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I'm wondering if any set of hypothetical conditions could result in us gravitating in a different direction. Specifically a reversal of gravity on the surface of Earth (or at least part of it), so that for people there, the ground would be above their heads, and everything unattached to it would fall into the sky.

At first I thought along the lines of a black hole passing by our solar system, but realized that could probably not suck things off the Earth's surface. Could anything other than a biblical rapture do this without breaking obvious physical laws? I know it's far fetched but thought I'd try anyway.

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You suggest a black hole passing the solar system as a possibility, and indeed this could do it.

Suppose the Earth was falling into a black hole. To a first approximation the Earth and the people on it will be accelerating at the same rate, so people would not be sucked off the surface. After all, the Earth and the people on it are accelerating towards the Sun but we are not pulled off the surface every midday.

But the acceleration of the Earth and the people on it is only the same when tidal forces can be neglected. At midday on the equator your centre of mass is nearer the Sun than the Earth's centre of mass, so the Sun is pulling on you slightly more strongly than it pulls on the Earth. However the difference is tiny compared to Earth's gravity.

But suppose a small black hole, say one the mass of the Earth, were to pass just over your head. In this case the tidal forces would be strong enough to pull you off the surface of the Earth (and to a messy death!).

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So that u say gravity can be reversed? –  Sajin Shereef Jan 30 '14 at 9:53
@SajinShereef: I think reversed is the wrong word to use. Suppose you have a piece of metal stuck to a magnet. If you bring up a much more powerful magnet you can pull the piece of metal free. However this doesn't mean that magnetism has been reversed. –  John Rennie Jan 30 '14 at 9:55
Nice answer with bringing up tidal forces, but would be nice to see an explicit statement that the gravitational force is strictly attractive. –  Kyle Oman Jan 30 '14 at 19:20

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