This idea requires one of the various theories that theorize gravity as a result of the bending of space time to be correct. If the Graviton theory is correct, I suppose you could find some way to block gravitons from travelling through an area, but that gravity shield would react so much differently than the one I am suggesting that I won’t talk about it here. You are welcome to ask me about it in the forums.
Basically, my gravity shield uses manipulation of space-time to isolate an area from the gravitational effects of everything outside of that area. To begin with, I would like to do a quick recap of Einstein’s theory for gravity. He theorized that matter warps space-time around itself, creating a “gravity well” that things would fall into. This is usually explained with the analogy of objects on a pillow, with the pillow being a two dimensional representation of four dimensional space-time, where a marble rolls into the indentation created by a larger object compressing the pillow.
So the space-time around objects is warped, creating a gravity well. Now, what if we could manipulate space-time, and restore it to its “resting” state? I have no idea of how we would do this, but if we could, we could use this ability to “unwarp” space-time abruptly (with respect to distance) in a sphere. Everything on the inside would be isolated from other gravitational forces. Going back to the two dimensional representation, a gravity well is a crater, and the space-time manipulator would be building an island in the middle of the crater. This might end up creating a shield that prevents the gravity from reaching things past it (like the Earth does to the Sun’s light in a lunar eclipse), and it might not.
Now, what are the ramifications of this? It would create a zero-G environment inside of it, to be sure, but what else? Let’s go back to the idea of an island in the middle of a crater. That island is surrounded by cliffs, representing how space-time is warped abruptly around it. If a car is speeding towards the island, and didn’t stop, what would happen? It would crash against the cliff, and never make it to the island. For the gravity shield, this means that if something is moving towards it, it would experience the full force of the exterior gravity all at that one infinitesimal point in its path, and it would be pulled away from the gravity shield by the earth’s gravity. It is kind of like the way a black hole operates.
A black hole can be made of any mass, because it is not the mass that is causing the immense gravity, but how close you can get to the center of gravity. Long story short, it is the density that decides whether something is a black hole or not, because you have to be able to get close enough to its center of gravity without passing any of it (at which point everything you pass will be pulling you towards itself and away from the center of gravity). In our crater example, think of the crater starting out with a very gentle slope, and getting steeper as you get towards the center. If you can get close enough to the center, the slope will be so steep that nothing can escape falling into it. That would be the event horizon.
Back to the island in the crater (the gravity shield), the slope of the cliff would be infinite (mathematically speaking, it would be undefined, but let’s just say it goes straight up), and would behave like a reverse black hole (I have heard something about white holes, and that might be how this would act, but I don’t know anything about white holes, so I won’t say for sure), essentially creating a force field. Now, from the inside, it would behave like a black hole that has no pull until you touch it, and then it shoots you through at a very high speed (and possibly spaghettified). You could compare it to one of those Hot Wheels car launchers that have the spinning wheels, so that when a car touches them from one side, it shoots it out the other side, but a car can’t go through from the other side.
Now, light is affected by gravity (which I guess I touched on in my mention of black holes), so light would be affected by the force field/gravity shield in the same way as other matter. So a gravity shield would be perfectly reflective.
Also, since nothing can penetrate the outside, and it acts like a solid, indestructible wall, and when you push against it you aren’t actually pushing against anything (just gravity), it would be impossible to move from the outside. I would say it is an immovable object, except that it’s not an object, just a warping of space-time.
One last thing. Since gravity affects time, so that the more gravity you experience, the faster time moves, what happens when there is no gravity? I would assume that there would also be no time. Anything inside the gravity shield would still experience time, because they have gravity themselves, but it would be such an infinitesimal amount of time compared to the outside world that, for all intents and purposes, anything inside of a gravity shield is in complete stasis.
So. A gravity shield “unwarps” space-time, creating a force field and time lock.