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Ripples in the Dirac Sea

by Geoffrey A. Landis

A physics guy invents a time machine that can go only backward and must always return the traveler to the exact same present from which he left.
— Michael Main
  1. Travel is possible only into the past.
  2. The object transported will return to exactly the time and place of departure.
  3. It is not possible to bring objects from the past to the present.
  4. Actions in the past cannot change the present.
DEBUT
“Ripples in the Dirac Sea,” in Asimov’s Science Fiction, October 1988.
VARIANTS
1 English variant
TRANSLATIONS
6 translations
TAGS(SPOILERS!)
Time Periods Timeline Models Time Travel Methods Themes
  • Grandfather Paradox: Yes, he does kill an ancestor (his father) twenty-three years before he was born. [. . .] [Y]ou have to try these things. How else could I know for sure?a paradox in this variant of branching timelines.
  • Self-Visitation: at least once, when he tries to talk himself out of going to Santa Cruz
Real-World Tags
  • Dinosaurs: Quite a disappointment.
  • Jesus: Christ is mentioned, although it sounds like the narrator didn’t actually see the crucifixtion because he got ambushed.
  • Paul Dirac: Oddly enough, he never mentions visiting Dirac, but he tells us about Dirac’s sea of negative energy.
Groupings
TIME TRAVEL ITINERARY (SPOILERS!)
  1. From a windowless lab, unknown time ⋙ to the same lab, 30 minutes earlier. Round trip. Note: The first test run with a camera pointed at a clock.
  2. From a windowless lab, unknown time ⋙ to the same lab, various earlier times. Multiple round trips. Note: Rats used as guinea pigs.
  3. From a windowless lab, unknown time ⋙ to the same lab, one hour earlier. Round trip. Note: The narrator’s first trip.
  4. From unknown place and time at an unknown time ⋙ to San Francisco Bay area, 8 June 1965. Round trip. Note: The first time the narrator met Dancer, aka Dan. It’s possible that this first meeting was in the real world rather than using the time machine.
  5. From unknown place and time at an unknown time ⋙ to the Cretaceous. Round trip. Note: The narrator goes “to see dinosaurs.”
  6. From unknown place and time at an unknown time ⋙ to the early 20th century. Round trip. Note: The narrator attempts to kill his own grandfather.
  7. From a hill outside Jerusalem, unknown time ⋙ to the Cruxifiction. Round trip. Note: By the time of this trip, the narrator’s time travel mechanism has become portable.
  8. From unknown place and time at an unknown time ⋙ to the Sierra Nevadas, September 1853. Round trip. Note: The narrator observes the Hapland family.
  9. From unknown place and time at an unknown time ⋙ to San Francisco, 18 April 1906. Round trip. Note: The narrator observes the earthquake.
  10. From a Santa Cruz hotel room on fire at an unknown time ⋙ to the same hotel room, five days earlier. Round trip. Note: One of many attempts to change the fire that is destined to kill him.
  11. From a Santa Cruz hotel room on fire at an unknown time ⋙ to that same Santa Cruz hotel room, a few minutes before the fire. Round trip.
  12. From a Santa Cruz hotel room on fire at an unknown time ⋙ to San Francisco, 1965. Multiple round trips.
  13. From a Santa Cruz hotel room on fire at an unknown time ⋙ to city hall to argue with the mayor, possibly about fire codes at an unknown time. Round trip.
  14. From a Santa Cruz hotel room on fire at an unknown time ⋙ to the narrator’s own house to argue himself into not going to Santa Cruz at an unknown time. Round trip.
  15. From a Santa Cruz hotel room on fire at an unknown time ⋙ to the same hotel, perhaps when it was being constructed, with dynamite at an unknown time. Round trip.
  16. From a Santa Cruz hotel room on fire at an unknown time ⋙ to many more times and places to try to stop destiny or re-meet Dancer or just live a long life after Dancer’s death. at an unknown time. Round trip.