Theories that attempt to unify gravity with the Standard Model or to explain the nature of dark energy suggest that gravity may deviate from the Newtonian form 1⁄r2 at micron length scales. We use mesoscopic optomechanical systems to measure sub-attonewton forces at micron length scales to test these theories. Our current work uses optically levitated microspheres to measure gravitational interactions between masses separated by less then 20 microns. Optically levitated microspheres provide a powerful probe for short-range forces because they can be precisely controlled with optics and do not need to be mechanically coupled to the surrounding environment. We are also exploring the use of mesoscopic optomechanical force sensors to search for other new particles and interactions. Recently, we have used optically levitated microspheres to search for millicharged particles (charged particles with much less than an electron's charge) bound in the bulk of the microspheres. Mesoscopic optomechanical systems are an important tool for probing new physics that may be most easily observed at the length scales between subatomic and laboratory length scales.