The flat faced wormhole that they describe in the paper would require negative energy equivalent to the size of Jupiter. They do point out that the mass would be contained in the wormhole rather than either reference frame so there would be no local mass to contend with. The paper also talks about creating negative energy by the casmir effect and also by using LiNb (as I recall) frequency doubling crystals that emit alternating photons of positive and negative energy.<br /><br />So I guess the answer is that if you pay someone $29K they will write you a paper and tell you that it is possible. The paper is actually good science fiction until it enters the realm of the spoon-bender...