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I explained it in detail on my website.
A dyson's sphere can be built above the sun just far enough so the gravity matches Earth. the outside of the shell will be habitable. It will be smaller than a traditional Dyson's, but it would need a lot less building material.
space structure
http://home.comcast...e.thacker/dyson.htm
describes the smaller dyson's [the great unknown, Jan 08 2007]
How strong does it need to be.
http://www.nada.kth...onFAQ.html#STRENGTH
The FAQ linked on the web page, showing how pressure can be calculated [caspian, Jan 16 2007]
Earth-Space Web
Earth-Space_20Web
An idea to support a stationary ring using something going faster than orbital speed [caspian, Jan 16 2007]
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how? Sounds like science fiction to me. |
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would it not start hoovering up all that
space dust ? |
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Er... wow. To the nearest hundredth of a millimeter. Guess you missed the discussion on significant digits, eh? |
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//While the surface of that planet can get a high temperature of almost 500 degrees, while the dark side goes down to –115. The temperatures for the shell can be just as extreme.// Well, no, they can't. At no point will your mini-Dyson have an entire planetary mass between it and the sun. It's full broad noon, all the time, tanning your little shell's backside. Your shell will be absorbing all of the energy output of the sun, and then re-radiating it all as infrared. I would normally run down the math to show what happens, but you have shown some math ability, so I'll let you have first crack at it: given the energy output of the sun and your sphere's radius, what will be the equilibrium temperature of the outside? (Note that holes don't matter, as they will have the same energy flux as the shell material - they just don't absorb/re-emit the energy). |
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When you complete that part, then we'll talk about using the mirrors to bring the sunshine back. |
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I suspect that, once we have the technology and power requirements needed to accomplish this feat that a regular Dyson sphere would be equally within our abilities to complete and it would require far less fiddling than this design also construction would be significantly less problematic. Either way we would need to have mastered interstellar travel and mass transportation on an epic scale to achieve either project. |
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Patent it and you could really Clean Up! |
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You're right, Lurch, the heat absorbtion/radation would be different from Murcery because of the thickness, but this is also a problem with a regular Dyson's. That is why it is suggested to look for bodies emitting infra red if we want to find one.
It will still provide a lot of the advantages of a Dyson's while using less material, or using that extra material to increase thickness to the shell.
In the case of both types, the technology may be advanced enough to direct that excess energy to other means besides radiating heat. |
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I don't see the need for interstellar travel to construct these spheres. |
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You're ducking, so I did the math for you. 2500 K. Think of the flame on a butane lighter. That's about what your equilibrium temperature will be. |
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//... the technology may be advanced enough to direct that excess energy to other means besides radiating heat.// |
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//I don't see the need for interstellar travel to construct these spheres.// |
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You don't see the contradiction between those two statements? If you had such extremely advanced technology to do one, it would imply the other. Or, as many have said about Dyson spheres, "If you can build one, you don't need it." |
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Holes do matter: some of the energy absorbed by the shell is re-emitted inside. On a complete shell, the amount emitted from one point on the shell will be about the same as it catches from the rest of the shell, so it cancels out. On a shell with holes some goes out the holes, and the amounts don't cancel out. I've ignored the amount that goes straight back into the sun which could be causing trouble... |
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//Holes do matter:// point granted. I got hung up on the "cavity radiation" idea, and didn't take into consideration the re-emitted infrared. Numerically, they do matter quite a bit. However, even if the shell is riddled to the point of being mostly holes, the equilibrium temperature is far higher than [tgu]'s max estimate, and beyond habitability. Reducing the shell to a lacework would bring your temperature down to something more like hot coals rather than an open flame. |
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//it would imply the other.// You can "imply" a lot of things. Years ago people "implied" that we would be buzzing around in flying cars by now. We could be colonizing the moon now but no leaders in the past took the challenge; just because you have the means don't mean it's being done. We could be doing a lot of things but we're not. Don't ask why. |
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We may have Interstellar travel by the time we build a Dyson's, (or maybe not) but there's is no need for it to make a Dyson's. |
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There is one problem with Dyson's Spheres that doesn't have an elegant solution: Because it completely encloses the star, its gravitational effects on the sphere as a whole are nil. That is, although all parts will be subjected to gravitational stresses (proportional to its mass, so increasing thickness won't help much in resisting this stress) there will be no force keeping the sphere in position. It could drift into the sun. |
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So you read Ringworld too. |
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By "imply", I don't just mean to make an unsupported stupid statement. I mean a logical consequence: You will note, perhaps, that there is *not* at this moment a Dyson sphere of some three and a half million klicks radius enclosing the sun. If there were to be one, it would require that the material necessary for its construction be moved from its current location to the place in which the sphere will be created. Its motion would have to be altered from whatever it's doing currently to the motion you wish it to have around the sun. |
