Time/Space Relativity Explained
Interstellar is based on the ideas of theoretical physicist Kip Thorne - specifically the notion that while we observe the universe in three dimensions, there could be at least five dimensions. In certain theories, it is posited that certain forces (in this case gravity) bleed through dimensions - meaning that, based on Newton's Laws, what we perceive as a finite calculation could actually have infinite implications.
The concept is outright exemplified in the first planet that the Endurance team visits. In general, time on our side of the wormhole moves faster than time in the uncharted side. Due to close proximity with gravitational anomalies from a nearby black hole (Gargantua), time on the other side is exponentially slower - relative to the distance between an object and the black hole's gravitational pull. As a result, time on Miller's planet moves significantly slower: for every hour that the team spends on the water planet, seven years pass back home - a primary reason that Cooper is motivated to get off the planet as soon as possible (even before they realize it's a death trap). Cooper knows that three hours on planet's surface will cost him decades of time with his family.
As Amelia suggests, the effect of gravity from the black hole on time was to blame for the Endurance team's unfortunate visit to Miller's planet in the first place - since what they perceived as years of positive beacon readings were actually mere minutes for Miller (who was killed by a wave moments after she landed).
The concept is further hammered home when, following the mission, Amelia and Cooper reunite with Romilly, who stayed behind on the Endurance to gather data (far from Gargantua) - and, in the three hours his team was gone, has lived twenty-three full years alone without them. Similarly, the crew receives video messages from back home and we see that Cooper's children, Tom and Murph have also aged - now full grown adults (played by Casey Affleck and Jessica Chastain, respectively).
As the team moves farther from the black hole, the disproportion in spacetime reduces - meaning that when they arrive at Mann's planet, there's significantly less urgency (though the once brave astronaut has been twisted by his longer stretch of time alone).
A tangible effect of gravity on spacetime is also responsible for Cooper's ability to communicate with young Murph when inside the Tesseract. Inside the machine, gravity bleeds through to other dimensions in time and space, allowing Cooper to spell out a message ("S-T-A-Y") by pushing books off of Murph's shelf - or communicate map coordinates to the past version of himself by spreading dust across the floor (in binary language). Most importantly, the fifth-dimensional communication through gravity (made visible by three-dimensional objects back on Earth) enables Cooper to gently manipulate the hands on Murph's watch - transferring the data that TARS acquired with morse coded watch ticks. Subsequently, translating that coded data gives Murph all the information she needs to drastically advance humanity's understanding of space and time - as well as complete Plan A.
As for how Cooper survives his time inside the Tesseract, and how he intends to reunite with Amelia? Nolan simply reapplies the same theory that has been present the entire film. Given that time moves slower near the gravity pull of the black hole, Cooper's ejection from the Tesseract is only seconds for him, but over half a century for the rest of humanity. Keep in mind, if the ratio of time on Miller's planet was 1 hour for every 7 years on Earth, the proportion would be skewed exponentially at the absolute center of the Tesseract singularity. As a result, while it appears to Earthbound humans that TARS and Cooper have been floating out in space for nearly ninety years, they were actually only out there for mere seconds as they perceived it.
The disproportionate relativity allows Cooper to survive and reunite with Murph - who, living on the faster moving side of the wormhole, is now over one-hundred years old. Knowing that Cooper has nothing left to live for in a post-Earth existence (since his son Tom is presumed dead and Murph will soon join him), Murph reminds her father that, through the wormhole, Amelia is just beginning to set-up Plan B on Edmonds' planet. At the same time, it is revealed that even though Edmonds' planet is actually habitable, the astronaut himself did not survive the landing - leaving Amelia alone at the colonization site.
Using a reversal of the film's primary relativity theory, Cooper hops into a ship, with the knowledge that even though nearly one hundred years have passed since the Endurance first set out, time on the other side of the wormhole is moving much slower - meaning that a second trip should allow him to reunite with Amelia on Edmonds' planet only a short time after Cooper first sacrificed himself and dropped into the singularity. We don't actually see the reunion, so Cooper's actual fate is left up to some interpretation, but there's reason to be optimistic that he reaches Amelia and helps ready the colony for humankind.
Interstellar is a film about shoving-off into the unknown. Much like Nolan's mind-bending sci-fi drama Inception (read our explanation of that ending), the main takeaway from the end of Interstellar is not that Cooper and Amelia will be reunited (though it's possible that they will).
Rather the ending, and Cooper's departure to find Amelia illustrates what Prof. Brand regularly suggested by way of poet Dylan Thomas:"Do not go gentle into that good night, old age should burn and rage at close of day. Rage, rage against the dying of the light." The movie seems to posit that humanity is at our best when we throw ourselves passionately into the unknown - in search of love and discovery.
...And that's exactly what Coop intendeds to do.
UPDATE: Here's a chart that explains what happened in visual terms (CLICK TO SEE FULL SIZED VERSION):
SEE ALSO: Interstellar Review
Interstellar runs 169 minutes and is Rated PG-13 for some intense perilous action and brief strong language. Now playing in IMAX theaters with a full release Friday, November 7th.
Have your own theory? Feel free to share it in the comments below! Want to discuss SPOILERS? Head over to our Interstellar Spoilers Discussion. Have questions about the film we didn’t answer here? Check out our Interstellar episode of the Screen Rant Underground Podcast.
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