Time travel is a domain of paradoxes.

The most familiar — the grandfather paradox, according to Space.com — involves a traveler going back in time to kill their own grandfather before he conceived children. The result is that the time traveler is never born, thus never goes back in time to kill anyone — so that the time traveler is born after all, and can go back.

But in the field of time travel, this is basic, first-semester stuff. To really get your head spinning, you need to take it to the next level with the Bootstrap Paradox, which raises basic questions about causality.

Giving Albert a Helping Hand

As Astronomy Trek reports, one (relatively) simple version of the Bootstrap Paradox involves a time traveler who goes back in time and explains the theory of relativity to a young Albert Einstein. The famous scientist then writes up the theory and publishes it as his own. Eventually, the time traveler learns it in his own "native" time and goes back to teach it to Einstein.

But in this timeline, who actually comes up with the theory of relativity? And when? Not Albert Einstein — he learned it from the time traveler. Not the time traveler, who comes from an era when the theory of relativity is well known and explained by numerous books and articles. And not by any of the authors the time traveler read; they all learned it (directly or indirectly) from Einstein.

By Whose Bootstraps?

The Bootstrap Paradox takes its name from the expression "lifting yourself by your own bootstraps." According to Astronomy Trek, in a general, non-time-travel context, the expression goes back to an 18th century fantasy classic, "The Surprising Adventures of Baron Munchhausen" (though the Baron pulled himself up by his own hair, not his bootstraps).

The Bootstrap Paradox came to science fiction in a 1941 story by Robert Heinlein, "By His Bootstraps." At one point in the book, the protagonist copies information out of an old notebook; later in the story, he realizes that the notebook he copied out of is the same one he himself made as a copy many years earlier.

The classic British sci-fi show Doctor Who also tackles this question of causality. A Beethoven fan goes back in time to meet his hero, taking along copies of all Beethoven's sheet music for the maestro to sign. But he can't find Beethoven anywhere and ends up copying and publishing the music himself, "becoming" Beethoven, but leaving the actual authorship of the music as a paradox.

Time Enough for Paradoxes

The paradoxes of time travel are not only a problem for science fiction and fantasy writers.

As Space.com notes, mathematician Kurt Gé¶del demonstrated that, under certain circumstances, Einstein's theory of general relativity allows for the possibility of time travel into the past: "... if the expansion of the universe was accelerating (which it is) and the universe is also rotating ... we could travel into our past on a whim." There's just one small issue with this theory — the universe isn't rotating.

For now, we'll have to leave the causal time loops to the realm of books and television, wherein a main character discovers that she is the mother of her own mother (see: Netflix series Dark).