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Our current space-going vehicle, the Space Shuttle, is very much limited by the amount of impulse it can generate - also known as "delta-V", but that term is after the mass is factored out. If you want to have more delta-V with the Space Shuttle, there are only two ways to get it: take more fuel (which you can't because it already launches with the tanks full) or carry less weight. Going on a Hubble servicing mission? The all-up weight has to be less than an ISS mission. We don't have the technology to get more "push" while we're out in space. |
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With current technology, if we were going to try to build a Dyson sphere, it would require a mass of fuel many times greater than the mass of the stuff we're making the sphere out of. Since the Dyson sphere will require pretty much all of the mass in the solar system, we can't get enough fuel. Sorry, I don't mean to seem negative, but it just isn't available no matter the price. |
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But - imagine someone came along with a new idea that allowed us to generate fuel out of nothing while we're in space. (You may be tempted to say that such magic wouldn't be required - perhaps a way to turn rock into fuel, rather than nothing, but nay - won't work. You can't have that rock - we need it for the Dyson sphere.) If we can generate fuel in such a way, then we would have the energy necessary - the "delta-V" to move everything to where it needs be to build the sphere. |
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On the other hand, what is keeping us from travel to the stars? Lack of delta-V. If we could generate fuel from nothing, then we could. |
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In other words, the solution to problem A implies a solution to problem B because they are at heart the same problem. You cannot solve one without your solution being applicable to the other. Hence, A implies B. If you can build a Dyson sphere, the capability exists for you to travel to the stars - whether you decide to take advantage of it or not is immaterial. |
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I'm sure I put a Dyson Vacuum link up
here... perhaps it got sucked up
someone ;-( |
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[david_scothern], Solar wind might be sufficient to compensate for the drifting sphere. As one side got closer to the sun, the effects of solar wind would increase, pushing it away from the sun. Meanwhile, on the other side, the increasing distance would decrease the solar wind effects. The two effects combine, and hopefully provide enough push to keep things in equilibrium. Selectively making portions of the sphere reflective/transparent could supplement this. |
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I suspect that gravity would prove far more of an influence than solar wind in this circumstance. |
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Two things - firstly, why not post the text of your idea here at the 'bakery? |
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It's 11am in the morning and I'm horribly hung over, but from what my battered cerebellum can determine, the idea is for a dyson sphere, with holes in. Is that what we're talking about? Because if you're going to do that, why bother with the whole sphere thing anyway, just get a bunch of floating platforms to orbit the sun. Or, make do with the existing, naturally formed platforms we call the planets. |
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//instead of taking the Earth and using it for part of the raw material for this project, I would like to see it preserved.// This bit made me chuckle, very kind of you to consider not destroying the Earth, thanks. |
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[jhomrighaus] Not so, surprisingly enough. I can't think of a succinct way to illustrate it (and don't have the time or money to build a dyson's sphere just to prove a point...) but if you have one spherical heavy object inside another, the net gravitational interaction between the two is zero. Essentially, the inner object is equally pulled in all directions. Surprisingly, this result holds even if the dyson's sphere drifts such that one area is much closer to the sun than the remainder of the sphere. |
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How do you determine that? If the distance is small then the difference in gravitational energy will be minimal, but if the imbalance grows beyond a certain point then the gravitational pull on that portion of the object will be greater and cause a corresponding decrease in the section that is farther away, since the force varies by a logarithmic function the forces will rapidly begin to imbalance each other, eventually this will grow into a wobble and later a shimmy the eventually a full on shake then the whole thing will fall apart due to shear. |
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In this case though the Dyson Spheres gravitational influence in negligable I understand that the acceleration due to gravity towards the sun will be fixed based upon the mass of the sun however since the sphere is not spinning it will be free to drift about at which point wouldnt you have to look at the sphere as a ballistic projectile in a sense. Or would you have to look at the distribution of mass, if there is an area of greater mass than another then it will be drawn with greater strength than another area of lesser mass, while both would accelerate at the same rate the heavier are would experience a slightly greater rate of acceleration when you figure in the solar wind. A structure of this size in such close proximity to the sun would experience tremendous tidal forces and be subject to deflection and distortion I would think. |
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//just far enough so the gravity
matches Earth.// |
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Eh? If you mean what I think you
mean (and not what I fear you
mean) that's going to be mighty
close to the sun, methinks. |
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Anyone got a figure for the
acceleration due to the sun's
gravity at Earth's orbit? |
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Wait, I can do this: 93 million
miles times 2 pi times 1600 over
365 ... |
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//just far enough so the gravity matches Earth.// |
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// Eh? If you mean what I think you mean (and not what I fear you mean) that's going to be mighty close to the sun, methinks. // |
