Time Travel - Ideas From Fiction - Time Travel or Spacetime Travel

Time Travel or Spacetime Travel

An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, the theory of relativity rejects the idea of absolute time and space; in relativity there can be no universal truth about the spatial distance between events which occur at different times (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference and therefore no frame's perspective is physically better than any other frame's, and different frames disagree about whether two events at different times happened at the "same position" or "different positions". In the theory of general relativity, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance".

Nevertheless, the idea that the Earth moves away from the time traveler when he takes a trip through time has been used in a few science fiction stories, such as the 2000 AD comic Strontium Dog, in which 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-appear in space. Much earlier, Clark Ashton Smith used this form of time travel in several stories such as "The Letter from Mohaun Los" (1932) where the protagonist ends up on a planet millions of years in the future which "happened to occupy the same space through which Earth had passed". Other science fiction stories try to anticipate this objection and offer a rationale for the fact that the traveler remains on Earth, such as the 1957 Robert Heinlein novel The Door into Summer where 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 coordinates 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 around the Earth's orbit, with the intention of landing at some chosen spatial location, though seldom hitting the mark precisely. In Piers Anthony's Bearing an Hourglass, the potent Hourglass of the Incarnation of Time naturally moves the Incarnation in space according to the numerous movements of the globe through the solar system, the solar system through the galaxy, etc.; but by carefully negating some of the movements he can also travel in space within the limits of the planet. The television series Doctor Who avoided this issue by establishing early on in the series that the Doctor's TARDIS is able to move about in space in addition to traveling in time.

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