Don’t confuse gravity with suction.
Take off, eh.
Hmm. So you have a Dyson sphere with an internal albedo of 1. Put a big friggin light source in the middle. How long before the sphere fills with light and bursts?
That would be one theory of the big bang. A science project gone horribly wrong.
I keed! I keed!
The only way that you could have gravity on the inside of the sphere and have it be stable around the central star would be if you could somehow tweak gravity so that the star exerted a push (effectively) against the sphere rather than a pull. Perhaps it could be done this way:
Suppose through some unimaginably advanced gravity technology you could engineer a local reversal of the star’s gravity gradient: that is, out to a certain point the star’s gravitational pull declines as the square of the distance, as usual. Then, at distance X, the star’s gravitational pull actually increases for a short distance; then further out goes back to declining again. You wouldn’t need at any point for gravity to actually repulse instead of pull; rather, the energy potential would be greater within the gradient reversal than on either side of it. An analogy might be the way “shepherd” moonlets in Saturn’s rings tend to concentrate material into narrow zones as if they exerted a repulsion, even though no actual antigravity is involved. The net result would be that matter would tend to be compressed into that zone, with objects on the outside and inside of the shell experiencing a force towards the middle zone of the shell. It would also be dynamically stable and not require the material to have mechanical strength other than resisting compression- rock would do.
I have no way to calculate what combination of star mass, luminosity, shell radius, shell thickness and sharpness of gradient reversal would be required to get 1G at the surfaces.
Yeah Dyson didn’t really propose a sphere. In his original article, he proposed that an advanced civilization may break apart a large planet in order to set up habitats in solar orbit. The confusion is understandable because of some other things he says in the article, but he cleared it up by saying: “A solid shell or ring surrounding a star is mechanically impossible. The form of “biosphere” which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star. The size and shape of the individual objects would be chosen to suit the inhabitants. I did not indulge in speculations concerning the constructional details of the biosphere, since the expected emission of infrared radiation is independent of such details.”
I think his idea was simply that an advanced civilization would tap their sun to feed their energy needs by migrating to orbital habitats. These would become so dense that the light from the star may be effectively blocked off to us on earth.
The only reason scrith had to have such an unreasonable strength is because of the rotation imparted to get 1g acceleration at the surface. If you’re not spinning it to simulate gravity then the speed required to obtain sufficient tension to keep it from kinking is much lower.
However, if you have the wherewithal to exert enough energy to put together a (continuous) Dyson sphere you probably already command more energy than would make it worthwhile. As previously mentioned, Dyson’s original scheme was more like a swarm of satellites orbiting a star in ball-of-yarn orbits that concealed the star. This is somewhat more plausible, though I suspect that a civilization with even this capability is going to be on the verge of starting to explore other systems, either by severe modification of their own biology to a form suitable for long-term interstellar transits or by robotic proxy.
Stranger
Indeed, as I said way back up there. But too many writers got entranced with the idea of a solid Dyson Sphere that the concept caught on. Why, I don’t know, because I’ve yet to encounter any book that made the idea worthwhile or interesting. Larry Niven did a hell of a lot more with his Ringworld than any of the writers of stories about walkabout Dyson Spheres did with theirs.
I think in part it’s because the concept is unimagiably huge. The Ringworld is already unbelievably huge – enough land area so that you can effectively lose the Earth on it. A Dyson Sphere is that unimaginably huge area squared. (Well, to the 3/2 power, but you know what I mean). What the hell do you do with that area? Shaw, at the end of Orbitsville, simply suggested that, with that much free land and resources, nobody had to work any more, and the damned thing was a civilization-killer.
I think they’re all short-sighted. People will expand to fill the space allotted, and wll probably wastre resources, given the chance. There’s a story in there, waiting to be told.
No. The gravity pull towards the center will decrease from 1g to 0g, but it will not because of the gravitational effect of the Earth surrounding you, it will be because of the gravitational effect of the Earth still remaining below you. A Dyson sphere is hollow, while the Earth is solid. The solid stuff below keeps pulling until you get below/within it.
In Larry Niven’s article “Bigger Than Worlds” (which has been reprinted in several of his short-story collections), he discusses ringworlds, sphereworlds, and even bigger stuff.
