Dear PF, I have two questions What is non-perturbative vacuum? A state - characterized by many non-vanishing condensates? (As explained in wikipedia). In what this non-perturbativeness is manifested when there are non -vanishing condensates? So why such vacuum is calld non-perturbative? And I was reading something where was written "In the spontaneous summetry breaking one has to use the symmetry broken vacuum when evaluating the S-matrix" My question is how/why the evaluation of S-matrix is related whether the vacuum is broken or no? Always thank you for assistance.