I have an idea for a DS-like world setting mostly set after the year 4200. The Earth was destroyed around 2500 and humanity moved from earth to an asteroid best and built a giant dyson sphere to live on that has 6 holes in it- North, south and equator. Its radius is 1 AU (average of Earth's orbit) and it's 12500 km thick (roughly Earth's diameter). I have calculated its surface area, volume of its 4 oceans, and a rough population estimate. I'm not sure about the physics aspects of something like this, I want the gravity to make sense and be similar to Earth's on the inside and outside but everything I've found says it wouldn't have internal gravity and ridiculous gravity on the outside. Could it have gravity if it was big/dense enough?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Jul 23 '18 at 5:03
  • $\begingroup$ @JBH I think we have covered all the salient issues. Going to chat would add very little. Some chats peter out very quickly once they go there. Although some do go on and on. $\endgroup$ – a4android Jul 23 '18 at 5:03
  • $\begingroup$ ZoneWolf, RE your edit, most of the discussions surrounding this question are the result of dozens of questions similar to this one. (a) The gravity on the inner surface of a hollow sphere is zero. It doesn't matter how much mass or density we're talking about, it's always zero. (b) The gravity on the outer surface in the condition you're describing would be enormous unless the material used was ridiculously low density. $\endgroup$ – JBH Jul 23 '18 at 5:04
  • $\begingroup$ Do you want your population to live on the inside or outside of the sphere? Because those are two separate problems. $\endgroup$ – ltmauve Jul 23 '18 at 5:36
  • $\begingroup$ a Dyson Sphere is not a massive sphere, but more like a swarm of satellites. a closed sphere is unstable. $\endgroup$ – ths Jul 23 '18 at 8:41

So, I did some calculations and this is what I came up with:

If we assume both structures to be perfectly spherical ( earth included) and both bodies to have the same gravitational attraction ( as you suggested), then with the Earth's mass of 5.972*10^24 kg, for your structure to have the same mass (and therefore, same gravitational attraction), it would have to have an average density of 1.698*10^-6 kg/m^3 in order to have an earth-like mass, with its volume of 3.5151*10^30 m^3

This implies either one of two things:

Humanity has either discovered some material with a super low density or they have some sort of gravity altering device to simulate an earthlike gravity. In today's tech, this is not possible. Keep in mind, the gravity will also vary significantly at different points since you have mentioned that your structure is 12500 km thick.

Since this will most likely require handwavium to make it work, I propose Cavorite from H.G. Wells' "First Men in the Moon". While this material violates physics as we understand it currently, this might help mitigate the gravitational effects of such a massive object especially if it's made of something that doesn't possess a super low density.

Something a little more logical and realistic perhaps is to have your people live high above the surface of the sphere where the gravity will be weaker.

A third option, while still difficult to pull off is to use levitation currents. Superconductors have been known to levitate objects so this may aid in maintaining an earthlike gravity.

Or finally, you could use Larry Niven's gravity generators if you choose to make your sphere out of some super light weight material.

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    $\begingroup$ The third alternative is, by 2500 and later, Earth has imported absolutely colossal amounts of mass into the solar system to build their supermassive dyson sphere. I can guess what you're thinking. It worries me too. $\endgroup$ – a4android Jul 23 '18 at 4:29
  • $\begingroup$ @JBH OP has specifically stated that their Dyson Sphere has a radius of 1 AU and a thickness of 12500 km. For a structure of that volume to have a gravitational attraction similar to that of Earth, it is going to be a lot less dense than styrofoam. It might be thick, but it won't be very dense at all. $\endgroup$ – ironduke97 Jul 23 '18 at 4:35
  • $\begingroup$ So, what's the answer to the OP's question? "Possibility of a ridiculously colossal structure?" $\endgroup$ – JBH Jul 23 '18 at 4:40
  • $\begingroup$ He wanted the gravity to make sense and I tried to help show how it could make sense. @JBH $\endgroup$ – ironduke97 Jul 23 '18 at 4:41
  • $\begingroup$ @a4android I downvoted and I did so because Ironduke97 didn't answer the question. Pointing something out is a comment. Note that I didn't go so far as to flag the post as "not an answer." $\endgroup$ – JBH Jul 23 '18 at 4:52

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