Most time travel movies don’t sweat the math — or, more accurately, the quantum physics of how time travel might actually work. The writers and directors include the time-travel concepts that will help their story and ignore the ones that won’t. For example, when Doc Brown takes Marty and Jennifer to the future in Back to the Future Part II, they can’t meet their kids because Marty and Jennifer would have been absent from the timeline for the past 30 years. Wait. What?