As topics go, “nuclear war” might be the last one of which we’re all in agreement. It’d be a bad thing. Very. Bad. Deadly, even. 50 Cities — 50 Traces highlights the diversity, beauty, and daily life in Mayors for Peace cities, of which Pittsburgh is one. Basically, if you’re against the destruction of cities by way of nuclear weapons, Mayors for Peace has the platform for your liking. If you’re in favor of nukes end-gaming everything — well, good luck with that. Mayor Bill Peduto leads a group of guest speakers at a reception that opens this exhibit that runs through December 9. 7:30 p.m.