Time Travel Theme in Science Fiction
H. G. Wells' "The Time Machine" is considered the literary masterpiece of the genre. Mark Twain's A Connecticut Yankee in King Arthur's Court is another early time travel classic. Probably the most elaborate demonstration of supposed time travel paradoxes is Robert Anson Heinlein's "All You Zombies."
Time travel themes in science fiction can generally be grouped into two types (based on effect--methods are extremely varied and numerous), each of which is further subdivided.
Time Travel in a type 1 universe does not allow any paradoxes, although in 1.3, events can appear to be paradoxical.
- 1. The time line is consistent and can never be changed.
- 1.1 One does not have full control of the time travel. One example of this is The Morphail Effect.
- 1.2 The Novikov self-consistency principle applies. (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University)
- 1.3 Any event that appears to have changed a time line has instead created a new one.
- 2. The time line is flexible and is subject to change.
- 2.1 The time line is extremely change resistant and requires great effort to change it.
- 2.2 The time line is easily changed.
In 1.1, Time travel is constrained to prevent paradox. If one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. Michael Moorcock uses a form of this principle and calls it The Morphail Effect.
In 1.2, The Novikov Self-consistency Principle asserts that the existence of a method of time travel constrains events to remain self-consistent (i.e. no paradoxes). This will cause any attempt to violate such consistency to fail, even if extremely improbable events are required.
Example: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 PM, then no bits for thirty seconds after that. If you send a bit back to 10:00:00 PM, everything works fine. However, if you try to send a bit to 10:00:15 PM (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 PM. Etc, etc. An excellent example of this kind of universe is found in Timemaster, a novel by Dr. Robert Forward.
In a universe that allows retrograde time travel but no paradoxes, any present moment is the past for a future observer, thus all history/events are fixed. History can be thought of as a filmstrip where everything is already fixed.
See block time for a detailed examination of this way of considering the nature of time.
In 1.3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated, unless the mechanics of time travel required that mass-energy be exchanged in precise balance between past and future at the moment of travel. An example of this kind of time travel can be found in David Gerrold's The Man Who Folded Himself.
Time Travel in a type 2 universe is much more difficult to explain.
The biggest problem is how to explain changes in the past.
One method of explanation is that once the past changes so do all memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past.)
This would make it hard to tell whether you are in a type 1 universe or a type 2 universe. However, you could infer that you were by knowing that
a) communication with the past was possible and b) it appeared that the time line had *never* been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often. An example of this kind of universe is presented in Thrice Upon a Time, a novel by James P. Hogan.
Larry Niven suggests that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number time travelers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered. However, many other "stable" situations may also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.2 scenario.
In The Restaurant at the End of the Universe Douglas Adams does not see a big problem in becoming his own father, since this is nothing a well adjusted family can't deal with. The big problem is grammar - the tense formation for time travellers.
Robert Heinlein's story All You Zombies shows the possible results of taking this concept to its logical conclusion ad absurdam: the time travelling protagonist is/was/becomes his/her own father, son, mother and daughter.
In many science fiction books about time travel, there is a physical machine for transporting people through time but there is a minority which involve time travel through mental disciplines. Jack Finney's Time And Again is one such book. Poul Anderson's There Will Be Time portrays time travel as an ability some are born with. Some people affiliated with the UFO movement say that the ability to time-travel lies latent in everybody's brain, and that that ability is "turned on" in the minds of the Greys, who supposedly have the ability to unlock it in human brains too. Other people believe that both time travel and teleportation can be learned through practice in a similar manner.
In 1992 Harry Turtledove published the novel The Guns of the South which became popular with its story about South African white supremacists using a time travel machine to go back to the days of the American Civil War and equip the dispirited rebel army with 20th century weapons such as the AK-47. They soon win every battle and gleefully march into Washington, D.C. to capture Lincoln. The limits of his time travel machine are ludicrous, however, because it can only take people back a set number of years. This allows him to prevent the white supremacists from making another trip to cure the ills of the first, which (ahem) goes wrong at the end.
It can be argued that the Book of Revelation, describes a form of "spiritual time travel". In contrast to most science fiction conceptualizations of time travel, the Revelation states that John (while on the Greek island of Patmos) had a vision that took him, in spirit, to the future end times in world history and that future events were revealed to him by an angel sent by Jesus Christ.
Time Travel, or Spacetime Travel?
The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats the earth like it is stationary in absolute space. The idea that you can go into a machine that sends you to "1865 A.D.", and you exit from a door that leaves you in the same spot in Poughkeepsie that the time machine was when you entered it ignores the issue that the earth is moving through space around the sun, which is moving in the galaxy, etc. So, if you think of spacetime as 4 dimensions, and "time travel" is just "moving" along one of them, you couldn't stay in the same place with respect to the surface of the earth, because the earth is a moving platform with a highly complicated trajectory! If you only moved "ahead" 5 seconds, you might materialize in the air, or inside solid rock, depending on where the earth was "before" and "after". If you moved "behind" one year, you'd end up in cold outer space, where the earth was a year earlier - in the same part of the sun's orbit, yes, but where has the sun gone over that year? So, to really do what they make look so easy in films like Back to the Future and The Time Machine, your time machine would have to be a very powerful spaceship that could move you large distances and that kept track of the earth's motion through space as part of the solar system, galaxy, etc. But how can you decouple the ship from momentum? If you try to move forward in time, is your ship automatically going to be propelled by the momentum gained by riding the earth? Or does it decouple? But doesn't that bring back the idea of absolute space? Again, even to move one millisecond forward or backward in time, the ship would have to be far beyond anything humans can build, not to mention the acceleration-deceleration problems and what that might do to your blood pressure. You might even use this to argue, Zeno-style, for the impossibility of time machines. In 1980 Robert Heinlein published an amusing novel The Number of the Beast about a ship that lets you dial-in the 6 (not 4!) coordinates of space and time and it instantly moves you there - without explaining how such a device might work.
Scientific references include:
Paul Davies, About Time
Paul J. Nahin, Time Machines
Clifford A. Pickover, Time: A Traveler's Guide
The Time Machine (full text), by H. G. Wells
A Connecticut Yankee in King Arthur's Court (full text), by Mark Twain
See also: Anachronism and time travel, anachronism, grandfather paradox.