If it [time travel] could be done, someone will eventually learn how. If that happens, history would be littered with tourists. They’d be everywhere. They’d be on the Santa Maria, they’d be at Appomattox with [cameras], and they’d be waiting outside the tomb, for God’s sake, on Easter morning.”45 The question the title of this section asks is an echo of the one the physicist Enrico Fermi (1901–1954) asked in the 1950s, about the possibility of interstellar space travel and of alien intelligent life in the universe—if such travel is possible and ‘they’ exist, then where are they? Why haven’t we at least received radio signals from them?
For many, the apparent lack of time travelers among us is similar evidence for the impossibility of time travel. As one famous science fiction writer put it, “The most convincing argument against time travel is the remarkable scarcity of time travelers. However unpleasant our age may appear to the future, surely one would expect scholars and students to visit us, if such a thing were possible at all.
Though they might try to disguise themselves, accidents would be bound to happen—just as they would if we went back to Imperial Rome with cameras and tape recorders concealed under our nylon togas. Time traveling could never be kept secret for very long.”46 Clarke’s idea is that, from the moment after the first time machine was constructed, through all the rest of civilization, there would be numerous historians, to say nothing of weekend sightseers, who would want to visit every important historical event in recorded history.
They might each come from a different time in the future, but all would arrive (according to Clarke) at destinations crowded with temporal colleagues, crowds for which there is no historical evidence! Long before Clarke the science fiction writer Robert Silverberg had already used the same idea in his 1969 novel Up the Line, where it’s called the cumulative audience paradox.
That paradox claims that as time travelers to the past continue to visit certain historically interesting dates and places, there will be an everincreasing number of people present. As it is presented in the novel, “Taken to its ultimate, the cumulative audience paradox yields us the picture of an audience of billions of time-travelers piled up in the past to witness the Crucifixion, filling all the Holy Land and spreading out into Turkey, in Arabia, even to India and Iran … Yet at the original occurrence of [that event] no such hordes were present!” And later in the same work, we read