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WD_245/ 2006
(  Satoshi Kinoshita )
| Series: | Works on paper: Drawings 3 | | Medium: | oil pastel on paper | | Size (inches): | 30.1 x 21.3 | | Size (mm): | 765 x 542 | | Catalog #: | WD_0245 | | Description: | Signed, date and copyright in pencil on the reverse.
Note: Continued from the preceding "page" as "WD_244/ 2006".
Time travel in fiction: Types of time travel
Time travel themes in science fiction and the media can generally be grouped into two main types and a third, less common type (based on effect—methods are extremely varied and numerous), each of which is further subdivided. These type classifications do not address the issue of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of the time line.
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. This concept of time can be referred to as circular causation. For exampes of circular causation, see Robert A. Heinlein's story By His Bootstraps.
1.2 The Novikov self-consistency principle applies (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University). The principle states that if you travel in time, you cannot act in such a way so as to create a paradox.
1.3 Any event that appears to have changed a time line has instead created a new one.
1.3.1 Such an event can be the life line existence of a human (or other intelligence) such that manipulation of history ends up with there being more than one of the same individual, sometimes called time clones.
1.3.2 The new time line may be a copy of the old one with changes caused by the time traveler. For example there is the Accumulative Audience Paradox where multitudes of time traveler tourists wish to attend some event in the life of Jesus or some other historical figure, where history tells us there were no such multitudes. Each tourist arrives in a reality that is a copy of the original with the added people, and no way for the tourist to travel back to the original time line.
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. Small changes will only alter the immediate future and events will conspire to maintain constant events in the far future; only large changes will alter events in the distant future.
2.2 The time line is easily changed (example: Doctor Who, where the time line is fluid and changes often naturally).
3. The time line is consistent, but only insofar as its consistency can be verified.
3.1 The Novikov self-consistency principle applies, but if and only if it is verified to apply. Attempts to travel into the past to change events are possible, but provided that:
-They do not interfere with the occurrence of such an attempt in the present (as would be the case in the Grandfather Paradox), and
-The change is never ultimately verified to occur by the traveller (e.g. there is no possibility of returning to the present to witness the change).
There are also numerous science fiction stories allegedly about time travel that are not internally consistent, where the traveler makes all kinds of changes to some historical time, but we do not get to see any consequences of this in our present day.
Immutable timelines:
Time travel in a type 1 universe does not allow any paradoxes, although in 1.3, events can appear to be paradoxical.
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 the time-travel stories of Connie Willis, time travelers encounter "slippage" which prevents them from either reaching the intended time or translates them a sufficient distance from their destination at the intended time, as to prevent any paradox from occurring.
Example: A man who travels into the past and intends to shoot his grandfather as a young boy finds himself snapped back to the present as he's about to pull the trigger.
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 p.m., then no bits for thirty seconds after that. If you send a bit back to 10:00:00 p.m., everything works fine. However, if you try to send a bit to 10:00:15 p.m. (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 p.m., etc. Two examples of this kind of universe is found in Timemaster, a novel by Dr. Robert Forward, and the 1980 Jeannot Szwarc film Somewhere In Time (based on Richard Matheson's novel Bid Time Return).
An example which could conceivably fall into either 1.1 or 1.2 can be seen in book and film versions of Harry Potter and the Prisoner of Azkaban. Harry and Hermione go back in time to change history. As they do so it becomes apparent that they are simply performing actions that were previously seen in the story, although neither the characters nor the reader were aware of the causes of those actions at the time. This is another example of the predestination paradox. It is arguable, however, that the mechanics of time travel actually prevented any paradoxes, firstly, by preventing them from realizing a priori that time travel was occurring and secondly, by enabling them to recall the precise action to take at the precise time and keep history consistent.
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 traveller or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated for the origin timeline and the destination timeline. A possible solution to this is to have the mechanics of time travel require that mass-energy be exchanged in precise balance between past and future at the moment of travel, or to simply expand the scope of the conservation law to encompass all timelines. Some examples of this kind of time travel can be found in David Gerrold's book The Man Who Folded Himself, the Robert Zemeckis film Back to the Future Part II (1989), The Time Ships by Stephen Baxter and the (1994) film Star Trek: Generations.
