[NoGravitas]

If you can produce negative mass, you can (in theory) make a faster-than-light warp drive, so that would certainly have serious implications.

[ben_w]

Also a time machine (assuming there really isn't any fundamentally preferred frame of reference), and a bag of holding.

Fig 5, page 19: https://arxiv.org/pdf/gr-qc/0207057

Put both together and call it a TARDIS.