[SPOILERS]

At the end of the film Interstellar, it’s revealed that future descendants of humans built the “Tesseract” to save humanity. With the ability to manipulate space and time, these future humans built and placed the Tesseract in the past where their ancestors would find it. Cooper (Matthew McConaughey’s character) enters the Tesseract while falling through a black hole.

The Tesseract is not an actual space, but, rather a projection of a human being’s lifespan from the fifth-dimension, which our future descendants simplified for Cooper’s (and our) fourth-dimensional understanding. The projection portrays a physical representation of a bedroom bookshelf that watches over an entire lifespan, in this case, Murphy’s (Cooper’s daughter). Through the bookshelf, Cooper communicates with Murphy, passing along data that allows her to save humanity.

This storyline begs the question - can human descendants even exist in the future when Cooper hasn’t actually saved humanity yet? This raises a futuristic version of the classic “chicken and egg” dilemma: can a chicken send an egg back in time, where the egg then becomes the same chicken. Thus, there seems to be a logical misstep, a fairly common paradox in science fiction films like Back to the Future and The Terminator (movies we love, but don’t actually work when we pull back the curtain and meet the wizard).

If there is a logical justification, it would be the way time functions within something like a Tesseract. It would not exist in linear fashion, like orderly points on a dotted line, but rather all points would overlap and all of time could exist simultaneously. Essentially, time is a circle. All of this is completely theoretical and there has been extensive debate over this concept, but it’s possibly the best explanation for how the ending could play out.