Regarding the concepts of wormholes and black holes, Kip Thorne said he "worked on the equations that would enable tracing of light rays as they traveled through a wormhole or around a black hole—so what you see is based on Einstein 's general relativity equations ". Early in the process, Thorne laid down two guidelines: "First, that nothing would violate established physical laws. Second, that all the wild speculations... would spring from science and not from the fertile mind of a screenwriter." Nolan accepted these terms as long as they did not get in the way of making the film. At one point, Thorne spent two weeks arguing Nolan out of having a character traveling faster than light before Nolan finally gave up. According to Thorne, the element that has the highest degree of artistic freedom is the clouds of ice on one of the planets they visit, which are structures that would go beyond the material strength that ice could support.