A Science of Signals: Einstein, Inertia, and the Postal System

What do the speed of light and inertia have in common? According to the famous physicist Arthur Eddington, who led the expedition to prove Einstein’s Theory of Relativity, they had a lot in common: “[the speed of light] is the speed at which the mass of matter becomes infinite,” where “lengths contract to zero” and—most surprisingly—where “clocks stand still.”1 The speed of light “crops up in all kinds of problems whether light is concerned or not,” reaching all the way into the concept of inertia. In Einstein’s work, the most seemingly ephemeral and fleeting of things—light— could not escape from the grasp of inertial and gravitational forces. From 1900 onward Einstein became acutely concerned (personally and professionally) with communications media, and, in particular, with their speed. Could love be sent through the mail?, wondered Einstein. Did kisses arrive at their destination?, asked his contemporary, Franz Kafka. How strange, both men remarked as they perused complicated train schedules and jotted down times and places in their notes and letters, that nearly contiguous places were so far apart once all the stops, bureaucrats and customs officers were overcome, while other distant places could be so easily reached. “How on earth did anyone get the idea that people can communicate with one another by letter,” wrote Kafka to his lover in the 1920s. During those same years, Einstein ended a letter expressing a similar concern: “I won’t write any more about it, in order not to confuse things even further.”2 Einstein’s famous 1905 theory of relativity paper dealt centrally with the problem of sending and receiving time signals. It formed part of a much broader concern with time coordination that involved many other scientists.3 But soon after its publication, scientists started to ask how the exchange of “light signals...through empty space” investigated by Einstein fit with other forms of communication, including those for determining time but not limited to them. Einstein expanded his work from its initial focus on time signaling to signaling in general. In the process, he learned that neither love nor time could travel at speeds faster than that of light. Einstein often claimed that his theory seemed strange only because in our “everyday life” we did not experience delays in the transmission speed of light signals: “One would have noticed this [relativity theory] long ago, if, for the practical experience of everyday life light did not appear” to be infinitely fast.4 But precisely this aspect of everyday life was changing apace with the spread of new electromagnetic communication technologies, particularly after WWI. The expansion of electromagnetic communication technologies and their reach into everyday life occurred in exact parallel to the expansion and success of Einstein’s theory of relativity. Kafka, who used similar communications system as Einstein and whose obsessive focus on “messengers” and their delays paralleled Einstein’s focus on “signals” and their delays, described the radical change he was seeing around him in the 1920s: