# Articial gravity?

1. Jun 6, 2006

### scott1

Is it possible to create artificial gravity without using coriolis forces?

2. Jun 6, 2006

### Danger

Well, Coriolis forces are a result, not a cause. I assume that you mean centrifugal force in place of gravity, imparted by spinning a spacecraft. As far as I know, the only way that you could 'create' gravity would still involve 'real' gravity as opposed to artificial. That would mean either harnessing gravitons, if they even exist, or using condensed matter such as neutronium or micro-black holes to impart a field.

3. Jun 6, 2006

### Mk

Right now its still science fiction to create gravity without having mass.

4. Jun 7, 2006

### vanesch

Staff Emeritus
I suppose you mean inertial forces (of which the coriolis force is a very special one - mostly one uses the centrifugal force).

According to GR, gravity forces ARE inertial forces. This is the equivalence principle. On the surface of the earth, we feel an m.g force, because the reference fixed to the surface of the earth is accelerating upward (according to the metric of spacetime here) with an acceleration of 1g wrt a local inertial frame.

So, within the framework of GR, there's no way to create artificial or real gravity forces which are not inertial forces, because both are one and the same.

5. Jun 7, 2006

### rcgldr

What about accelerating a very huge number of electrons or protons to near light speed? (Ignoring the tremendous amount of energy this would take).

6. Jun 7, 2006

### vanesch

Staff Emeritus
This is supposed to do what ?

7. Jun 7, 2006

### rcgldr

Wouldn't the mass of highly accelerated particles increase and therefore increase the strength of a gravitaional field?

8. Jun 7, 2006

### LURCH

The mass would indeed increase, making this regular gravity, not artifical. To get a 1G gravitational field from such particles, one would have to accelerate tham 'till they were almost as massive as the Earth.

Also, their has been cnsiderable debate as to wether relativistic mass (the kind of mass gained by accelerating
) even has a gravitational effect. Alot of that debate has been right here in these Forums.

9. Jun 7, 2006

### pervect

Staff Emeritus
I've seen proposals by Robert Forward to create something resembling artifical gravity

The technical reference is:

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000031000003000166000001&idtype=cvips&gifs=yes [Broken]

but I don't have access to it. I do have the popularized version published in "Indistinguishable from Magic", there is also some information available online in

http://www.aleph.se/Trans/Tech/Space-Time/art_grav.txt

The basic idea relies on gravitomagnetic effects. Unfortunately, there are several problems with the idea.

First of all, I don't think the idea will work as he describes it (more later).

Second of all, if it does work the way Forward thinks it will, it requires white-dwarf star density material accelerated to near-light speeds in a few milliseconds (Forwards own description, from "Indistinguishable from Magic".

Third of all, the field is not constant, it's an "AC" effect. So if it worked the way Forward thought, as you pumped the white-dwarf star fluid throught the coils, you would "shake" anything in the center up and down.

The problem that I have with Forward's idea is that the the magnetic field of a torroid is zero, except in the region within the windings.

Thus there will be no electric or magnetic field at the center of a torroid from what I can see. So I think that Forward has probably made a mistake in borrowing electrical engineering, and that his gravitational version of a torriod with hyperdense fluids in tubes replacing currents will have no fields at its center, either.

Last edited by a moderator: May 2, 2017
10. Jun 7, 2006

### rcgldr

It would be less if the center of mass of the accelerated particles very much closer to the observer. Anyway, this seems more like sci-fi than fact.

11. Jun 7, 2006

### Quaoar

Not entirely zero, depending on how the currents are set up. If you have one continuous spiral around the toroid (as illustrated in your link), you'll have SOME field in the middle of the torroid due to the "drift" current.

Last edited by a moderator: Apr 22, 2017
12. Jun 7, 2006

### pervect

Staff Emeritus
The kinetic energy of a body does contribute to it's "gravitational mass". As others have mentioned, this has been discussed here before. See for instance

http://arxiv.org/abs/gr-qc/9909014

There are some tricky points to this issue that I will gloss over, but I will say that it is definitely a mistake to think of the field of a moving body as being uniform in all directions. It is closer to the truth to think of the "field" of a moving body as being "squished", strong in the direction transverse to the motion, and weak in the direction parallel to the motion.

I'll point out that this is only an approximation. The reason that it is an approximation is that the concept of a "field" isn't really quite right, gravity is not really a force in GR but a curvature in space-time. In a static system, the force explanation can be made equivalent to the curvature explanation, but in a non-static system the curvature explanation turns out to be more general than the "force" explanation.

The end result, however, is that hot gas (consisting of moving atoms) does "weigh more" than a cold gas (consisting of less-rapidly moving atoms). This end result is not an approximation because a hot gas (in a container) is a static system.

While theoretically correct, this result is not of much practical utility to those wanting to create artifical gravity, though. One still needs something very heavy to cause significant gravity, and it is more convenient to use normal dense solid objects than to attempt to contain a relativistic gas.

For static systems without significant time dilation, one can even use Gauss's law to say that a closed surface of area A surrounding a mass M will have the surface intergal of normal-force*Area of surface A equal to 4*Pi*G*M, where M is the enclosed mass. Given a desired gravity g, and a desired area A, Gauss's law says that one needs a mass of at least g*A/4*Pi*G to generate it.

This equation breaks down if one has significant time dilation, or if the system is not static, however. The existence of "negative mass" would be another way around this constraint.

Last edited: Jun 7, 2006
13. Jun 7, 2006

### pervect

Staff Emeritus
If I'm understanding this right, that's only because of the finite resistance of the wire. I don't this this sort of parasitic effect is going to help Forward's propsal any. There isn't really any "wire" or "resistance", the electrostatic equivalent of Forwards proposal is a bunch of charge forced to flow around the torus. There will certainly be forces due to the net (high) charge on the torus, these are just the gravitational field of the torus. There won't be any gravitomagnetic fields in the center of the torus, Forward's proposal demands a strong, time-varying gravitomagnetic field.

Last edited: Jun 7, 2006
14. Jun 7, 2006

### Quaoar

Well, I probably shouldn't have used the word "drift" considering it has another meaning in the same context.

I'm just saying that since the coils in the toroid are not parallel to the cross-section of the toroid (since we wound the wire around the loop in a spiral), we'll get some field in the middle.

To illustrate: Set our cylindrical coordinates at the center of the toroid, with z pointing up. In a perfect toroid, all the turns have current running in only two directions: z and s. But since we have one long wire precessing along the entire toroid length, the current will also have a phi component. This generates a field in the middle of the torus.

Basically, all I'm arguing is that the field is zero in the middle only if the torus is perfectly made.