Could someone explain to me in a Dysan Sphere what's to stop anything on the surface getting pulled towards the star ?
Centrifugal force around the equator because it rotates. But that is not the real problem. What keeps the sphere from shifting out of position and colliding with the star. Any orbital structure that completely surround another astronomical object is always in an extremely unstable orbit and would need precise correction thrusts all the time.
Surely that would only work on the equator and anything north or south would be dragged torwards the equator. The whole premise just kind of bothers me.
The first thing to realise is that a Dyson Sphere is *HUGE*. The most common suggestion is for one to have a circumference of approximately 1 Astral Unit - that's the same distance as from the Sun to the Earth - so the gravitational pull of the star at the centre of the sphere would not be enough to drag things off of the inside surface of the sphere.
As Mephane has said, part of the gravitational effect keeping things held down on the sphere's internal surface would be the centrifugal force of the sphere's spin. So with the right amount of rotational velocity the equator would have a marginally higher than 'normal' gravity (in our case, slightly higher than 1G), which would then drop off as you moved further away from the equator. This would then allow for low-G and possibly even zero-G environments to exist in the northerly and southerly latitudes, useful for various forms of scientific exploration.
In addition, depending on what the shell of the sphere is made of and how it's constructed, there could be areas of hypermass that use massively dense materials to increase local gravitational effects. If a civilisation has reached a level of technological know-how capable of constructing something as mind-blowlingly complex as a Dyson Sphere then it's not hard to imagine that they would also have access to materials we can only speculate about (unobtanium?). One hypothesis using the Kardshev Scale as a base line suggests that a civ would need to be at Type-II on the scale in order to be capable of building one of these things, and at that same level they should (according to the scale) have reached a point where they're able to manipulate the material of their local star, so making hypermass and hyperdensity materials shouldn't be too much of a stretch for them.
This last point also negates the argument that there wouldn't be enough raw material in the local solar system to build a Dyson Sphere; any civ that has reached a level where they can figuratively pick up and play with the stuff stars are made of would have absolutely zero difficulty in harvesting materials from nearby star systems, whether they do it the slow way (sub-light drones going out and collecting materials to fuel the project) or the fast way (some form of FTL travel that allows them to gather materials from a larger volume). And that's before we consider the theoretical possibility of energy-to-matter transformation.
With respect to why we haven't seen any evidence of such Type-II civilisations so far, there could be any number of reasons for this:
- If the civilisation has only recently reached the Type-II boundary then we won't see them because light travels at a finite speed - if they're 1000LY away and only started playing with star stuff 500 years ago then it's going to be another 500 years before we even come close to being able to see that in action.
- If the civilisation has already completed a Dyson Sphere then it immediately becomes harder to see because it will be blocking out any radiated energy from the local star; for all intent and purpose it would probably have the same 'appearance' to us as a low-emission star such as a brown dwarf, and we have enough trouble 'seeing' those already. The further away it is, the harder it becomes to see.
- Even if we did 'see' the Type-II civ playing with star stuff, how would we know that's what we're seeing? While we have an excellent knowledge of what's going on out there in space, the extent of what we know is tiny compared to what there is to learn, and it may end up that a lot of what we think we know is actually wrong anyway. Until we get out there and see it for ourselves, we won't know whether we're right or not.
- Given the age of the Milky Way Galaxy, there's a strong possibility that a Type-II civilisation could very well have developed and died out before we even started crawling out of the primordial ooze. As a result, we might not see any evidence of their passing until (again) we actually get out there and look for ourselves. In this hypothesis, those now-extinct Type-IIs may even have already been to Earth, but so long ago that there was nothing here to convince them to hang around.
- Perhaps (and this one's my personal favourite) any active Type-II civilisation really doesn't want to have anything to do with us primitive apes, in which case they would do everything in their power to hide their presence from us. Given that their abilities outweigh ours by a massive amount, it's not hard to imagine that they'd be more than capable of staying off our radar indefinitely. An off-shoot of this argument even suggests that there's one highly advanced civilisation out there (Type-III or above) that is so predatory (you know, just like us humans) that it wipes out any other civ that surpasses a given level of advancement (i.e. Type-I) in order to maintain it's top-dog status - if this last bit is how it actually works then, well, to put it bluntly, in a couple of hundred years we're going to have one hell of a problem.
Another comment I've seen made is that Type-II civilisations would have the ability to physically move planets around at will, and then argued that we haven't seen this so it can't be happening. My response to this is: of course we haven't seen it.
We can only just 'see' all of the planets in our own solar system, and have no way of directly observing extra-solar planets. Yes, we have identified what we believe to be hundreds or thousands of extra-slora planets, but we haven't 'seen' them. What we've seen is a temporary dimming of their parent star, which we believe to be caused by a partial occlusion of said star by a large body moving between us and the star. Through prolonged observation we have confirmed that this occlusion is happening with a measurable periodicity, and *to the best of our knowledge* that suggests that the large body in question is a planet.
In addition, we can, to some degree, measure the 'wobble' of a star and use that data to infer that a large body is having a gravitational effect on the star, and again can infer that *to the best of our knowledge* that gravitational effect is being produced by a planet. There are also other methods of observation that allow us to guess whether the hypothetical planet in question has an atmosphere or not, and if so what that atmosphere may be made of, but we still can't escape the fact that we can't actually 'see' these planets to confirm our hypotheses. Ergo, of course we're not going to be able to 'see' the hypothetical Type-II civilisations playing billiards with planets (assuming that's what they're doing).
Whatever the truth, whether there are any of these hypothetical Type-II and above civilisations out there or not, it's still a lot of fun to throw around all this speculation and guess-work, and I for one would much rather believe we're not alone in the galaxy than face the slightly depressing possibility that we're all there is.
