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Would it be possible to build an "Inverted Dyson Shell"


daniel l.

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So, while watching an Isaac Arthur video, I was suddenly struck by an idea. What if you built a Dyson Shell around a star, and terraformed the outer surface of the shell.

At a distance of approximately 0.025 AU, the Sun's gravity is about 1G. This means a shell built at this altitude over the Sun's surface would have Earth gravity on its outer surface. Meaning it could hold an atmosphere, oceans, continents, and life. The surface area would be approximately 1.75769E+20 m2 . These figures vary with the size and mass of the star, though I'm using Sol as a reference point.

The inner surface of the shell could be coated with solar cells or some other form of energy collection technology, this would make it possible to harvest nearly the entire energy output of the star.

A tiny portion of the harvested output could then be somehow transferred to an orbiting "artificial-sun", which would orbit at such an altitude as to have a 24hr cycle, and illuminate the surface of the shell.

From a distance, such a structure would look like a weird solar system, with a gigantic planet orbited by a tiny sun. However, from the outer surface of the shell, it might be indistinguishable from the surface of the Earth.

What do you guys think of this idea? Is it even possible?

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About as possible as a regular Dyson Sphere, give or take. Once you reach a level of tech where the challenges presented by a Dyson Sphere are surmountable, then you probably have the ability to modify the design however you please.

 

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56 minutes ago, insert_name said:

I suspect heat would be a problem

Heat is a problem in a regular Dyson Sphere. But anyway, the whole point of a Dyson Sphere is that you use the energy captured from the sun - Habitable area is not the main driver for building Dyson Spheres.

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10 hours ago, daniel l. said:

So, while watching an Isaac Arthur video, I was suddenly struck by an idea. What if you built a Dyson Shell around a star, and terraformed the outer surface of the shell.

At a distance of approximately 0.025 AU, the Sun's gravity is about 1G. This means a shell built at this altitude over the Sun's surface would have Earth gravity on its outer surface. Meaning it could hold an atmosphere, oceans, continents, and life. The surface area would be approximately 1.75769E+20 m2 . These figures vary with the size and mass of the star, though I'm using Sol as a reference point.

The inner surface of the shell could be coated with solar cells or some other form of energy collection technology, this would make it possible to harvest nearly the entire energy output of the star.

A tiny portion of the harvested output could then be somehow transferred to an orbiting "artificial-sun", which would orbit at such an altitude as to have a 24hr cycle, and illuminate the surface of the shell.

From a distance, such a structure would look like a weird solar system, with a gigantic planet orbited by a tiny sun. However, from the outer surface of the shell, it might be indistinguishable from the surface of the Earth.

What do you guys think of this idea? Is it even possible?

With our current understanding of gravity and entropy it would be hard either rigid, particular, inverted doesn't matter.

Arthur is presenting idealized scenarios of space colonization. The basic problem is that most of what a Dyson sphere will be growing crops and sunlight is a benefit, except the green component has to be removed.
But he shows planes floating around the star. In reality you would want either cylinders that can rotate (or artificial gravity) or spheres (variable artificial gravity). Its simply easier to pressurize a sphere than a flat plate.
Solar panels would pipe power to into the sphere and else radiators would radiate the heat. Its all problematic because as it grows in size its progressive harder to get rid of the heat.

He also makes claims of Dyson spheres harvesting the hydrogen from the star and using it for fusion. Of course first you have to have fusion that is much more power efficient relative to todays fusion, and, on top of that you would need to put a huge magnetic field around the star causing it to degas at the poles, and then some how get scooped up. 

The solar panels are also endanger close to a start due to the electro-magnetic intensities of ion storms. And on might want to have an interior belt of ion capture, providing fuel for putative fusion.

Again, before you can think about Dyson aggregates you first need to get into space and bring asteroid and comets together in such a way you can harvest minerals and gases, Dyson would be way late in a sentients evolution.

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1 hour ago, PB666 said:

With our current understanding of gravity and entropy it would be hard either rigid, particular, inverted doesn't matter.

Arthur is presenting idealized scenarios of space colonization. The basic problem is that most of what a Dyson sphere will be growing crops and sunlight is a benefit, except the green component has to be removed.
But he shows planes floating around the star. In reality you would want either cylinders that can rotate (or artificial gravity) or spheres (variable artificial gravity). Its simply easier to pressurize a sphere than a flat plate.
Solar panels would pipe power to into the sphere and else radiators would radiate the heat. Its all problematic because as it grows in size its progressive harder to get rid of the heat.

