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It's well known that because the gravitational attaction at one point on the inside of a hollow shell is zero, because it is cancelled by the attraction from all the other points on the interior.

Here is a wikipedia page on this 'Shell Theorem' which might explain it better.

A Dyson sphere is an example of one of these hollow spheres. I've not read any fiction based on them, but suddenly I'm interested; how would such a device work (for habitation, mainly), and how has it been depicted? Is this gravitational issue mostly ignored or are there clever ways around it? The only way I found explained is to rotate the sphere so that the equator experiences a pseudo-gravitational force. This is the page I've read most of, about this topic.

Is that rotational solution the only one used in the sci-fi literature or are there other methods? Which method did Dyson use?

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    The shell theorem says that the Dyson sphere does not attract bodies on the inside. You still have gravity from the sun in the center. This again does not affect the sphere, but everything else, so the inhabitants will still be drawn to the sun. – Till B Nov 17 '12 at 16:35
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    I can't decide if this is more a physics question (off-topic) or a list question (still off-topic). – Kyle Jones Nov 17 '12 at 22:38
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    @KyleJones: "Which method did Dyson use?" Neither physics nor list question. – bitmask Nov 18 '12 at 4:36
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    @bitmask What method Dyson used is still off-topic because Dyson is a mathematical physicist, not an sf author, and so at best we're still talking about physics divorced from fantasy and sf. And the answer is easily found by looking at the Wikipedia article on Freeman Dyson so this question is pretty basic even if it were on-topic. – Kyle Jones Nov 18 '12 at 4:52
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    Dyson himself admitted he hadn't thought the concept through far enough, after he had read Ringworld by Larry Niven. – Mr Lister Nov 18 '12 at 14:15
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There are several possible approaches.

Sidestep the problem. Some books find uses for Dyson spheres that don't require them to have internal gravity. Some examples of this include:

  • If your Dyson sphere is actually a Matrioshka brain (as in Accelerando by Charles Stross), you don't need any gravity. Computation works fine in zero-G.
  • The sufficiently advanced aliens in John Scalzi's Old Man's War books use a Dyson sphere to power a shield around their home planet, so again gravity is irrelevant.
  • Farthest Star by Frederik Pohl and Jack Williamson has a Dyson sphere where the outside is inhabited. So the Shell Theorem doesn't apply, and the inhabitants experience (a very small amount of) gravity from the sun and the sphere itself.
  • I can't think of any examples at the moment, but there are almost certainly books out there where people simply inhabit a zero-G environment inside a Dyson sphere. The problem with this is atmosphere -- pumping the entire sphere full of breathable atmosphere leads to crazy stuff happening near the sun, but if there's nothing gravitational going on there's no obvious way to keep the atmosphere near the surface and away from the sun.

Ignore the problem. At this point you're not really writing hard sci-fi anymore, but this is a possible approach.

  • Weis and Hickman's Death's Gate books have a Dyson sphere in a fantasy setting that has entirely unexplained gravity.

Create a gravity source. Rotating the sphere is one approach to doing this, but there are others.

  • Fred Saberhagen's Berserker books have a Dyson sphere where the sun at the center is gravitationally repellent, leading to exactly the kind of behavior you'd intuitively want on the surface of the sphere.
  • If you had a Dyson sphere in a universe with four spatial dimensions, you could rotate it in such a way as to cause the entire surface to experience uniform pseudogravity. I am not aware of any book that does this (and unlike the other "I can't remember" bullet point, I'm sure I'd remember if I'd read one).
  • Isn't this typically explained by having the sphere rotate at an incredible velocity? That's why the poles are usually uninhabitable. – Gorchestopher H Nov 17 '12 at 20:05
  • If you can find a way to keep the sphere in position, there should be sufficient gravity from both the sun and the sphere if you live on the outside of it. Also, if the sphere is built in layers, each layer would experiment gravity from both the sun and the layers closer to the core. A Dyson sphere is theoretically needed to harvest all of a sun's energy. It's a lot of wasted effort if 100% of the inner wall is not covered in solar panels, no? – Dungarth Nov 17 '12 at 20:38
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    @Dungarth: If the sphere is at an Earthlike distance from the sun, the sun will give its outside a tiny fraction of Earth's gravity. Since the sun contains most of the mass of the solar system, the only way to get Earthlike gravity on the outside of a Dyson sphere at an Earthlike distance from the sun is to disassemble a few thousand solar systems. – Micah Nov 17 '12 at 21:18
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    @Micah Why exactly would you assume an earth-like distance from the sun? Placing the sphere closer not only easily solves the gravitation problem, it also requires exponentially less materials... – Dungarth Nov 18 '12 at 21:12
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    @Dungarth: I don't assume you'd be living on the inside of the shell; I assume you'd be living on the outside (because that's what you wrote). Unless you also have some way of violating the second law of thermodynamics, you still need your shell to be at least some minimum size to avoid being cooked by the waste heat from your energy-collection activities. (And if you do have some way of violating the second law of thermodynamics, you don't need a Dyson shell to collect energy, since you can do degenerate things like running your civilization off its own waste heat.) – Micah Nov 19 '12 at 15:14
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There are different types of Dyson Spheres, with the solid shell ones, Type II, being most physically problematic. Dyson proposed (or stated that what he really meant was) that the shell would not be solid, but a virtual shell of orbiting objects (Type I). Type III are non-orbiting and held in place by solar wind, but those are so fragile that you don't have to worry about gravity because you can't live on the surface.

