Establishes the semiclassical (bounce/instanton) theory of quantum tunneling from a metastable “false” vacuum to a lower-energy “true” vacuum in field theory, computing the nucleation rate per unit volume for bubbles of true vacuum. Foundational formalism for all later electroweak-vacuum-metastability calculations. relevance_note: the theoretical machinery every Higgs-vacuum-decay catastrophe argument is built on; without it there is no quantitative decay-rate to compute or bound.