ALT.SF4M Big Structures • s******j@**.com 11/08/1996 00:00:000 UTC Spheres, Dyson: I found a copy of Dyson's autobiography. In a chapter about space settlements, and the energy needs of civilizations trying to blanket the galacy with radio waves, he modestly disclaims credit for dyson spheres, saying his inspiration came from a "tattered copy" of _Star Maker_ he found in 1945. The quoted passage refers to the collectors as a "Gauzy shell of light traps." Dyson, besides being embarassingly fair-minded, is a decentralist. He likes the idea of colonies clinging to comets and forming weird little societies that can cook up ingenious ideas. To folks such as these, a central energy collecting sphere would be a rather dull, but vital, source of power. Niven is certainly responsible for the ringworld, and solid dyson spheres. I still doubt such things would be built; they have no practical purpose I can think of. This leaves irrational ones, which could be quite interesting. Which I guess is what SF is about. Done to Death: By this I meant endless discussions justifying / rationalizing solid spheres. All sorts of weird ideas have been proposed to give a sphere gravity, support the non-spinning polar regions, etc. The same stuff gets worked over time and time again. I'll try to find the FAQ about spheres that collects some of this material. Death Star / Orbital big is still plenty big, and building such is close enough to today's engineering that meaningful discussions are possibe. -- +-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-+ ***@***.com ~ s*****s@a*****.***u.edu ~ s******j@**.com http://www.ini.cmu.edu/~sjones/ • j********o@***.com 13/08/1996 00:00:000 UTC In article <4um6es$***@x*****.**o.com>, s******j@**.com (Stefan E. Jones) writes: >Niven is certainly responsible for ... solid dyson spheres. Prove it! (An exhaustive search of the literature to eliminate any other possibility will be necessary.) You asked about the necessary strength of the "pieces" of a Dyson sphere: no pieces, no strength (and you still haven't told me what Dyson himself envisioned! If he's just quoting from Staplegun, then he should get no credit at all!) >I still doubt such things would be built; they have no practical purpose >I can think of. This leaves irrational ones, which could be quite >interesting. Which I guess is what SF is about. Again, little is done because it is necessary. If they can do it for the Hell of it, then a milennia earlier, they can do it because it's useful. (e.g., your supernova fireworks: I assume there's some money in it for them.) If you want my honest answer about the future, I'll tell you that no sentient has ever left a solar system, nor ever will. They achieve immortality, get bored, and let themselves die. There's probably never been, nor ever will be, a successful space habitat: a species inhabits those habitable planets that nature gives them in a star system, and that's all. I'm not married to these giant beasties, but they would put an absurd amount of land in one solar system, in one easily accessible strip. Really advanced future-boys might want to have such huge tracts of land to divide among a small or a huge population. In terms of known fundamental physics, FTL travel is completely ruled out, but materials strength could give a lot more room for expansion, so instead of colonizing the galaxy, you build a galaxy at home. And the advantage over habitable rings over spheres is still that rings can provide spin gravity without artificial gravity, which is also a big deal (even if it's not real gravity). I still say that we can't meaningfully speculate about the social motivations and technological capabilities of arbitrarily advanced societies. Given a time frame, that's different. An orbiting ring remains a logical development of a highly developed orbit (as around Earth in the Fountains of Paradise, if memory serves). Brian said: >In response to Gorno: we know a lot more today about fundamental physics than >primitive savages of the past did, so the comparison is invalid. One society's fundamental physics is another's out-moded model. I sure won't rule out the possibility of exhausting fundamental physics, but mechanical strengths aren't hyperdrives: there might be all sorts of weirdness possible. The other point of the comparison is to social scale and use: the abo couldn't conceive of a societal need for the World Trade Center. The Roman would argue that the weight of chimneys alone would limit it to 10 stories! Future societies will surely look back at us with the same rightful sneer. >The solution is simply to live on the outside, and place the atmosphere there as well. But it would be dark (not necessarily cold)! Hmmm, big hollow lighthouses with convex mirrors in the top, rotating once a day... The Sun's "surface" g is around 2.5, if memory serves, so the effective temperature at 1 G would be around 1000 K: you could pick a more suitable star or use the excess energy to do stuff... That is a neat twist: still need advanced materials (it doesn't matter whether g is in or out, or exerted between the AG shell and the surface: the strength's the same for a given size). This still assumes a need for gravity. >By the way, what are the plasma conduits for? I think they replace electricity. They may be some weird sort of plasma-like thing (to their science, the new label makes perfect sense)... This at least ties into the Old Trek technical mindset: an organic evolution from our day. >So a fleet is far more flexible, and therefore a better investment, that a Death Star. Again, depends on the engineering concerns. Armor is an obvious square-cubed advantage: you can put a kilometer depth of armor on a Death Star, but not on a million Star Destroyers that are a kilometer long. Put them in a box to fight it out, and the big guy comes out alone... If one Death Star costs less than "half the fleet," yet packs as much firepower, it's economical. Gorno Near as I can figure, the generalized stress at some point along an axis in a body is the integral of the forces (projected along the axis) acting on each volume element in either portion of the structure as divided by a plane perpindicular to that axis. (The cross-sectional analysis.) A point lies in an infinity of planes, but we are interested in stress, which has the units of pressure when considered over a surface yet is not a pressure in the normal sense (that is, it can be unequal in different directions: we have already considered all projections). Hence, I am taking a building solid to be compressable but rigid (until it yields), as opposed to a fluid that is incompressable and non-rigid. We can identify the weakest possible plane (where the most force acts on the smallest area), and if the material holds there, it holds everywhere. For the kinds of structures I'm investigating, the plane(s) are obviously through the geometrical center. A narrow ring is approximately equivalent to a vertical column: the forces that pull in must be supported by the cross-section, so the stress is along the ring; the length is the limiting factor and cross section is irrelevant. Since circumference (hence weight) is proportionate to r, while g is proportionate to r squared, the bigger the stationary ring, the less the materials strength required (if in a circular orbit, there are no circumferential stresses acting on it). Still working (leisurely) on this. Also, center of mass ceases to be a useful thing if g> varies meaningfully over a body.