Empty vacuum - pure "emptiness", so to speak - is filled with rivers and oceans of seething energy, just as Nikola Tesla pointed out. It is running off the fact that vacuum space-time itself is nothing but pure massless charge. That is, vacuum has a very high electrostatic scalar potential - it is greatly stressed. To usefully tap the enormous locked-in energy of that stress, all one has to do is crack it sharply and tap the vacuum oscillations that result. The best way to do that is to hit something resonant that is imbedded in the vacuum, then tap the resonant stress of the ringing of the vacuum itself. In other words, we can ring something at its resonant frequency and, if that something is imbedded in the vacuum, we can tap off the resonance in vacuum stress, without tapping energy directly from the embedded system we rang into oscillation, So what we really need is something that is deeply imbedded in the vacuum, that is, something that can translate "vacuum" movement to "mass" movement.