PDA

View Full Version : Megastructures



Michael_FJS
2004-Oct-26, 10:17 AM
I've been reading about real big structures built around stars like ring worlds and dysons spheres and stuff. I was wondering how big can a structure get, not just ring worlds but any kind of structure.

There are a few main factors which would limit a structures size,
- The resource available
- The strenth of the material
- The effect of gravty from stars as well as the structure

Any thing larger then a dysons sphere would require the resources of more then one solar system.

Would things like massive wall be possible, e.g if to alien races are at wall and wanted to keep each other out of their teritory. Maybe not soild structure but a mine field that would span lightyears.

What other structures could exist? Would they be possible to build?

The univese would have an enormous amount of resources available, the only problem is that its spread out too far. Unless some sort of FTL travel is possibe, structures like this will only stay in sci-fi.

Michael_FJS
2004-Oct-26, 10:24 AM
I think the only reason an alien race would need to build a structure larger then a dysons sphere would be for military purposes. But I would think a large fleet of ships would be alot better then a big structure, unless it housed some kind of weapon.

eburacum45
2004-Oct-26, 11:50 AM
Dyson spheres as generally described are impossible; the gravity of the star will pull everything of the inside of the sphere into the star; and in fact there is no material strong enough to make either a dyson sphere or a ringworld.

But there are alternatives; ring worlds could be built up to 2000km in diameter using buckytube; a dyson swarm of habitats could entirely surrond a star, but you have to control their orbits very carefully so as not to have them collide with one another;

and the biggest structures of all can use dynamic compression members as described by Paul Birch- this technique involves using rotation beams of magnetic particles to support structures in a gravity field- given enough of these and you could build a structure around Jupiter, or around the Sun; gravity would still be towards the central planet or star, so you live on the outside.

http://www.orionsarm.com/eg-article/4847416b8b0ad

Taking Birch's dynamic compression member techique to its limits, you could set up a loop of magnetic particles between stars; then enclose this loop in magnetic hoops and hang structures of the loops- eventually you could build a web between the worlds, and fill space with strands of megastructures- not exactly a wall, but just about as close as you can get without using impossibly strong materials.

Michael_FJS
2004-Oct-26, 01:53 PM
Thanks,

I always thought that there would be not material strong enough to build a type 2 dyson sphere.

How much stress would be produced in a ring world?

antoniseb
2004-Oct-26, 07:40 PM
Originally posted by Michael_FJS@Oct 26 2004, 01:53 PM
How much stress would be produced in a ring world?
It depends on how different the ring-world's spin is from the orbital velocity of a particle in the same orbit. If the difference is zero, there is not much stress, and also not much apparant gravity, or forces to hold in the atmosphere. If it spins so there is one-gee, every ton of material has a ton of force trying to break the ring apart.

As a side note about mega structures, take a few minutes and calculate the largest diameter that a very long [an AU or more] Iron bar in deep space could have before it would collapse simply beacuse of its own gravity. It is surprising! Giant structures are hard to design without giant parts.

Duane
2004-Oct-26, 11:40 PM
How would one go about making that calculation antoniseb?

antoniseb
2004-Oct-27, 01:21 PM
Originally posted by Duane@Oct 26 2004, 11:40 PM
How would one go about making that calculation
Approximate the length as infinity. Pick a central point and integrate the total force, this gives you a formula based on density and diameter. Find the diameter that produces a force greater than the compression strength of Iron [or steel if you prefer].

Dave12308
2014-Oct-14, 08:39 PM
Why are we not allowed to speculate on ETI in here, yet we are allowed to talk about Dyson spheres/etc. as if they were something that really exists?

AFAIK no one has ever built one or even proven that it can be done. So why do we throw these terms around like they are fact, yet something like ETI we have to "prove"?

SkepticJ
2014-Oct-14, 08:54 PM
We are allowed to speculate on ETI. What we're not allowed to do, outside of Against the Mainstream, is present our speculations as fact.

Megastructures are open to being analyzed using mathematics and physical laws. Unless we're fundamentally mistaken about most of physics, some varieties of megastructures should be possible. If they actually exist is a different question. Many things are possible that don't exist, because no one has bothered to make them.

PetersCreek
2014-Oct-14, 08:57 PM
Dave12308,

If you want to question board policy, do so in the feedback forum. Resurrecting a thread that's been dormant for nearly 10 years to air such a grievance is not appropriate.

eburacum45
2014-Oct-20, 01:01 PM
The largest speculative structure I have ever heard of is Paul Birch's Supraself; rather than building a shell around a planet or star, and relying on the gravity of that object to hold it together, a Supraself simply rises above itself.

http://www.orionsarm.com/fm_store/CEMS%20Syposium%20_Bringing%20Worlds%20to%20Life_% 20at%20...pdf

....each shell is dynamically supported within the gravity of the shells beneath. Supraself has a mass of about 2 x 10e12 solar-masses, as massive as the largest galaxies. The outer shell is about 1.2 light years in radius, and 14 square light years in area, which is quite a lot of land to find in one chunk. And of course that's only the first layer; there's another 30 million shells below that. The total habitat area is thus about 2 x 10e23 Earths.

swampyankee
2014-Oct-20, 07:21 PM
How would one go about making that calculation antoniseb?

For something like a ringworld or a Banks Orbital, the stresses can be calculated by treating the structure as a thin pressure vessel under internal loading. You'll have to do a little algebraic jiggery-pokery to get everything to work out, but pressure is replaced by the centripetal acceleration times the areal density of the shell material.

mkline55
2014-Oct-20, 07:50 PM
How much stress would there be on the structure of a ringworld where its spin rate matched the orbital velocity? For example, if the ringworld were built around Sol at Earth's distance, and rotated around at a leisurely once per year, wouldn't it be mostly stress free? And atmosphere free unless someone put a lid on it. Never mind for now just how it stays in position.

