h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think that it's an engineered sun, so it doesn't overheat the world.

The sphere itself is made from a super dense material, so it creates enough gravity to hold everything down.

It was created by a very ancient race of godlike beings, who then abandoned it for some reason. Currently, I'm writing a story about the colonization of it. Basically, it was discovered by a group of people put in stasis and sent out on a colony ship.

And I'll put a list of who's doing what on the main page later.

You should also read "Captive Universe" by Harry Harrison. That would give you some insight on how to create a colony ship where the inhabitants think they are on a normal planet.

Hi Future World Dictator, must of the problems you mention have already been sorted in Larry Nivens Ringworld. Also, you might be surprised but there are plenty of scientists looking into Dyson spheres as well, I've read the odd thing in New Scientist on them, you can find loads more if you google.

I've been meaning to read the Ringworld novels for some time now, but haven't as yet. Too much to read for English class, as well as too many good books in my room where I don't have to look for them.

I'll have to look around for the Harry Harrison book. He's the one who wrote the Stainless Steel Rat series, is he not? That's also on my list, but I haven't read it yet for the same reasons I haven't read Ringworld.

And I have been meaning to look for more information on Dyson Spheres, but I haven't been on the net at home for quite some time.

Perhaps I should join the procrastinators society...nah, I'll do it later

I refer you to the first google result under the search "dyson sphere", in particular the bit about halfway down where it says:

"A nonrotating dyson shell would have just two sources of gravity: the shell itself and the star. As mentioned above, on the inside only the gravity of the star would be felt and everything would fall down into it, while on the outside there would be weak gravity (for a 1 AU sphere centred around the sun, the gravity would be 6e-3 m/s^2).

The only ways to make a rigid Dyson shell habitable on the inside would be either to provide it with some sort of antigravity (which is unlikely) or to rotate it, which would make only the equatorial band habitable unless the interior was terraced. A rotating dyson sphere would be under immense strains; see the section about the ringworld for a simple calculation. Niven pointed out that if you want to spin a Dyson sphere, it is better to build it like a film canister for reasons of structural strength, and then you have a Ringworld."

I am doing a maths degree, so I can tell you that this is correct.

The situation with the Earth is completely different, because we are on the outside of the Earth, and so feel its gravitational pull.

Spinning the sphere about two axes will not help because there will still be points that are not moving.

The Rinbgworld exerts no gravity on its inhabitants; they are held on by centrifugal force, which as explained, does not work perfectly for a sphere.

This is all beside the point really, but it is better to get it straight.

How would the gravity be if the sphere was made from super dense material? (similar to that of a neutron star, although not that dense)

If the material has the mass to give it a surface gravity of 9.8 m/s^2 or thereabouts, wouldn't that solve the acceleration/gravity problem? The concept was explained in a Charles Sheffield story I read once, although that was a while ago.

You didn't read the page I reffed, did you?

Suppose you have some mass that is distributed in space in a spherically symmetric way (so if you stand at the centre, it looks the same in every direction. For example a star with a Dyson sphere around it: if you stood at the centre of the star you would see the same thing in every direction). There is a law of physics (it's called Gauss's Law) that says that in this situation the force of gravity always points inwards and that you can find its size at any point by pretending that all the mass *closer to the centre than the point* was actually at the centre, and then using Newton's Law of Gravitation.

So if you are outside the sphere gravity points inwards and has the size that it would if all the mass of the star and the sphere were squashed together at the centre of the star. It is therefore true that if you make the sphere heavy enough, you can make the gravity as strong as you like.

*But* if you are inside only the star counts. It is as if the sphere was not there. This is weird but true. It was verified in an experiment by Michael Faraday (he used electric force becuase it's much stronger than gravity, but the principle is exactly the same).

Hi Future World Dictator (13)

You pose some very interesting points which I would like to discuss if I may. Worth pointing out that I do not have anything like a degree in anything so common sense and a bit of magic will have to apply.

First point, I understood that there is, by strict dictionary definition, no such thing as centrafugal force. The effect of gravity generated by the spin of an object is centrapetal force (I think the spelling is correct). This is simply Newtons Laws of motion expressed on the inside of a rotating strip. Picky but true.

Second point is that not only is Electromagnetic force stronger than that of gravity, but I understand that they are two completely different types of force and could therefore behave in different ways, especially when scaled up to a D Sphere scale.

Third point is that the technology used to create such a device is well in advance of our own and therefore would use techniques well beyond our current understanding. The rules that you quote are rules based in normal space / time. The D Sphere world here is far from normal and could distort space / time to a point where such rules no longer apply.

It would make perfect sense in this scenario to take the space / time continuum contained within the inner surface and distort it in such a way so that it behaved as if it were a planet surface rather than the inside of a sphere. The distortion would be stable because of the spherical shape and the continuous harmonic it creates.

Another very important point is that the inhabitants believe that the gravity keeps them stuck to the inside of the sphere and faith is an incredibly strong force (ref: Discworld)

How does that grab you?

Wow...I'm learning a lot of physics we haven't covered in class yet. This is really interesting. I'm still not sure how to work out the gravity problem, but I'm learning something.

I am perfectly aware that in an imaginary world, you can do what you like. For example, you could have a universe that consisted only of the interior of such a sphere, with nothing outside at all. You can even fiddle it (and the maths works) so that this universe is still infinite in size!

