A technologist friend, on learning about the Casimir force and energy, immediately designed a reactionless drive based on the Casimir cell ( see pic). (This is useful - http://en.wikipedia.org/wiki/Casimir_effect) The cancellation of vacuum energy in the cell, by his reasoning would be like negative energy and thus have a mass -E/c^2. In other words, the assembled parts would have less mass than the unassembled parts. Also by varying the separation, he reasoned he could change the mass, and thus by spinning two cells ( or more) and varying their masses appropriately, a reactionless force would appear. Obviously this is false, but why ? If the cancelled energy does not have equivalent mass there's no problem. But if it does we're in trouble. Eventually I came up with this counter-argument. The more energy is cancelled, the greater the pressure on the separators. This means the potential energy in the separators has increased. If it increases by the same magnitude as the cancelled energy we are home and dry. There's no nett change in mass. So we can conclude a) if the cancelled energy in the cell decreases the mass, then the energy in the separators must contribute the same amount back b) neither the cancelled energy nor the potential energy add or reduce the mass. Any thoughts ?