Why Hollywood had to Fudge The General Relativity-Based Wormhole Scenes in Interstellar

Interstellar is the only Hollywood movie to use the laws of physics to create film footage of the most extreme regions of the Universe. Now the film’s scientific advisor, Kip Thorne, reveals why they fudged the final footage

Wormholes are tunnel-like structures that link regions of spacetime. In effect, they are shortcuts from one part of the universe to another. Theoretical physicists have studied their properties for decades but despite all this work, nobody quite knows if they can exist in our universe or whether matter could pass through them if they did.

That hasn’t stopped science fiction writers making liberal use of wormholes as a convenient form of transport over otherwise unnavigable distances. And where science fiction writers roam, Hollywood isn’t far behind. Wormholes have played starring roles in films such as Star Trek, Stargate and even Bill & Ted’s Excellent Adventure. But none of these films depict wormholes in the way they might look like in real life.

All that has now changed thanks to the work of film director Christopher Nolan and Kip Thorne, a theoretical physicist at the California Institute of Technology in Pasadena, who collaborated on the science fiction film Interstellar, which was released in 2014.

Nolan wanted the film to be as realistic as possible and so invited Thorne, an expert on black holes and wormholes, to help create the footage. Thorne was intrigued by the possibility of studying wormholes visually, given that they are otherwise entirely theoretical. The result, he thought, could be a useful way of teaching students about general relativity.

So Thorne agreed to collaborate with a special effects team at Double Negative in London to create realistic footage. And today they publish a paper on the arXiv about the collaboration and what they learnt.

Interstellar is an epic tale. It begins with the discovery of a wormhole near Saturn and the decision to send a team of astronauts through it in search of a habitable planet that humans can populate because Earth is dying.

A key visual element of the story is the view through the wormhole of a different galaxy and the opposite view of Saturn. But what would these views look like?

One way to create computer generated images is to trace all the rays of light in a given scene and then determine which rays enter a camera placed at a given spot. But this is hugely inefficient because most of the rays never enter the camera.

A much more efficient method is to allow time to run backwards and trace the trajectories of light rays leaving the camera and travelling back to their source. In that way, the computing power is focused only on light rays that contribute to the final image.

So Thorne derived the various equations from general relativity that would determine the trajectory of the rays through a wormhole and the team at Double Negative created a computer model that simulated this, which they could run backwards. They also experimented with wormholes of different shapes, for example with long thin throats or much shorter ones and so on.

The results provided some fascinating insights into the way a wormhole might appear in the universe. But it also threw up some challenges for the film makers.

One problem was that Nolan chose to use footage of the view through a short wormhole, which produced fascinatingly distorted images of the distant galaxy. However, the footage of travelling through such a wormhole was too short. “The trip was quick and not terribly interesting, visually — not at all what Nolan wanted for his movie,” say Thorne and co.

But the journey through a longer wormhole was like travelling through a tunnel and very similar to things seen in other movies. “None of the clips, for any choice of parameters, had the compelling freshness that Nolan sought,” they admit.

In particular, when travelling through a wormhole, the object at the end becomes larger, scaling up from its centre and growing in size until it fills the frame. That turns out to be hard to process visually. “Because there is no parallax or other relative motion in the frame, to the audience it looks like the camera is zooming into the center of the wormhole,” say Thorne and co.

But camera zoom was utterly unlike the impression the film-makers wanted to portray, which was the sense of travelling through a shortcut from one part of the universe to another. “To foster that understanding, Nolan asked the visual effects team to convey a sense of travel through an exotic environment, one that was thematically linked to the exterior appearance of the wormhole but also incorporated elements of passing landscapes and the sense of a rapidly approaching destination,” they say.

So for the final cut, they asked visual effects artists to add some animation that gave this sense of motion. “The end result was a sequence of shots that told a story comprehensible by a general audience while resembling the wormhole’s interior,” they say.

In other words, they had to fudge it. Nevertheless, the remarkable attention to detail is a testament to the scientific commitment of the director and his team. And Thorne is adamant that the entire process of creating the footage will be an inspiration to students of film-making and of general relativity.

Of course, whether wormholes really do look like any of this is hard to say. The current thinking is that the laws of physics probably forbid the creation of wormholes like the one in Interstellar.

However, there are several ideas that leave open the possibility that wormholes might exist. The first is that wormholes may exist on the quantum scale, so a sufficiently advanced technology could enlarge them in some way.

The second is that our universe may be embedded in a larger multidimensional cosmos called a brane. That opens the possibility of travelling into other dimensions and then back into our own.

But the possibility that wormholes could exist in these scenarios reflects our ignorance of the physics involved rather any important insight. Nevertheless, there’s no harm in a little speculation!

Ref: arxiv.org/abs/1502.03809 : Visualizing Interstellar’s Wormhole