He also makes claims of Dyson spheres harvesting the hydrogen from the star and using it for fusion. Of course first you have to have fusion that is much more power efficient relative to todays fusion, and, on top of that you would need to put a huge magnetic field around the star causing it to degas at the poles, and then some how get scooped up. 

The solar panels are also endanger close to a start due to the electro-magnetic intensities of ion storms. And on might want to have an interior belt of ion capture, providing fuel for putative fusion.

Again, before you can think about Dyson aggregates you first need to get into space and bring asteroid and comets together in such a way you can harvest minerals and gases, Dyson would be way late in a sentients evolution.

This so much, an dyson swarm who was the original idea on the other hand makes way more sense. he also describe them. The planes are solar collectors in balance with the solar pressure I assume. 
An dyson swarm is also something who could evolve, you start of with habitats in orbit around the planet, this get crowded so you move toward L3 and L4 and then all around in orbit, before this some start with inclined orbits, you would need rules for shadowing others. 
Downside of an dyson swarm is limit of growth, you can only eat one chicken a day, probably an limit on how long keeping up with the Joneses would be relevant and unless you start doing real energy expensive stuff like large scale interstellar colonization you will slow down. 
 

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3 hours ago, magnemoe said:

This so much, an dyson swarm who was the original idea on the other hand makes way more sense. he also describe them. The planes are solar collectors in balance with the solar pressure I assume. 
An dyson swarm is also something who could evolve, you start of with habitats in orbit around the planet, this get crowded so you move toward L3 and L4 and then all around in orbit, before this some start with inclined orbits, you would need rules for shadowing others. 
Downside of an dyson swarm is limit of growth, you can only eat one chicken a day, probably an limit on how long keeping up with the Joneses would be relevant and unless you start doing real energy expensive stuff like large scale interstellar colonization you will slow down. 
 

Dyson swarm is impracticable because there is no resource that can be drawn to build it. And even if there was we don't have a power supply capable of moving it. Efficiencies on all types of generators would have to improve markedly.

All of Arthur's arguments are based on a pretense that if we had an unlimited supply of hydrogen, a perfect fusion-electric generator . . . . . we could have this that and the other. Its one of those if and if and if and if then arguments. The failing of any one aspects and the whole thing becomes impossible. For a Dyson sphere to work you need

1. Fusion (much much more power produced than required to operate the reaction)
2. Greater than 50% efficiency in the heat conversion to power (we can maximally at current convert 30% in space and this may not suffice to keep a fusion reactor operating)
3. A source of reaction mass (e.g. argon, xenon, magnesium)
4. An abundant source of hydrogen (Don't say Earths oceans)
5. An abundant source of structural mass that is not deep within a gravity well.

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18 hours ago, daniel l. said:

What if you built a Dyson Shell around a star, and terraformed the outer surface of the shell.

Why not both sides ? Grow crops on the inside, sleep on the outside... (unless Nutrimatic comes first ofc)

 

Still, biggest problem is the fact that such arrangement is only statically stable. Give it a small nudge and it's over.

Edited by YNM
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25 minutes ago, YNM said:

Still, biggest problem is the fact that such arrangement is only statically stable. Give it a small nudge and it's over.

That could easily be resolved by placing thrusters on the outer shell. In fact, all you'd need is some doors to the inside that can open on command, thus releasing some of the star's energy in a chosen direction -- mirrors could be used to focus the light.

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50 minutes ago, daniel l. said:

That could easily be resolved by placing thrusters on the outer shell. In fact, all you'd need is some doors to the inside that can open on command, thus releasing some of the star's energy in a chosen direction -- mirrors could be used to focus the light.

Orchestrating something that takes a while to know is... quite difficult ?

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7 hours ago, daniel l. said:

I would appreciate a further explanation of these words. I am stumped.

Because space-time propagates at the speed of light. Therefore the best approximation is always off be d/c.

The way to solve the problem is to have magnetic interleaves between sections. But the reality is that best a Dyson sphere can ever be is  a ring a few hundred kilometers wide. IMO even this would tap every available resource in the inner solar system that is not already deep in a gravity well .

There is a saying learn to walk before learning to run. Dyson anything is way-way ahead of our technology or our ability to predict how likely it would be able to be done (.e.g will thermodynamics even transiently allow it).

