Repulsive gravity

IANAP, so feel free to berate me for thinking apocryphal thoughts! Just as magnetism has two charges, in which particles of like-charge repulse and particles of dissimilar charge attract, might gravity have two charges in which particles of like-charge attract and particles of dissimilar charge repulse?

In practice, the state of magnetism means that there is no system composed of many particles in which all particles attract. Rather, there is a net 0 charge if there are equal numbers of each particle type.

My silly theory regarding gravity would mean in practice that there would be two (or more) "clumps" (or universes) in existence, which are racing away from each other. So in our clump (universe) we see only attracting particles, because all the opposing particles have long since separated out and are racing away beyond the boundary of the observable universe. Just like the alien who lands in China and assumes that all humans have slanted eyes, we only observe the attracting particles (or "charge") and disregard the other, unobservable, "charge".

Is there any way to disprove this idea, or like string theory can I go one believing it as it can never be disproved?

Thanks.

Answer

Let's start by assuming you believe General Relativity (on the grounds that it has given the right answers every time it's been tested).

There are various ways to get repulsive gravity in GR. The most obvious way is to have matter with a negative mass. See http://en.wikipedia.org/wiki/Negative_mass for a discussion of this. Such matter has never been observed, so I suppose we can't absolutely rule it out. However it seems unlikely. Your example of two "universes" racing away from each other would mean the universe is uneven on large scales, and the observations so far show the universe is basically the same in all directions. In addition you'd have to come up with a mechanism for the positive and negative matter to have separated in the first place.

There are ways to get repulsive gravity that don't require negative mass. For example there appears to be dark energy (http://en.wikipedia.org/wiki/Dark_energy) generating a gravitational repulsion. Also it's widely believed that at very early times the universe underwent a massive expansion, called Inflation (http://en.wikipedia.org/wiki/Inflation_(cosmology) ) due to repulsive gravity.