Time is a fundamental dimension of human existence and comes in many forms. Using a comparative approach, philosopher and physicist Norman Sieroka looks at what distinguishes them, using time travel and music.

Have you ever come face-to-face with yourself before? Obviously not. But it is the kind of thing that might happen during time travel in —for example, in films such as Back to the Future. In this 1980s blockbuster, Marty McFly travels back in time from 1985 to 1955. This is the year when his parents fell in love. When Marty intervenes in the plot, he changes the past—and runs the risk that his parents will neither fall in love nor get married. If that had happened, he never would have been born. But how could Marty have travelled through time if he doesn't exist?

It is from these contradictions that the film gets its entertainment value. "Back to the Future is a good example of the internal conflict that stories about time travel have to struggle with," says Norman Sieroka. As a physicist and philosopher, he has studied the concept of time extensively. "Contradictions such as those Marty McFly experienced occur in particular when the sequence of cause and effect is reversed. The question of whether this is even possible makes the subject of time travel an interesting philosophical issue.

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