Do these things actually exist? Basically a thing that pushes back on another force in order to help cancel out the pressure pushing on it by applying an equal yet opposite force to it. Basically a device based upon Newton's third law of motion. If it does, what is the physical limit on the amount of force it can apply? Can it be increased? If it is in the vacuum of space, will it still work? What about handling a steady output so that the time between the output intervals approach zero, in order to achieve the impression of working constantly? In advanced calculus terms, when the derirative of the force approachs zero. Can it handle mimicking more complex forces like encountering a handful of bumps among the road, or the constant pull of a black hole? I have a lot of questions. Thanks for reading.