Spoiler warning: Plot and/or ending details follow.
Example: In Back to the Future Part II, Marty McFly and Doc Brown decide (after Doc returns from the 21st century to 1985) to travel to 2015 to save McFly's future son. While there, McFly buys an almanac of sporting events from 1950-2000, hoping to use it for financial gain. The book is stolen by the aged Biff Tannen, who takes the time-traveling DeLorean back in time to give the almanac to his 1955 younger self. When McFly and Doc Brown use the DeLorean to go back to 1985, they soon discover what Tannen had done; the younger Tannen has used the almanac for financial gain and changed the timeline. The alternate 1985 that McFly and Brown have returned to is now the future of a tangent that started in 1955, where Hill Valley is now corrupt and its citizens' lives changed because of Tannen.
Mutable timelines:
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 too do the 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. You could, however, infer such information by knowing if a) communication with the past were possible or 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 of 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. This is sometimes referred to as the "Time Dillution Effect."
Gradual and instantaneous:
In literature, there are two (commonly used) methods of time travel:
1. The most commonly used method of time travel in science fiction is the instantaneous movement from one point in time to another, like using the controls on a CD player to skip to a previous or next song. There is not even the beginning of a scientific explanation for this kind of time travel; it's popular probably because it is more spectacular and makes time travel easier.
2. In The Time Machine, H.G. Wells explains that we are moving through time with a constant speed. Time travel then is, in Wells' words, "stopping or accelerating one's drift along the time-dimension, or even turning about and traveling the other way." To expand on the audio playback analogy used above, this would be like rewinding or fast forwarding an analogue audio cassette and playing the tape at a chosen point. This method of gradual time travel fits best in quantum physics, but is not popular in modern science fiction. Perhaps the oldest example of this method of time travel is in Lewis Carroll's Through the Looking-Glass (1871): the White Queen is living backwards, hence her memory is working both ways. Her kind of time travel is uncontrolled: she moves through time with a constant speed of –1 and she cannot change it. This would make Lewis Carroll the inventor of time travel. T.H. White, in the first part of his Arthurian novel The Once and Future King, The Sword in the Stone (1938) used the same idea: the wizard Merlyn lives back in time, because he was born "at the wrong end of time" and has to live backwards from in front. "Some people call it having second sight".
Time travel, or space-time travel?
The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats Earth as the frame of reference in space. The idea that a traveller can go into a machine that sends you to "A.D. 1865" and leave through a door into the same spot in Poughkeepsie ignores the issue that Earth is moving through space around the Sun, which is moving in the galaxy, etc. So, given space-time as four dimensions, and "time travel" referring to just "moving" along one of them, a traveller could not stay in the same place with respect to the surface of Earth, because Earth is a moving platform with a highly complicated trajectory. A vessel that moves "ahead" 5 seconds might materialize in the air, or inside solid rock, depending on where Earth was "before" and "after." In the 2000 AD comic Strontium Dog, Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the movement of the Earth causes the unfortunate victim to re-materialize in space. To really do what filmmakers make look so easy in films such as the Back to the Future series and The Time Machine, the device might have to be a very powerful spacecraft which could move across large distances in space to compensate for the offset of position associated with the change in time.
A possible rebuttal to this criticism is the fact that cars and airplanes manage to move around the surface of the Earth with it, despite the surface itself moving with an astronomical speed. One could postulate that a time traveller experiences a combination of spatial temporal inertia that makes him move along with the Earth.
In the 1957 Robert Heinlein novel The Door into Summer Heinlein essentially handwaved the issue with a single sentence: "You stay on the world line you were on." In his 1980 novel The Number of the Beast a "continua device" allows the protagonists to dial in the six (not four!) co-ordinates of space and time and it instantly moves them there—without explaining how such a device might work. The television series Seven Days also dealt with this problem; when the chrononaut would be 'rewinding', he would also be propelling himself backwards along the earth's orbit, with the intention of landing in the same place (in space) that he originated.
-en.wikipedia.org/wiki/Time_travel
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