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According to the website:
almost 3 million (2,984,855.19637063) kilometers above the surface of the sun |
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Wikipedia says the sun is 1,392,000 km [edit: in diameter], so yeah, that's close. |
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//Solar wind might be sufficient to compensate for the drifting sphere. As one side got closer to the sun, the effects of solar wind would increase, pushing it away from the sun. Meanwhile, on the other side, the increasing distance would decrease the solar wind effects. The two effects combine, and hopefully provide enough push to keep things in equilibrium. // |
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It can't be as simple as just letting the solar wind on one side travel further and spread over a greater area before being absorbed. Having stated it that way, it's obvious that the reduced intensity is cancelled out by the increased area. But there are other factors. The wind should theoretically lose energy as it travels away from the sun and its gravity. An intense wind of charged particles could get absorbed differently from a more diffuse wind. |
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//Selectively making portions of the sphere reflective/transparent could supplement this.// |
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There's that material they use for sunglasses that darkens in stronger light. That would get pushed away more strongly when it got closer. |
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Lurch: fine, so you're saying that if we had the tech & resorces to do one, we can do the other; but since they are both big projects we have to choose. I'm sure history is full of excamples of that happening. |
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A lot of the probs people are pointing out about my Dyson's are the same for a regular size one. |
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Cap, I was comparing the size to the orbit of Mercury, not the size of the sun itself. |
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I added holes to my Dyson's so that light can get through, redirected w/mirrors & shine on the otherwise dark liviable surface. I should need at least 2 on opposite sides to cover all the surface, but the furtherest edge of the shell will get the lest amont of light. intead of polar icecaps, it'll have a ice ring. |
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sounds a lot like Stepen Baxter's "Lakes of Light." He spun it to simulate gravity, and the gravity increased as you went north, but overcoming the heat of the star was done by some unexplained method. |
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Didn't Niven cover just about every permutation of the Dyson sphere idea? |
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[jhomrighaus] Inside a sphere, as you approach the surface at one point, you will be strongly pulled towards a relatively small area of its surface (the nearest bit) and weakly pulled towards the large remainder. As you get closer, the small area gets smaller and pulls harder (per unit mass), while the large area gets larger and pulls less hard. The net effect is that they both cancel out. This I have been taught. |
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Further, from wikipedia: Divergence Theorem: |
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Spherically symmetric mass distribution: |
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In the case of a spherically symmetric mass distribution we can conclude from this that the field strength at a distance r from the center is inward with a magnitude of G/r² times the total mass at a smaller distance, regardless of any masses at a larger distance. |
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For example, a hollow sphere does not produce any gravity inside. The gravitational field inside is the same as if the hollow sphere were not there (i.e. the field is that of any masses inside and outside the sphere only). |
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See also the shell theorem. |
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I agree that deflection of the sphere alters the problem, but I put it to you that that was outside the scope of the initial problem. |
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From the website: // I came up with almost 3 million (2,984,855.19637063) kilometers above the surface of the sun. // |
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From above: //Cap, I was comparing the size to the orbit of Mercury, not the size of the sun itself.// |
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//Cap, I was comparing the size to the orbit of Mercury, not the size of the sun itself. // Right, the comparison to the sun's diameter was my own idea. And I wasn't being sarcastic calling it close if anyone thought that. Two and a bit diameters from the surface seems pretty close. I wonder if trapping heat and/or solar wind inside the sphere could make the sun swell up into contact with the sphere. |
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By the way, it's an interesting idea, unknown, even if I think it's impractical. |
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Here's my main problem with it: I think reducing the size increases the pressure it must hold up against. Increasing the thickness proportionally increases the mass, so that cancels out in the pressure calculation. Decreasing the sphere diameter decreases the mass and increases the gravitational force per unit mass in a way that cancels out (square of diameter). That just leaves the area you spread the weight over, which is less for an equal thickness smaller sphere, and as I said above, increasing the thickness makes no difference to the pressure (it should help against buckling though, I'd expect). |
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Maybe you can support it from inside with stuff going at faster-than-orbital speed. |
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It sounds like some of you are thinking that the living area of my Dyson's is on the inside like a regular one. No, the living area will be on the OUTSIDE surface. From the distance I caluated, the sun's gravity will be equal to that of earth's. Since it is smaller than the 1 AU radius Dyson's the shell can be made thicker & give more sturctural support. |
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I have lost the will to live. |
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[the great unknown], you seem to suggest in a couple of annos that making the shell thicker would help. A simple analysis would suggest that it won't (a more advanced analysis might reveal some hidden advantages; I don't know). |
