This is where Matthew McConaughey's Cooper encounters the Tesseract: an artificial construct that allows him to perceive time as a physical dimension.
The design and execution was a total collaboration between Nolan, theoretical physicist and exec producer Kip Thorne, the art department led by production designer Nathan Crowley, and VFX studio Double Negative led by co-owner/supervisor Paul Franklin. "We looked at works from Gerhard Richter, who has this technique of scraping the paint across the canvas and leaving these trails, so there's this sense of a historical record," Franklin explains. "The other thing I looked at was slit scan photography, and of, course, the Stargate in '2001,' but it goes back a lot further than that.
"Slit scan is this process that records one specific location across a whole range of moments. You have a slit and an aperture and you move the negative behind it so the neg is recording everything in a continuous stride as it slides past. For me, what was interesting is that you're turning time into a horizontal axis of the photograph as a record of the time of the exposure," Franklin says.
"So I thought that might give us some sort of lead in to how we would represent the concept of these world lines. A world line is something that comes out of Einstein's theory of relativity: It's the idea that every object, every person, and every piece of matter in the universe is leaving a trail in space-time behind it because we live in a four-dimensional universe. We have three physical dimensions and one dimension of time, which is always moving forward. And we had this idea that the Tesseract was a concept that shows time as a physical dimension, so Cooper could interact with it and move back and forth across the timeline and find specific moments in the history of his daughter's bedroom."
And the world lines were the obvious way to create this timeline: each object and all the people in the room extrude these timelines in all three axes, and the tricky thing was representing it in such a way that it wasn't too complicated and hard to follow or that it didn't obscure what was in the rooms.
"So we spent time thinking about how we might arrange this. Chris came up with the idea of an alternating lattice where each of the rooms has two sets of world lines coming in and the third axis alternates along the routes. If you look carefully at the Tesseract, you have these world lines that stretch through the lattice and the rooms are embedded as a series of moments in time along the world lines. It's either a moment in the future or a moment in the past, depending on which way you go," Franklin says.
They explored several concepts. One version was an infinite matrix but they decided to make it much more intimate in this father-daughter story about the transcendence of love. "It's about this room and the infinity of possibilities within this child's bedroom. The whole idea is that the Bolt Beings that created this thing know this place is important but because they are detached from space-time -- they're outside of our four-dimensional universe -- have no direct emotional connection to any specific moment. They need Cooper, who has the emotional connection to his daughter and also the faith that his daughter is going to respond to this and come back for the watch."
Cooper's past and future are already predetermined but they won't happen without his active participation. Early on, Franklin showed Nolan a test demonstrating world lines. They had a little figure walking around and leaving a trail extruding behind it. "And we worked out how to sample the material properties of the 3D model with the little walking figure and then attached that to the world sheets, the tube coming off was made of the same material as the little figure. It wasn't just a motion trail in space."
"And Chris thought that was fascinating," Franklin adds. "We gave the geometry to the art department to physically build the Tesseract -- cut them out and then extrude them in physical space. And then we rendered what the surface of these tubes should look like and we turned those into very high-res laser prints and they pasted the computer-generated imagery physically onto the set to create all the textured colors on the world lines.
"And the result was we had this set that was a collaboration between the art department and Double Negative. They built a physical set with axis cameras placed inside it and steel supported the set. They built a Tesseract volume with four rooms around it with different walls facing into a central void, and then digitally we extended it off in all directions. But we also added all the overlays, all the streaming information traveling along the timelines.
"We used projectors on set and projected animated images and at one time had 18 projectors working. We then enhanced that with animation streaming along the timelines. And then all the objects in the rooms are connected to the world lines by these very, very thin, diaphanous threads, which are essentially solid extrusions that we've culled out along the geometry. We had to body track all the actors in the room as well and and then generate threads off them, and also sample the photography so it picks up the correct curve."
Franklin admits the Tesseract is a challenging concept, which you can read more about in Thorne's book, "The Science of Interstellar" (W.W. Norton & Co.). "Hopefully, it will stimulate all sorts of debates, which have already begun on the internet," he concludes.