So I know you can make a "negative density" region between two plates due to the Casimir effect. I know there's probably not, but is there any theoretically possible way to create a region of such negative density that the total density of the plates and region would be less than vacuum? I had a dream about little personal vehicles with fist-sized negative density regions that were so much less dense than anything that they floated buoyantly through the sky like little hover-scooters. Like a hot-air balloon, but smaller. :-) If this is not possible, what kind of magic "negative energy" or "negative mass" or whatever would be needed to make it happen?