This paper situates Einstein's theory of relativity within broader networks of communication. The speed of light, explained Einstein, was an unsurpassable velocity if , and only if , it was considered in terms of »arbitrary« and »voluntary« signals. Light signals in physics belong within a broader set of signs and symbols that include communication and military signals, understood by reference to Helmholtz, Saussure, media philosophies from WWII to '68 (Lavelle, Ong, McLuhan) and Derrida. Once light signals in physics are considered (...) in relation to semaphore, print, telegraph, radio, computers and tape recorders, Kittler and Habermas provide us with conflicting ways for understanding science and technology, rationality and consensus. We conclude with a study of »flash and bang« in popular accounts of relativity theory to understand the role of theoretical science in the transmission of information and violence. (shrink)