general relativity - Would a closed field of gravity neccesarily lead to paradoxes?

I've asked wether artificial gravity, as seen in some SF-Movies, would violate known laws of physics. To recap, my idea of an Artificial Gravity (AG) system was like this:

• 
  

  A Device that creates an 'artificial gravity' (AG) inside a Box, with these properties:

  
  


• 
  

  the system does not use inertial forces (like centrifugal force) or a huge mass to create AG

  
  


• the AG 'feels' just like gravity/inertia over most of the volume of the box


• the Device, the Box and its content are still subject to any gravitational forces that may be present, but the AG force adds to other gravity/inertia forces, net force may be zero


• Matter, Energy and Information can leave and enter the box there is a smooth but strong gradient in the AG at the walls of the box -


• conservation of energy still holds: if the AG does work, this energy has to be supplied to the device in some way, vice versa for work done against the AG



• conservation of momentum still holds: for every force due to the AG on an object, there is a corresponding counteracting force on the device Basiclly, I tried to rule out all the obvious ways such an AG would violate laws of physics (or could be used to costruct another device that does so), and see if one can find a less obvious way und thus rule out the possibility of AG.

If you want to talk about the original question, please head over (and please do!). I may have thought of a way a device like this could create an impossible situation, but I don't understand enough relativity to know wether I'm right.

Suppose you take a number of boxes as describet, and cut open two sides. Then you arrange the boxes in a rough circle or polygon. The AG in each box is adjusted so that the net 'gravity' (AG + gravity at the place the box is) result in a sort of circular, homogenous field of gravity. The devices are anchored somwhere to account for any momentum. An observer placed inside the sytem would not 'fall' freely around the circle, but would bump into the wall after the first bend since there would be no centripetal force present. However, we can build a circular rail through the boxes and attch our oberver to that, then he could travel round and round while constantly getting faster. Whee!

Now, I think there would be no meaningful way to define gravitational potential in such a situation, so we could not tell how fast the clock of the observer on our merry go round is going. So the device would create a prardoxical situation is thus impossible.

On the other hand, either our observer is in free fall or we know the accelration she is subject to (either from the rail, or from bumping into the walls of the boxes). So her watch will behave like any other wach subject to the same acceleration.

I tend to believe the second interpretation is correct, our AG would not create an impossible situation, but since I don't realy undersant gravity, relativity and all the rest I thought I ask.

p.s. I've thought about posting this thought experiment as an answer to my first question, but since I'm totally unsure I want to exp.ose my little thought experiment to more scrutiny.

p.p.s Why is there no 'thought experiment' tag?