Feynman diagrams are now fundamental for string theory and M-theory, and have even been extended topologically. The world-lines of the diagrams have developed to become <answer>tubes</answer> to allow better modeling of more complicated objects such as strings and membranes. Shortly before his death, Feynman criticized string theory in an interview: "I don't like that they're not calculating anything," he said. "I don't like that they don't check their ideas. I don't like that for anything that disagrees with an experiment, they cook up an explanation—a fix-up to say, 'Well, it still might be true.'" These words have since been much-quoted by opponents of the string-theoretic direction for particle physics.
The diagrams have been made from lines into what 3 dimensional shape?
tubes