Variable speed of light "c"/"c" where "c" is light speed in flat space. Peter Bergmann did not agree with Einstein, but left the dispute out of his earlier book in 1942 to get Einstein’s endorsement. After Einstein died Bergmann wrote a new book in 1968 claiming that vector light velocity could change direction but not speed. This has become a prevailing opinion in science, but not in agreement with Einstein’s unambiguous math. Bergman did acknowledge that the apparent speed of light would change in a gravity field and go to zero at an event horizon as viewed by a distant observer. Max Born agreed
Speed of light (cellular automaton) is because it's impossible to build a spaceship that can move every generation. (This is not true, though, for cellular automata in general; for instance many light-speed spaceships exist in Seeds.) It is, however, possible for objects to travel at the speed of light if they move through a medium other than empty space. Such media include trails of hives, and alternating stripes of live and dead cells. Certain patterns can appear to move at a speed greater than one cell per generation, but like faster than light phenomena in physics this is illusory. An example is the "Star Gate",