Early in pre-production, Dr. Kip Thorne laid down two guidelines to strictly follow: nothing would violate established physical laws, and that all the wild speculations would spring from science and not from the creative mind of a screenwriter. Christopher Nolan accepted these terms as long as they did not get in the way of the making of the movie. That did not prevent clashes, though; at one point Thorne spent two weeks talking Nolan out of an idea about travelling faster than light.