I know what the definition is, but the concept it's conjuring for me seems in my experience of learning about these things, to indicate that I'm missing the point completely. What does it mean for something to be oscillating when it's in a ground state? I was under the impression that such a thing is impossible unless there's some external thermodynamic input. Is this a hypothetical structure that is believed *not* to exist, but is interesting as a thought experiment? Naively it sounds like a PPM, which is why I assume that I must be missing quite a bit.