This site has good information on the properties and classification of Dyson Spheres.

Anyway, with Type 1, since you have a bunch of orbiters, you can have each orbiter rotate (at least that part which is supposed to provide an feeling of gravity).

(Note also that if you wanted to collect all the light from the sun with Type I orbiters, they would occlude each other (be in each others' shadows) (pi-2)/pi of the time, or ~36%.)

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To quote Arthur C Clarke: "any sufficiently advanced technology is indistinguishable from magic".

So the solution becomes simple: use magic, AKA sufficiently advanced technology, to generate the required gravity. There's no need for an explanation in terms of our own current understanding of the laws of physics (and an attempt to cook up such an explanation may look horribly dated within a quite short time).

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    Poor answer.. It can be answer to most of sci-fi questions on this site.. – I Love You 3000 Nov 18 '12 at 15:30
  • Well the physically correct answer is "they all die" because a Dyson sphere using the hollow shell model is also unstable and has no gravitational interaction with it's parent star either. It has a risk of eventually colliding with the star, in other words: en.wikipedia.org/wiki/Dyson_sphere#Dyson_shell In order to circumvent that we must begin by invoking "sufficiently advanced technology". – user8719 Nov 18 '12 at 22:18
  • @mh.: Doesn't solar wind pressure equalize the shell's distance from the star? As one side closes in it should experience higher pressure and move away. – Zan Lynx Nov 18 '12 at 22:42
  • @ZanLynx Actually, these things could equalize at any given radius. Both photons and gravity propagate in the same way, with a 1/r^2 force. It will balance at a certain value of mass per solid angle. So the shell could be thicker closer if the radius were smaller, but regardless of where it is, the total mass needed to be balanced would be constant. – AlanSE Nov 19 '12 at 1:40
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I think it's much simpler than that...

You can build the dyson sphere, say, at the orbit of Mars; you can then have a planet (or planets) at the orbit of Earth that has its own gravity just as ours does now.

Any society advanced enough to build such a thing could figure out the orbital mechanics and effects of the gravity of the planet on the sphere.

You don't have to LIVE on the surface of the sphere...you can just use it to capture the output of the sun.

Classic experiment: take a square piece of ice and measure its mass. Set it in the sun and time it as it melts. Knowing how much energy it takes to melt that mass in that amount of time, multiply this figure by a factor equal to a sphere of ice with the same radius as the sun to disk distance. This will give you an approximation of the energy output of the sun and it's pretty freaking staggering.

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The problem isn't 0g, it's that a Dyson sphere is unstable. You have to constantly use energy to keep it centered, even if it is spinning. Otherwise the sun will slowly drift into the sphere, and vice versa. (Same with ring worlds.) The solution is to use a Dyson swarm. It's a bunch of self-contained units that orbit the sun and there are enough to block out the sun. This video will explain it.

The Fermi Paradox & the Dyson Dilemma:

They may have gravity generators, or they may be in rings that rotate to create artificial gravity.

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Use gravity generators.

Being able to generate an artificial gravity field is a common feature of sci-fi spaceships, so that they don't all have to be rotating cylinders. Ships in Star Trek have artificial gravity.

Since your society has built a Dyson sphere, they shouldn't have any trouble powering the generators, especially since that's made up technology, so you can say it takes as much power as you like.

For some degree of validity, you could attempt to determine how much force the gravity generators are applying to the surface, and come up with a source for that much energy. I would expect that the energy collected from the sun at the center of sphere would be enough.

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