is used for most needed internal things and that the most of it is successfully harvested and redirected to defenses/shields.

If the Dyson Sphere has a star similar to our Sun, it would output around 384 YW (Yotta watts - 1000^8 ) of energy. Different stars may have different output yields, but still limited.

In such a case all you have to do is build an energy weapon that exceeds that amount of power. And you can do that in multiple ways: a big ship powered a singularity core, multiple ships that merge energy outputs into one combined shot (like these Species 8472 ships destroying a planet) or using a stationary very long range direct on indirect weapon that could be power by anything starting with a bigger start, therefore having more energy output to work with. Those would be some the "fire with fire" cases.

An alternate way is just to find something to destabilize that energy or even turn it against it's own system. Here you can find also multiple ways to do so, even with very limited power and/or specialized weapons.

And finally, you have the option of sabotage. When something cannot be directly defeated or it is too inefficient/expensive to do so, sabotage is the good way to go. A power-off of the defense system and all that super sun-powered-shield becomes useless enough for an attack to happen.

So clearly, that amount of power would not make it even close to invincible. At that civilization level, harvesting singularity gravimetric power could be a much more efficient way to have higher than a sun power levels using easier means of harvesting.

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Consider a Dyson sphere with a radius equal to the Earth's orbit. That's an area of 2.79E17 square km.

Currently there is something round-about some billions of processors on the Earth, more or less. So call it 10 billion, or 1E10. With even 1 processor per square km there would be 20 million times the total processing power of all of Earth. There would certainly be room for large numbers more, and there would be lots of energy to spare.

There are people who think the internet is getting close to waking up. Example, Example, Example, Example With 20 million times as many processors working together you would have a daunting potential AI. In other words, a trivial relative investment in computing power would probably produce something unimaginably smarter than humans. And almost certainly much smarter than anything that could be sustained on merely planet sized systems.

So it's pretty much impossible to predict what such an AI could or would do. It's smarter than any entity we have yet had experience of. Indeed, it would quite likely be able to do things we could not begin to comprehend. It would be like squirrels defeated by aluminum siding on a house.

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    $\begingroup$ This "internet waking up" is just a sci-fi thing. With this reasoning, any star should be expected to have intelligence of its own, just given by it's size. $\endgroup$ – Tomáš Zato - Reinstate Monica Sep 1 '20 at 22:56
  • $\begingroup$ @TomášZato-ReinstateMonica Computers have computing power by design and construction. There is no particular reason to think that some volume of plasma has any such ability. $\endgroup$ – puppetsock Sep 3 '20 at 18:56
  • $\begingroup$ There is no particular reason to think computers would have the same ability either. They are designed to compute stuff, not think. I suggest that googling the concept of boltzman brain may make it easier for you to understand my argument. $\endgroup$ – Tomáš Zato - Reinstate Monica Sep 3 '20 at 22:44
  • $\begingroup$ @Tomáš Zato - Reinstate Monica You should read the book Sundiver by David Brin. It's about attempts to contact the sentient life inside the sun with a ship designed to fly into the sun. Oldie but a goodie. $\endgroup$ – DWKraus Sep 4 '20 at 3:55
  • $\begingroup$ @TomášZato-ReinstateMonica Yeah, but you didn't provide an argument, only a transparently incorrect claim. There is a very good reason to think computers have such abilities, since they have already got such abilities. AI is a thing. Maybe you've noticed. $\endgroup$ – puppetsock Sep 11 '20 at 20:19

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