To get an earthlike environment on the inner surface of a big sphere, he suggested spinning it. The atmosphere would collect on the equator. The poles would still be in vacuum, but you would still have a usable surface area several million times that of Earth.
If you have enough energy to accelerate something with the mass of a Dyson sphere to the velocity necessary to simulate 1g, do you really need the Dyson sphere?
That’s an economics question. We’d have to do a return on investment calculation. Something like the total energy required to build the sphere divided by the power output of the star would give us the time required to recover the energy invested.
It might not be a bad investment.
All right, this is still hurting my head.
I understand being gravity neutral in the center. Is it that the closer you get to one sphere wall, the the greater gravitic attraction caused by proximity would be balanced by greater gravitic attraction caused by more stuff on the other side of you? Am I making sense?
That is exactly it.
Brian
That’s exactly it (without getting into the hairy math).
At the center of the sphere, there’s no net gravitation force because there’s an equal amount of stuff an equal distance away in every direction. Say you start there and move “left” in the sphere (using whatever reference frame you want, since it’s radially symmetrical). You’re now closer to the left side of the sphere and further away from the right side of the sphere. The gravitational force of the left side of the sphere is now stronger. However, there’s less stuff in that direction since more of the sphere is to your right. The reduced gravity from the sphere material on your right (due to the increased distance) is exactly balanced by by having more stuff in that direction. The gravitational force from any particular area of the sphere increases / decreases by the square of your distance from the sphere, but the amount of stuff in any given direction is also proportional to the square of your distance from the center. Those two proportions cancel out.
So anywhere inside the sphere, the gravitational force from the sphere in any one direction is balanced out by the rest of the sphere in the opposite direction. So there is no net gravitational attraction from the sphere itself. If there’s any other mass in the sphere (say, a star at the center), then that would impart it’s own gravitational attraction, but the sphere itself wouldn’t.
I saw it pointed out once that the Ringworld, by itself, has far more habitable area and population than Asimov’s entire Galactic Empire. Which just makes it absurd, to me, that Niven attached a hyperdrive to it in one of the later books: What’s the point of having a vehicle which is, itself, 99.9% of all possible destinations?
In the SF webcomic Schlock Mercenary, they dealt with a race that had built fairly plausible Dyson-ish structures out of a thin, Mylar-like material. The material acted like an all-over solar sail, being ‘inflated’ by radiation pressure. They had millions of habitats attached all over the bubble with untold living space, but they were small compared to the whole structure. I suppose each station had to generate its own artificial gravity, if it wanted to have something more than the pull of the star.
Then the attacking fleet ripped the fabric, and everything fell inward…
Not sure I follow what you are getting at. Asimov’s Galactic Empire is almost entirely empty space, so a very very tiny percentage of it is habitable. Ringworld is, presumably, almost all habitable. So, Niven just wants that habitable Ringworld to be able to wander around in all the empty uninhabitable space in the empire. Or, am I misunderstanding you?
Sure, but what’s the point of wandering around in empty space? One patch of empty space is much like any other. The only real distinguishing feature between points of empty space is how much habitable surface is close to them, and the Ringworld is already close to most of the habitable surface in the galaxy.
IIRC, the Night People protector put in a hyperdrive because the kzinti & humans (& others?) were fighting a war in the outskirts of the system for possession of the Ringworld. One ship (or asteroid) had already crashed into the ring and almost destroyed it. So he was going to move it outside of Known Space to protect it.
Charles Stross illustrated this concept well in the book Accelerando, in which he posited not a layer of habitat satelites surrounding the sun, but layers upon layers of computing elements, each one feeding off the waste heat/light of the previous layer, in what he called a Matrioshka Brain (he did not, as I see, come up with the idea originally). The outermost shells are effectively indistinguishable from normal cosmic background temperature.
One sort-of intuitive way that I’ve found to think about the gravity inside a sphere is to imagine jumping into the hole down to the center of the earth. Once you’re ten feet in, gravity is indeed a bit less. It’s less by the 10-foot thick shell of earth that’s above you (and all around the surface). You’re now only being pulled by the mass below you. If you remove that mass, then there’s no net gravitational pull due to the shell above you.