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9 hours ago, PB666 said:

Dyson swarm is impracticable because there is no resource that can be drawn to build it. And even if there was we don't have a power supply capable of moving it. Efficiencies on all types of generators would have to improve markedly.

All of Arthur's arguments are based on a pretense that if we had an unlimited supply of hydrogen, a perfect fusion-electric generator . . . . . we could have this that and the other. Its one of those if and if and if and if then arguments. The failing of any one aspects and the whole thing becomes impossible. For a Dyson sphere to work you need

1. Fusion (much much more power produced than required to operate the reaction)
2. Greater than 50% efficiency in the heat conversion to power (we can maximally at current convert 30% in space and this may not suffice to keep a fusion reactor operating)
3. A source of reaction mass (e.g. argon, xenon, magnesium)
4. An abundant source of hydrogen (Don't say Earths oceans)
5. An abundant source of structural mass that is not deep within a gravity well.

An dyson swarm uses solar as main power source if you have good fusion you don't need it much. 
Nobody is gone use an fusion reactor to produce heat to generate electricity for running an ion engine, you can just as well use fission for this in space 
You use part of the plasma directly, note that you do not need to go break even if you feed power to the process.

More an issue with need for an dyson swarm. However it has an benefit over many of his other idea in that it can grow over time however it require an setting there space around earth get to crowded. 

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40 minutes ago, magnemoe said:

An dyson swarm uses solar as main power source if you have good fusion you don't need it much. 
Nobody is gone use an fusion reactor to produce heat to generate electricity for running an ion engine, you can just as well use fission for this in space 
You use part of the plasma directly, note that you do not need to go break even if you feed power to the process.

More an issue with need for an dyson swarm. However it has an benefit over many of his other idea in that it can grow over time however it require an setting there space around earth get to crowded. 

You have to think about the thermodynamics, what you are trying to do is to move mass from where the structural mass comes from to the inner solar system. Remember Arthur starts most of his videos, if you have fusion power  . . . . .
So the principle problem of the dyson swarn is that enough is never enough. And at some point in his video he is talking about fusion H2-Fe, that is getting all the energy out of hydrogen, hydrogen that would be extracted from the sun with fusion power and structure being made with the fusion of hydrogen. But initially you have to convert mass into energy and then leverage that mass to move mass from the outer solar system (asteroid belt, kuiper belt, whatever) down to your dyson orbit. Since Fissile material is neither useful in construction and has a 4 fold lower energy/mass conversions ration as hydrogen, basically it increases the cost of moving things, so as things get harder and harder to move (further away and having less of the structural materials you want) they would theoretically become too expensive to move. Solar power will not move mass around, it can only provide the energy at the inverse square of the distance.

Or to put this another way, a Dyson sphere is not about what it is, but what it does . . .its a function of growth, growth needs mass. There is an inadequate amount of fissile material in the innersolar system to provide the energy to acquire the mass.

The problem is that based on what we know about nuclear conversions they are already 'theoretically' too expensive. For example, your basic thermonuclear engine with the maximum theoretical fuel has about 1/10th the dV required to move mass from the outer solar system, down into say a mercury/venus orbit and then return back to its station and do it again. The ion driven engines are more mass efficient, but the current limit to heat conversion in space is below 30% efficiency coupled with the ION drive efficiencies that is basically 21%, So if you need a million kw of power, you will produce 4 million kw of waste heat, and this adds weight.

Take some time with a big long spread sheet and try to move a cubic kilometer of substrate from objects in the asteroid belt/outer solar system into the inner solar system, then construct an object, then return the ship back to the outer system (again with how much fuel) and gather the resource and move it again. You will find that it takes alot of dV.

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On 2/21/2018 at 6:22 AM, Shpaget said:

A rigid Dyson sphere is not stable. Without active station keeping it would fall out of orbit, regardless of its invertedness.

10 hours ago, YNM said:

Still, biggest problem is the fact that such arrangement is only statically stable. Give it a small nudge and it's over.

Bear in mind that it's not unstable, either.  It's "neutrally stable".

That is:  if you have a spherical shell around a gravity source that's perfectly centered... there's net zero gravity effect on the shell.

And the same thing is true if the gravity source is off-center, too.  There will still be net zero gravity effect on the shell.  (Because of the shell theorem.  It's the same reason why the gravity inside a hollow shell is zero.)

This means if you built a Dyson sphere around a star in the center, there's nothing pulling it to keep it centered, true... but there's nothing pushing it off-center, either.  And that's true whether the star is perfectly centered or not.

