THORNE: Chris wanted a planet with time dilation unbelievably greater than I have ever seen in physics, and I just didn’t think that was possible. And he said, basically, “I gotta have it.” I went home, slept on it, did a calculation, and found that if you have a black hole that spins rapidly enough, and a planet that is very close to the last stable circular orbit, you could get the time dilation he wanted. It just amazed me.

NOLAN: Our meetings never ended with definite answers. They ended with questions. As a true scientist, Kip questions everything.

THORNE: Well as a true scientist I have been proved wrong so many times that I’m very humble.

NOLAN: Even if the audience can’t grasp the science, my goal was to make them understand it emotionally and for it to be clear that there’s a consistency and a reality to what we’re presenting. We went back and forth about time a lot, because Kip felt that I had done certain things at the end of the film that were at odds with the rules we had talked about. Kip thought we were seeing it different ways, and the truth is we weren’t. And I kind of knew we weren’t. So we bounced it back and forth—

THORNE: Yeah, a whole sequence of telephone calls.

NOLAN: Well, I didn’t want it to be dismissed as wild speculation. I felt very strongly that what we had done with the climax of the movie, we’re basically taking—I hate to use the term, in a way—an artistic approach to the key visual element. We tried to construct a rigid geometrical idea, thinking about the work of Escher or any number of artists, and build a set that can demonstrate it. I came up with this idea of an array, a matrix representing all the information of a four-dimensional world in three dimensions, or a five-dimensional world in four, depending on where you count time. Eventually I was able to say to Kip, “I am not violating the rules.”

THORNE: I was very happy with the whole tesseract scene—once he explained it to me. There is a fascinating scene earlier in the movie in which Brand (Anne Hathaway) says that to five-dimensional creatures, time is like mountains and valleys. You can go forward in time like climbing a mountain. You can go backward in time like going into a valley. It’s beautifully worded. But how does that fit with Chris’ rule set that says nothing can physically go backward in time? That’s where I was struggling.

NOLAN: I explained that her dialogue is about analogy, it’s about perspective. It all comes back to Flatland, I think. If you’re a two-dimensional being you can’t see two dimensions. You can see one. If you’re three-dimensional you can observe that second dimension. To me, time is like that. We can’t see it. We can feel it, and we can act accordingly. But if you were a five-dimensional being looking at our world, you could observe time as a spatial dimension.

2 A brane is, technically, an object in a spacetime. It’s a mathematical concept for thinking about multiple dimensions.

THORNE: This is where we were talking past each other. To me as a physicist there is time that flows in the fifth dimension, in the bulk, and there’s time that flows in the brane2—our reality. They’re intimately connected. But this whole business that Chris was getting at was if you live in the bulk and you’re looking at how time flows in the brane, you can go forward and backward in time just fine.

NOLAN: But you can’t enter the brane. And I was determined. He kept saying to me, “No, it’s fine, I’m not saying it’s impossible.” But I didn’t want to let him down. I didn’t want him to think I’d broken my rule set.

THORNE: He was sure we agreed. I was sure we disagreed.

NOLAN: The geometry we constructed is an honest attempt to explain to the audience that from a higher dimension our world would look very, very different. It’s an impossible task, but the attempt was tremendous fun. I wanted to be sure Kip knew we hadn’t just said, “Oh, here’s the crazy bit.”

THORNE: Oh, I never had any doubt about that. I just had a doubt whether or not what you were doing fit with general relativity in five dimensions.

3 The four-dimensional analog of a cube.

NOLAN: In a way, the tesseract3 is an analogy. Early in the film we talk about wormholes using a sheet of paper to represent the universe, and you can fold it over. But that piece of paper is representing three dimensions. You suppress the third to make it easier to understand.

WIRED: Like using two surfaces connected by a tube to diagram a wormhole, or a mass warping a sheet to represent gravity. The two-dimensional sheets stand in for three dimensions.

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NOLAN: Right, you’re not moving on the surface. You’re moving in the surface. You suppress one dimension to represent what you’re talking about. As Kip and I talked about the wormhole, I finally understood that it’s a four-dimensional hole in three-dimensional space. And since a three-dimensional hole in two-dimensional space would appear as a circle, a four-dimensional hole would look to us like a sphere. That’s why I put it in the film. It changed everything about my ability to understand dimensionality.

WIRED: But how do you get something like that across to an audience?

NOLAN: I know how to do that because of the way photographic perspective works, which is essentially two-dimensional, and the way eyes work, which is also two-dimensional. Because of diminishing perspective, because things look smaller when they’re farther away, it’s actually pretty damn simple to go, “OK, we jump into this sphere, and then there’s a smaller sphere, and then it grows and we jump into that one.” The idea that the spheres at either end of a wormhole would be the same size? Photographically that’s no problem. We filmmakers deal with that kind of thing all the time.

WIRED: Fundamentally, film always flattens a three-dimensional world onto a surface. It’s not just metaphor.

NOLAN: That’s why I object to the term “3-D” for stereoscopic imaging, because movies are already three-dimensional. A photograph represents three dimensions in two. And then a strip of film adds time. That’s how you take time and you represent it physically: a reel of film running through a projector.