Those are really for coherent gravitational waves (the same across locations across the world) not for interactions between individual small objects. Since they

Those are really for coherent gravitational waves (the same across locations across the world) not for interactions between individual small objects. Since they're looking for any defect in the inverse square law (e.g. due to extra dimensions of string theory) these would screw it up. Light actually exerts a force (even like tweezers) so it's not a good choice for this kind of measurement. They must have done an amazing job of removing any electrostatic or magnetic effects since those so quickly dominate at these dimensions. The gold ball isn't so bad to discharge, but typically glass fibers are used, which can build up charges. My expectation is that they used radically different frequencies on the two ball supports and very high oscillator Q (they're pendulums in a vacuum) to separate them mechanically since they have to be in the same chamber. People have tried placing "screening materials" between the masses to reduce electro-magnetic coupling, but those tended to cause their own non-gravitational effects.