When you run into Wick rotation for the first time, the fact that a QFT can be described by correlation functions of a probability measure in imaginary time seems like pure magic.
Is it true that Wick rotation works whenever you have a thermal state that somehow "covers all spacetime"? Is it also true that it *only* works in these situations? Also, is there a more rigorous definition for my intuition of "covering all spacetime"?
Finally, if all of that is correct, then does KMS thermality give an explanation for why the correlation functions are given by a probability measure? If I remember right, KMS thermality only requires the correlation functions to be analytic, and doesn't say anything about probability measures. If Wick rotation is indeed purely a consequence of KMS thermality, I would like to know what part of that thermality leads to the interpretation as a commutative random field. #science source