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If point masses could be generated in a controlled manner distorting the fabric of space-time objects could be made to move in any direction relative to each other and not simply attract as is normally experienced. Imagine a rubber sheet stretched tight with a heavy mass [m1] at its center creating a gravitational well. Now add a point mass some distance away [m2] that distorts the rubber sheet creating a second well. m2 and objects close to it would move toward it and away from the m1. Could this be the basis for an anti-gravity effect? Envision water flowing across the surface of a rubber sheet distorted by a mass at its center; now place another mass [a marble] on the sheet some distance from the center causing another distortion, the flow of water close to the second mass changes, and flows away from the center and toward the second mass. If this distortion is made in space the effect would be anti-gravity would it not?

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  • $\begingroup$ It doesn't matter where you place mass, gravity is always attractive. Can you place the planet above your head and be attracted in the opposite direction than your antipode? Of course, just fly to Australia, that's exactly what it feels like. That's why they call it down-under. $\endgroup$ – CuriousOne Apr 15 '16 at 22:55
  • $\begingroup$ Related: physics.stackexchange.com/a/13839/2451 $\endgroup$ – Qmechanic Apr 15 '16 at 23:56
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An example of the sort of system you describe would be the Earth and the Moon, with the Earth playing the part of your mass $m_1$ and the Moon $m_2$. Neither of these are point masses, but courtesy of Gauss' law we know that the gravitational field of a sphere is the same as the gravitational field of a point mass provided you are farther away than the radius of the sphere.

When the Moon is overhead it does indeed slightly reduce the force we feel from the Earth's gravity, but this is simply because the Earth pulls us one way and the Moon pulls us in the opposite direction. No physicist would describe this as anti-gravity.

Given that you mention water it's worth mentioning that the obvious effect of the Moon's gravity is the tides. If you progressively made the Moon more massive and brought it closer the tides would get bigger, and there would come a point where the Moon would pull water off the Earth (though by time it would be pulling the Earth to bits as well!).

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  • $\begingroup$ Thanks for your comments. Your example of the tides is a better one. Here is another thought experiment. What are the implications of having a hypothetical device[s] that can create mass (convert energy to mass) and vice versa, rapidly and in sufficient quantity to allow the direction and intensity of forces attracting its mass to others (e.g. Earth, sun, moon, etc. ) to be quickly changed thus allowing it to surf the gravitational wells. $\endgroup$ – Jim Apr 16 '16 at 15:41
  • $\begingroup$ @Jim: please post a new question if you want to ask anything further. However note that GR makes no distinction between matter and energy - they are treated as equivalent using the famous equation $E=mc^2$. Also I think you'd need to make your question a lot clearer than your comment i.e. what does surf the gravitational wells mean? $\endgroup$ – John Rennie Apr 16 '16 at 15:44

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