Thread:Ghelæ/@comment-29927902-20161001081010/@comment-47205-20170325090356

Mainly because it's a requirement of real-world suggestions for faster-than-light travel. Strictly speaking, you don't need the energy to be negative if the pressure if the more negative than the energy is positive. However, note that the argument I gave only forbids negative kinetic energies of particles that usually have positive energies; a field that is naturally negative-energy does not suffer from those problems.