Tiny rockets/propellant canisters attached on top of the weights and activated so they constantly push the weights toward the surface. No need for weights even; just attach the rockets to the end of an empty bar and the configuration of the rocket determines how forceful it pushes toward the surface to simulate a certain weight being lifted on Earth. EDIT: I was thinking of only a bench where the bar would only go up and down. A rocket pushing downward would be all that is needed. For free weights and doing curls, where the weight rotates, you would want to have the propellant attached to a gyroscope so you can do curls and the 'rocket' would always stay pointing toward the ground as the weight rotates. Otherwise, without a stabilizer, curling would change the angle of force and instead point the rocket exhaust toward you as you curl and push the weight away from you.