"When Worlds Collide" is the 18th episode and the season finale of the fourth season of the American television show Numbers. In the episode, two brothers, one a Federal Bureau of Investigation (FBI) Special Agent and the other a mathematician, disagree with each other on the issue of academic freedom after one of the mathematician's friends is arrested on terrorism charges. Series creators Cheryl Heuton and Nicolas Falacci, who wrote the episode, first mentioned the idea for the brothers' conflict during a season two commentary. When Heuton and Falacci wrote the episode for season four, their episode launched a story arc that was later resolved during season five.