Long-term, yes, you'd have to worry about "drift", but it's not going to be a runaway effect as long as the sphere stays rigid.  So you'd want to have some kind of station-keeping thrusters or something, but likely they could be fairly modest.

 

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36 minutes ago, PB666 said:

You have to think about the thermodynamics, what you are trying to do is to move mass from where the structural mass comes from to the inner solar system. Remember Arthur starts most of his videos, if you have fusion power  . . . . .
So the principle problem of the dyson swarn is that enough is never enough. And at some point in his video he is talking about fusion H2-Fe, that is getting all the energy out of hydrogen, hydrogen that would be extracted from the sun with fusion power and structure being made with the fusion of hydrogen. But initially you have to convert mass into energy and then leverage that mass to move mass from the outer solar system (asteroid belt, kuiper belt, whatever) down to your dyson orbit. Since Fissile material is neither useful in construction and has a 4 fold lower energy/mass conversions ration as hydrogen, basically it increases the cost of moving things, so as things get harder and harder to move (further away and having less of the structural materials you want) they would theoretically become too expensive to move. Solar power will not move mass around, it can only provide the energy at the inverse square of the distance.

Or to put this another way, a Dyson sphere is not about what it is, but what it does . . .its a function of growth, growth needs mass. There is an inadequate amount of fissile material in the innersolar system to provide the energy to acquire the mass.

The problem is that based on what we know about nuclear conversions they are already 'theoretically' too expensive. For example, your basic thermonuclear engine with the maximum theoretical fuel has about 1/10th the dV required to move mass from the outer solar system, down into say a mercury/venus orbit and then return back to its station and do it again. The ion driven engines are more mass efficient, but the current limit to heat conversion in space is below 30% efficiency coupled with the ION drive efficiencies that is basically 21%, So if you need a million kw of power, you will produce 4 million kw of waste heat, and this adds weight.

Take some time with a big long spread sheet and try to move a cubic kilometer of substrate from objects in the asteroid belt/outer solar system into the inner solar system, then construct an object, then return the ship back to the outer system (again with how much fuel) and gather the resource and move it again. You will find that it takes alot of dV.

he talk about lots of things, some pretty realistic at least in an long term, other not so. H-H fusion will be very hard for one, 

Still an I do not get the idea of using an thermal fusion reactor to run an ion engine it makes no sense. using the fusion plasma directly will give an way higher exhaust velocity.
Unless you replace the ion engine with an particle accelerator and says goodbye to trust, not that in this setting you can use anything as reaction mass, ionization energy can be ignored. 
Yes its more wasteful with fuel as its also reaction mass but it will have far higher trust and less heating issues. 

Now if you have a faction of an dyson sphere you could use laser pumped solar sails or ions powered by lasers on solar panels, you can make an solar panel designed for just one frequency very efficient 
 

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5 hours ago, Snark said:

This means if you built a Dyson sphere around a star in the center, there's nothing pulling it to keep it centered, true... but there's nothing pushing it off-center, either.  And that's true whether the star is perfectly centered or not.

You do realize stationkeeping around L-points is more "expensive" than in GEO right ?

Edited by YNM
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3 hours ago, YNM said:

You do realize stationkeeping around L-points is more "expensive" than in GEO right ?

Not sure how that's relevant to the current discussion.  We're not talking about L-points or GEO, we're talking about the stability-or-not of a spherical shell around a central mass.

Some earlier posts seemed to be suggesting that it was dynamically unstable, i.e. that if it ever gets off-center, there would be a runaway effect pulling it further off-center.  I was just pointing out that this is not actually the case (due to the shell theorem), and that actually it would be "neutrally" stable (i.e. an off-center sun would neither try to pull it back to center, nor pull it farther off-center).  Their motions wouldn't be gravitationally "coupled", so to speak.  That's all.

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1 hour ago, Snark said:

Not sure how that's relevant to the current discussion.

L-points are also "statically stable", ie. it's only stable for that one point, if nudged it'll tend to deviate.

So is a skin/shell of equal gravity (equipotential surface). Deviate the slightest and it's no longer that way.

Shell theorem doesn't apply well I think, given the potential is not the same. If anything, even if it's the same, that's actually alarming - it can continue to veer off and nothing is stopping it.

 

Also, extra problem for OP's idea : heat. At 0.025 AU, the equilibrium temperature around the Sun is about 1,600 K. You'd need one hell of an insulator.

Edited by YNM
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