Chuck
2014-Oct-20, 08:00 PM
Maybe a Dyson Sphere could be light enough to be supported by solar wind. When it drifts off center, vents can be opened in it to reduce the pressure on one side to move it back into position.

Nick Theodorakis
2014-Oct-21, 01:00 AM
ISTR that there is an astrophysicist that is mining WISE and Kepler data to look for signatures of megastructures like Dyson spheres but so far I don't think he had found any candidates. It's still fascinating to think it's even possible to look, though.

Nick

Githyanki
2014-Oct-22, 06:06 PM
I just assume the aliens are smarter than us; one question: Does a solar system have enough material to construct a sphere?

Noclevername
2014-Oct-24, 11:11 PM
I just assume the aliens are smarter than us; one question: Does a solar system have enough material to construct a sphere?

A sphere (in Dyson's original sense of an orbital cloud) or a shell?

publiusr
2014-Nov-03, 08:24 PM
I think David Egge or Bob Eggleton did some art for Science 82 or OMNI that showed the interior and exteriors of Dyson Megaspheres.

I can't find it on the web.

ngc3314
2014-Nov-07, 10:29 PM
There is a nice pair of recent papers doing a serious discussion and search for Kardashev-uprated civiizations, including proper spectral fits to the WISE data:

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification (http://arxiv.org/abs/1408.1133)

The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result (http://arxiv.org/abs/1408.1134), both by Jason Wright and colleagues. As they note, these searches are for Dyson swarms and their kin, since a filled sphere requires unobtainium and you get the same results with a swarm of objects in independent orbits.

(I was struck at how much the calculation to seek waste heat from a megastructure parallels that to tell whether a quasar is extant, obscured, or faded. They remark that quasars look like really excellent signs of cosmic engineering to generate energy efficiently, except for having a very unlikely history across cosmic time).

Van Rijn
2014-Nov-11, 10:03 PM
Those are interesting articles. I have some issues with some of his justifications in the first paper, but he makes a better argument for the galactic colonization idea than what I usually find, and does seem to mention more counter-arguments than typical as well. I still think there are holes in his argument. Having said that, I want to see astroengineering SETI searches go forward.


As they note, these searches are for Dyson swarms and their kin, since a filled sphere requires unobtainium and you get the same results with a swarm of objects in independent orbits.


There do appear to be two known ways a filled Dyson sphere could be possible: One would be by using statites to support a thin-skinned Dyson bubble. Another would be a dynamic support system, with rings of material moving at higher than orbital velocity to counter the collapse of a Dyson sphere.

It's true that it doesn't really matter from an observation standpoint, but there might be some technical advantages to having something more structured than a Dyson swarm.

eburacum45
2014-Nov-13, 04:02 PM
There are structures that are not Dyson Spheres that would still be very large, and possibly even more detectable. If a civilisation manages to acquire control over fusion reactions, then it could mine gas giants and even the local star for hydrogen/deuterium/helium and fuse it to create energy on tap. As Paul Birch pointed out, there are metals and impurities in every star, enough that it would be worth mining the atmosphere of a star to obtain them for building materials. Given very sophisticated fusion technology you could even fuse H/He into usable building materials. This would produce a lot of usable energy and observable waste heat.

Dyson spheres are one application of technology that might be observable, but they are far from the only possibility.

publiusr
2014-Nov-15, 09:30 PM
the biggest structures of all can use dynamic compression members as described by Paul Birch- this technique involves using rotation beams of magnetic particles to support structures in a gravity field- given enough of these and you could build a structure around Jupiter, or around the Sun; gravity would still be towards the central planet or star, so you live on the outside.

I wonder if you could surround a tiny black hole like this.

At the poles you would have some high energy labs where x-ray jets could strike--mercury to gold transmutation.

Feed a little to the black hole within to power everything somehow.

Noclevername
2014-Nov-16, 01:07 AM
I wonder if you could surround a tiny black hole like this.

At the poles you would have some high energy labs where x-ray jets could strike--mercury to gold transmutation.

Feed a little to the black hole within to power everything somehow.

If you can do it to a star, you can do it to a black hole. However, close enough to either one to produce significant "surface" gravity, would be too hot.

eburacum45
2014-Nov-16, 12:18 PM
If you can do it to a star, you can do it to a black hole. However, close enough to either one to produce significant "surface" gravity, would be too hot.

An Earth-mass black hole with no accretion disk would be colder than the CMBR, so you don't need to worry too much about that. Black holes are colder than the universe down to a mass of about 3 x 10e22 kg, similar to the mass of Triton. So you could have a shell over a black hole with the mass of Triton without having to worry too much about the temperature of the hole. Smaller holes would emit energy, which might help you to power the dynamic shell system; but if you make it too small the temperature of the hole is so high that it is mostly emitting exotic and dangerous particles.

publiusr
2014-Nov-16, 08:44 PM
That might be a good thing, if you want those particles to do work. I imagine it would only be dangerous near the poles of the BH--isn't that where the X-ray jets come from?

Have your alchemy done there at the poles.

Noclevername
2014-Nov-17, 12:26 AM
That might be a good thing, if you want those particles to do work. I imagine it would only be dangerous near the poles of the BH--isn't that where the X-ray jets come from?

Have your alchemy done there at the poles.

"Exotic" particles probably means stuff that can penetrate the shell all over, I think.