As for centrifugal force, it is common practice to use the term while knowing perfectly well that it has no physical existence. It is merely an intuitive way of thinking about rotation.

Electric force is identical to gravity. For example, a van der Graaf generator (a charged sphere) has an electric field that is identical to the gravitational field of Earth, just scaled down. The laws that govern electrical forces (when you ignore relativity and quantumness) are identical to those that govern gravity. In Faraday's experiment, he made a hollow metal sphere and sat inside it. He then charged it up to a squillion volts so that the electric field outside it was enormous. However there was no field at all inside so he was perfectly safe. (This is why you are safe in your car in a lightning storm - the metal chassis ensures that there can be no electric field, and so no lightning, inside the car. It has nothing to do with the tyres.)

Hope this helps.

Hi FWD(13),

You'll have to excuse my ignorance but I would like to discuss the following points. This is the simplistic view that I have which supports my points.

Electrical force is to do with electrons flowing along conductive routes according to potential difference. If there were no routes available then the electrons hang around and would be referred to as static.

I do not believe that gravity flowed down conductors and did not think that it could be stopped by some sort of insulative device. Thus how could it have similar properties to electrical force? I have heard that Gravitons (particles of gravity) have been discovered but that also makes little sense to me.

The analogy used to describe the behaviour of Gravity is the flat rubber sheet. As you put something on to the sheet, the sheet sinks down where the object is so that when you roll a marble across the sheet, its path will be affected by the dip and behaves as if affected by gravity. I don't see how particles could explain this behaviour.

The analogy used to describe the behaviour of electricity is water through a pipe, a completely different perspective.

BTW: What significance are Quantuum and relativity theory? and I didn't think you could conveniently hide laws away when they didn't fit the expected picture.

I look forward to your reply.

Well, here it is (sorry if it's in no obvious order)...

I did not say that electricity and gravitation are the same, but that electrical force and gravitational force behave the same. Electricity is the flow of electrons, not electrical force.

Imagine a particle (say an electron) with a charge q. Then the electric field at a distance d from it is:

F = (C * q) / d^2

where C is a constant (it's got pi in it and some other stuff) and ^2 means squared. This is an experimentally verified fact.

Now imagine a particle (say an electron again) with a mass m. Then the gravitational field at a distance d is:

F = (G * m) / d^2

where G is another constant. This is also an experimentally verified fact.

Both fields point inwards. You see that those equations are the same? What is confusing is that things with a noticeable amount of gravity are always enormous, so they aren't shaped like electrical circuits. Electrical circuits are complicated shapes so they produce complicated effects (plus there is the problem of permittivity making it even worse).

Therefore we can pretend that a van der Graaf generator and the Earth are the same thing. (Of course, they aren't.)

Relativity and quantum mechanics are of no significance. That's the point. Those equations above do not work in all situations. If things are moving very fast, you need relativity, and if you want to work on the atomic scale, you need quantum mechanics. However, Dyson spheres and so on are very large and very slow moving, so we can ignore such effects.

This is something that happens all over the place in science. Newton's law of gravity (the 2nd one above) is *almost* right - when people send probes to Mars they use it, and it's easily good enough for Dyson spheres - but about 100 years ago someone noticed that it didn't get the orbit of Mercury quite right. Then Albert Einstein came along with General Relativity and solved that problem. GR is very different from Newton at first sight, but if you assume that nothing extreme is going on, all the weird stuff magically disappears and you're left with Newton again. The situation is very similar with quantum mechanics.

Gravitons are strange things called virtual particles that are supposed to "carry" gravitational force, which as far as I can see means they don't actually exist, but are a useful tool for doing stuff (a bit like centrifugal force!). Insulators are materials whose electrons are tightly bound to their atoms, so it is hard to pull them free so that they can flow. They therefore block electricity, not electrical force.

The part about gravity working differently on the inside of the sphere is very confusing. Would it still work if the sphere is more massive than the sun? I understand the part about electricity, but I don't get the part about electricity and gravity behaving the same way on the inside of a sphere.

For gravity inside a sphere, if you're confident with calculus head over to the page I mentioned in post 25, where there's a proof that the force is zero. If you're not, you'll just have to believe me.

Again, electricity and gravity are completely different. Electrical force and gravitational force are analogous, so what is true for one is true for the other.

Having extracted my brain from the plug hole. I do find that I can follow what you are saying. I don't necessarily agree with it but don't have enough info to query it any further.

As I understand it, it's not that the gravity focus of a hollow sphere is at the center it's that there is no gravitaional force on the inside of the hollowed out sphere. The sun being the only gravitational force would therefore attract everything and gently cook it.

Plan B!

How about we cancel out the gravity of the sun. We would then have zero G. To do this I have a neat trick and it goes something like this.

A mass is a collection of energy. The energy makes up protons, neutrons, electrons or plasma. If you look at energy, such as light for example, it is a wave form. If you pass an identical wave form that is completely out of phase then although the light is still there, you will cancel out its effect. It will dissappear. So if you transmit a waveform which is completely out of phase with the energy of atom components then the object will not dissappear but you will cancel out it's effect in Space / Time. Thus with no mass, no gravity.

All you need now are fairly low powered gravity generators and we are cooking on Gas.

I headed over to the dyson sphere page yesterday. It's very interesting. Although it doesn't solve our problems.