For more than 20 months, the world watched as the five permanent members of the UN Security Council and Germany hammered out a deal with Iran regarding its nuclear program. The deal limits Iran’s capabilities to develop nuclear weapons, something the 1970 international Nuclear Non-Proliferation Treaty set out to do globally. But in recent years four countries that aren’t part of the treaty: Israel, North Korea, Pakistan and India have built their own nukes. On this edition of Global Journalist, we take a look at the lessons learned in the global effort to halt the spread of nuclear weapons. How have other countries been persuaded to give up pursuing the bomb? And will it ever be possible to achieve President Obama’s call for “A nuclear free world?” Our guests this week: Edwin Lyman, a nuclear proliferation expert with the Union of Concerned Scientists. Philip Coyle, a former assistant secretary of defense and co-author of the book “The Challenges of Nuclear Non-proliferation.” Rob Goldston, a professor of astrophysics at Princeton and the former director of the Energy Department’s Princeton Plasma Physics Lab.