This raises the (somewhat silly, I must admit) question: what if gravity had actually reversed itself outside my friends' office? Three scenarios are possible:
- gravity instantaneously reverses itself;
- the downward-pointing vector of gravitational acceleration decreases in magnitude, goes through zero, and then becomes an upward-pointing vector;
- the gravitational acceleration vector always has the same magnitude, but swings around through different angles to point up instead of down.
I claim that the third of these is not reasonable. Basically, this is for reasons of symmetry. Imagine gravity that neither points straight down nor straight up. How would gravity know in which of the many possible "sideways" directions to point? There are only two points that could possibly have any effect on the direction of gravitational acceleration, namely the point that you are at and the center of the Earth. The direction of gravity must be invariant under any isometry which fixes those two points -- thus it must point straight towards the center of the Earth or straight away from it.
Apparently I am more attached to gravity being a central force than to its particular strength or even to the fact that it is attractive, not repulsive.
(I'm not sure whether the first or second of the models suggested above is reasonable. This post is silly enough as it is.)