Physical meaning of the instability of NEC violating material Stephen Hsu and Roman V. Buniy published a paper in which they demonstrated that, for a large class of different models, any materials or fields that violated the Null Energy Condition would be "unstable". What does this mean in the physical sense? Does the material disappear, or does it fluctuate in strength? (See the section on the Casamir effect specifically).
 A: This is an interesting paper, thanks for bringing this up! I was unaware of it, and it seems to be important -- it has a lot of citations, and to me it seems like a big deal, since otherwise there are a lot of reasons to think that the energy conditions fail in quantum mechanics. I'll take a whack at answering your question, but others who know something about semiclassical gravity may be able to do better. The paper gives three results that seem to be different but related:

(1) classical solutions of scalar-gauge models which violate the NEC are unstable, (2) a quantum state (including fermions) in which the expectation of the energy-momentum tensor violates the NEC cannot be the ground state, (3) perfect fluids which violate the NEC are unstable. These results suggest that violations of the NEC in physically interesting cases are likely to be only ephemeral.

Result 1 is classical and involves fields whose quantization would be bosonic. I don't see where they ever give an explicit physical interpretation of what the instability really means. I think this is because they see this as a stepping stone to the semiclassical results 2 and 3, which tell us what would really happen.
In result 2, they extend this classical result to semiclassical gravity, considering first purely bosonic fields and then systems that also contain fermions. They show that the state isn't the ground state. Therefore it would presumably decay to the ground state, and the NEC violation would then go away, spontaneously.
In result 3 (perfect fluids):

... the fluid is unstable with respect to clumping ... We see that the system can decrease its free energy by clumping into over- and under-dense regions. This itself
  is an instability, which results in a runaway to infinitely negative free energy unless the ...  (violation of NEC) ... ceases to hold.

So since we don't believe in process that release infinite energy, we should expect that either these systems don't exist or that they decay into some other thing that obeys the NEC.
