determining time but not limited to them. Einstein expanded his work from its initial focus on time signaling to signaling A Science of 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 Signals only because in our “everyday life” we did not experience delays in the transmission speed of light signals: “One would Einstein, Inertia and the Postal have noticed this [relativity theory] long ago, if, for the System 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 Jimena Canales communication technologies, particularly after World War I. The expansion of electromagnetic communication technolo- What do the speed of light and inertia have in common? gies and their reach into everyday life occurred in exact According to the famous physicist Arthur Eddington, who parallel to the expansion and success of Einstein’s theory of led the expedition to prove Einstein’s Theory of Relativity, relativity. Kafka, who used similar communications system they had a lot in common: “[the speed of light] is the speed at as Einstein and whose obsessive focus on “messengers” and which the mass of matter becomes infinite,” where “lengths their delays paralleled Einstein’s focus on “signals” and their contract to zero” and—most surprisingly—where “clocks delays, described the radical change he was seeing around stand still.”1 The speed of light “crops up in all kinds of him in the 1920s: problems whether light is concerned or not,” reaching all the way into the concept of inertia. In Einstein’s work, the most Humanity … in order to eliminate as far as pos- seemingly ephemeral and fleeting of things—light— could sible the ghostly element between people and to not escape from the grasp of inertial and gravitational forces. create a natural communication … has invented From 1900 onward Einstein became acutely concerned the railway, the motor car, the aeroplane. But it’s (personally and professionally) with communications media, no longer any good, there are evidently inven- and, in particular, with their speed. Could love be sent tions being made at the moment of crashing. The through the mail?, wondered Einstein. Did kisses arrive at opposing side is much calmer and stronger; after their destination?, asked his contemporary, Franz Kafka. the postal service it has invented the telegraph, the How strange, both men remarked as they perused compli- telephone, the radiograph. The ghosts won’t starve, cated train schedules and jotted down times and places in but we will perish.5 their notes and letters, that nearly contiguous places were so far apart once all the stops, bureaucrats and customs officers From Time Signals to Signals In General were overcome, while other distant places could be so easily Einstein’s 1905 relativity paper was an investigation into reached. “How on earth did anyone get the idea that people the time taken by “a light signal” to reach an observer can communicate with one another by letter?” wrote Kafka “through empty space.”6 It initiated an unprecedented to his lover in the 1920s. During those same years, Einstein overhaul of physics that had not been seen since the time ended a letter expressing a similar concern: “I won’t write of Newton. How did an apparently simple account about any more about it, in order not to confuse things even basic, procedural techniques pertaining to the exchange of further.”2 “light signals” become so much more? And how did it efface Einstein’s famous 1905 theory of relativity paper dealt its lowly origins? In 1905 the “light signals” referred to by centrally with the problem of sending and receiving time Einstein were very elementary, especially compared to what signals. As such, it fit within research on time coordination they would become in the decades that followed. that involved many other scientists.3 But soon after its pub- Contemporary physicists and even philosophers often lication, scientists started to ask how the exchange of “light understood Einstein’s work as a science of signals with implica- signals … through empty space” investigated by Einstein tions for telecommunication technologies. Descriptions of fit with other forms of communication, including those for Einstein’s work as a study of light signals abounded during 12 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/thld_a_00156 by guest on 03 October 2021 the period of its emergence. The philosopher and mathema- tician Alfred North Whitehead, one of the most important thinkers to challenge the theory, called it both “signal- theory” and “message theory.”7 The connection of relativ- ity theory to telecommunications science and technology followed three stages. While they first appeared in 1907 as constitutive, Einstein later described them in 1910 as consequen- tial. Eventually these connections were completely effaced, to the point that most historians, physicists and philosophers forgot all about them. Einstein’s theory flourished in the era of electromagnetic communication technologies and debates over its validity were argued in terms of the possibilities and limitations of long-distance communication. While still working at the Patent Office, which was being flooded with applications for new wireless communications technologies, he cor- responded with his colleague Wilhelm Wien to determine if the exchange of light signals described in his relativity paper applied to other forms of information-transfer signals. His letters now described problems in physics in terms of the general scenario of signal sending (“emanation”) and reception (“perception”): “Let A be a point from which elec- tromagnetic influence can emanate, and B a point in which the influence emanating from A be perceived.”8 Einstein followed the results of Emil Wiechert, who determined that the velocity of an “optical signal” should always be less than the speed of light. This was true, Einstein claimed, “in any medium.”9 Einstein soon started defining “signaling” in physics in the way it was used by the communications industry, and distinguishing the term from previous definitions that included periodic and predetermined signals. Previously, the term “signal” was used frequently in physics to denote both a symbol and a sign, including periodic and predetermined causes, but Einstein increasingly defined it in narrower terms: as a communications signal. During the second half of the nineteenth century, the fa- mous German scientist Hermann von Helmholtz had urged his followers to consider the world as a system of signs; now Einstein urged his colleagues to think of it in terms of signals. In 1907, Einstein explained to a colleague that relativity theory was concerned with communication signaling—and not with other types of signs or signals: I now designate the kind of velocity that, accord- ing to the theory of relativity, cannot be greater Figure 1. Letter 49: Einstein to Marić [4 February 1902], with a keyed diagram of his lodgings in Bern. his lodgings of diagram to Marić [4 February 1902], with a keyed Einstein 49: 1. Letter Figure than the velocity of light in a vacuum as “signal 13 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/thld_a_00156 by guest on 03 October 2021 velocity.” This is a velocity which a one-time (not for Einstein’s expansion of the notion of time—from clock regularly recurring) influence, which is not yet time to time in general. determined by past electrodynamic processes, is propagated; thus, we are dealing here with the “Signal-theory” propagation of an influence that could, for ex- From 1905 to 1907 Einstein’s work changed from being ample, be used for sending an arbitrary signal. The an investigation of time signals to signals in general, but propagation velocity … in your analysis refers to a by 1910 Einstein reframed his research in yet another periodical process (periodical amplitude change, way. Its implications for signaling were described as a not an amplitude change of the most general “consequence” of a much broader physical theory—and a kind.)10 profoundly counterintuitive one at that. Conditions neces- sary for sending or receiving signals were what “follows From that time onward Einstein’s notion of signal was one immediately” from his theory—not its starting point. which could be “arbitrary” and “one-time (not regularly Einstein described the inability “to send signals that would recurring)” of “the most general kind” and which was “not travel faster than light in a vacuum” as a “consequence, yet determined by past … processes.” He focused on “the as strange as it is interesting” of his theory.17 Not everyone propagation of an influence that could, for example, be was convinced that this “consequence” followed necessar- used for sending an arbitrary signal,” which was different ily. An audience member in one of Einstein’s lectures pro- from “a periodical process.”11 tested that “he always comes to perceive the world around Einstein’s theory of relativity was based on a particular us by way of light signals.”18 notion of observation-based science, one that considered “Signal-theory” is what “we will call it,” wrote “observation” in terms of light sent and received, and Whitehead referring to Einstein’s theory of relativity.19 which contrasts starkly with how it was defined at other Whitehead’s reading of Einstein’s theory (and of what he historical periods.12 His descriptions of “simultaneity” considered to be its main inadequacies) centered intently explained, with precision, the behavior of rays of light on its relation to communication technologies. He chas- meeting at one source at the same time: “a coincidence at tised Einstein for giving an unwonted importance to light once spatial and temporal on the retina of the observer.”13 signals over others.