Time Travel: A Writer's Guide to the Real Science of Plausible Time Travel

CHAPTER 10 Faster-Than-Light (FTL) Into the Past

CHAPTER 1O Faster-ThanLight Into the Past Velocities greater than that of light. . . have no possibility of existence. Albert Einstein, 1905 "We cannot fight the laws of nature." "Nature be damned! Feed more fuel into the tubes. We must break through the speed of light. . . Give me a clear road and plenty of fuel and I'll build you up a speed of half a million miles a second. . . . What's there to stopit?" words exchanged by the first officer and the captain of a starship on its way to Alpha Centauri in N. Schachner's "Reverse Universe." The captain, we are told, "hadheard, of course, of the limitingvelocity of light, but it meant nothing to him." (Astounding Stories, June 1936) Some ofthe most intriguing paradoxes oftime travel involvenotraveler (at least no living one)—only information. Of course, any information flow at all involvesthe flow ofenergy and, as Einstein showed in his famous result in special relativity, energy and mass are different aspects ofthe same thing. Thus, information time travel involves 144 Time Travel the transfer of mass. So, a man in the twenty-fifth century sending a backward-in-time "temporal radio" message to a twentieth-century woman stating that he loves (will love?) her is sending much more than mere emotion (e.g., read Jack Finney's hard-to-fmd "The Love Letter" in Tales Out of Time—see the bibliography). Two points should be clearly understood and kept in mind as you read this chapter. First, the Einstein quote that opens this chapter applies only to local regions of space-time, an area physicistswould describe as a chunk of space-time sufficiently small that we can say it is flat and so the effects of space-time curvature (i.e., gravity) do not apply to it. A strong example would be a piece of land the size of a house lot which appears flat, even though the entire surface of the Earth is curved. In such a situation, special relativity (and the speed limitation) applies. On a global scale, however, where spacetime curvature cannot be ignored, general relativityrules and fasterthan -light speeds are possible, perhaps via wormholes. Second, the paradoxes of information time travel arise onlywhen backward time travel is involved.Writers do not always understand this fact, so let me say a little more on this point. Suppose a time traveler goes into the future and, before returning to his normal time, meets himself. Suppose further that the two quarrel and the time traveler kills his future-self. This is not paradoxical, but simply a delayed form of suicide. Now,suppose it is the time traveler that is killed. Then we have a variant of the grandfather paradox, and all the previous arguments against that red herring apply here, too. It is crucial to note that the very assumption that the time traveler finds an older self in the future implies that a backward journey will take place. This means that if a time traveler journeys forward and stays in the future, he simply will not find another, older version of himself there. You have to keep all of the above ideas in mind when writing about information time travel, too. Now, all forms of present-day communication are transmissions only to the future. If you speak to someone or send a radio message, there are delays depending on the distance of separation and the speed oftransmission of sound Faster-Than-Light Into the Past 145 and of light, respectively. No new super-science technology is required to talk to the future. If you want a character in a story to send a message to the one-hundred-twenty-fifth century, you can very easily have him or her write a letter and seal it in a pressurized bottle of inert gas (e.g., helium). A version of this idea is in Murray Leinster's "Dear Charles" (in the 1960 collection Time Twisters), where a character in the twentieth century sends a message to the thirty-fourth century by simply printing it in a book that he knows will be preserved in a library. This is all quite ordinary, but certainly not in the reverse time direction. What, for example, could be a more exciting message than the one received from the future by the young genius Cullen Foster, inventor of the first time machine in D. Stapleton's "How Much to Thursday?" (Thrilling...