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Firstly, the stresses you are trying to resist are those caused by the effects of the sun's gravity on the shell. Gravitational forces are proportional to mass. Thus doubling the thickness will double the mass and hence double the forces you are resisting. You're essentially back where you began. |
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Secondly, the sun's surface temperature represents an equilibrium temperature at which the power the sun generates is all radiated. If you make the shell thicker, this will trap the heat within the sphere. Consequently, the sun will actually start absorbing some of this heat as it is radiated off the sphere's walls. Thus it is giving out less power than it is receiving, so the sun's surface will heat up to regain equilibrium. Hence the sphere's outer surface temperature will rise. Guess what? It will put you precisely where you began. |
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again, these are the same problems that occur with a regular size Dyson's. with mine people would be walking on a surface 4 meters thick as opposed to 8 centmenters thick. |
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//Inside a sphere, as you approach the surface at one point, you will be strongly pulled towards a relatively small area of its surface (the nearest bit) and weakly pulled towards the large remainder. As you get closer, the small area gets smaller and pulls harder (per unit mass), while the large area gets larger and pulls less hard. The net effect is that they both cancel out. This I have been taught.// |
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Does this not assume a uniform distribution of mass throughout the sphere in question? What is the impact of a sphere of varying mass or a perforated sphere? |
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Three little words = One little fishbone: Coronal Mass Ejections |
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[tgu] You keep saying that the objections being raised are the same objections applicable to a full-size Dyson. If so, what have you accomplished with the mini-D? If you concede the points about the surface temperature of your mini-D, then you are left with a vast sphere with 1G surface gravity, but without atmosphere or surface water. There is no habitable area, while in the Dyson there are vast tracts of habitable surface on the inside. What we have before us then is an engineering marvel, but of less use than the full size Dyson. (Not that there's anything wrong with that.) |
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What happens to a radiant heater when you focus its heat back on itself? Wait, bad example...resistance rises with temperature, etc... |
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Making the Sun hotter certainly can't be a good idea. |
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"Where's the kaboom? There was supposed to be a Dyson-shattering kaboom!" |
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//With current technology, if we were going to try to build a Dyson sphere, it would require a mass of fuel many times greater than the mass of the stuff we're making the sphere out of. Since the Dyson sphere will require pretty much all of the mass in the solar system, we can't get enough fuel. Sorry, I don't mean to seem negative, but it just isn't available no matter the price.
But - imagine someone came along with a new idea that allowed us to generate fuel out of nothing while we're in space. (You may be tempted to say that such magic wouldn't be required - perhaps a way to turn rock into fuel, rather than nothing, but nay - won't work. You can't have that rock - we need it for the Dyson sphere.) If we can generate fuel in such a way, then we would have the energy necessary - the "delta-V" to move everything to where it needs be to build the sphere. // |
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There is one souce of energy that would be powerful enough to support the building of a Dyson, but would be impractical to use for interstellar travel: |
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//you are left with a vast sphere with 1G surface gravity, but without atmosphere or surface water. There is no habitable area//
If the outer surface has the same gravity as earth, it can contain an atmosphere , environment, etc. the same as earth. One difference is that sunlight is under the surface. that is why I have holes & mirrors to let light out & redirect it back to the surface. |
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There is one possible answer to both station keeping and surface temperature issues. Use heat pump technologies to concentrate the heat in specific areas which then radiate into space. By controlling which of these you radiate from, you also develop some thrust, which provides some station keeping capability. Admittedly, I'm not sure I want to live on a big sphere that could melt and drift into the sun if the control computers go down. |
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Also, people don't live on the inside surface of a normal Dyson sphere, they live in closed environments embedded in the shell, as neither the outer or inner surface is habitable. The inner surface has no gravity, and neither surfacer (most likely) can sustain an atmosphere. |
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//same gravity as earth, it can contain an atmosphere , environment, etc. the same as earth//
The gravity may be the same, but if the surface is 2500K (as per [lurch] above), there won't be any water. There may be an atmosphere, but I suspect it will be mostly vapourising particles of the sphere.
Alternative: use the small sphere to generate power, but leave a hole so Earth still gets the light it's used to. (I'll let the rest of you argue about how big the hole needs to be...) |
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if we get the technology to be able to do this, then surely we have the technology to be able to do this around a star that's of a size/output that works with this idea. |
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I never understood Dyson spheres; an interesting thought experiment maybe but if you had the unlimited (!) energy and resources to build one, why would you bother? Why not spend your unlimited resources and energy on colonizing space? Or hookers? |
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a working Dyson sphere probably wouldn't be used as a living space unless you had antigravity. On the other hand if you can find a star with the right specs, this idea would really shine... so to speak. |
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