You and the professor used a DeLorean to go back in time to solve problems with the spacetime continuum. But suddenly, another DeLorean appears, and it has older versions of both of you inside. The professor gets really worried and says that if both sets exist at the same time, it could make the whole universe fall apart. To fix the problem, you’d have to merge both timelines to travel back home. But is this possible? Dan Finkel explains how to do it.