A treaty to ban the use of nuclear weapons becomes international law on January 22, 2021.  The treaty seeks to do to nuclear weapons what previous international treaties have done to chemical and biological weapons -- that is, prohibit their use on humanitarian grounds.  Nobel Peace Prize winning Beatrice Fihn, executive director of the International Campaign to Abolish Nuclear Weapons, discusses exactly this treaty obliges of its member states and also the broader politics surrounding the effort to get countries to sign onto the treaty.           Beatrice Finh -- exec dir of int'l camp to abolish nuclear weapins