Time Travel

#REDIRECT Time travel

Time travel

Time travel is the concept of travelling forward and backward to different points in time, much as we do through space. Humans are in fact always travelling in time — in a linear fashion, from the present to the immediate future, inexorably, until death. Some theories, most notably special relativity and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future if they themselves are possible. It has been confirmed that the effects of relativistic and gravitational time dilation can cause a traveller who starts at and returns to a point of origin that remains stationary, to arrive at a time farther in the future in that reference frame than their subjective elapsed time would indicate (a constrained form of time travel into the future). Often it is a plot device used in science fiction and many movies and television shows to set a character in a particular time not their own, and explore the character's interaction with the people and technology of that time—as a kind of culture shock. Other ramifications explored are change and reactions to it, parallel universes, and alternative history where some little event took place or did not take place, but causes large changes in the future. Famous fictional time machines include the TARDIS from the long-running BBC science fiction on television series ''Doctor Who'' and the titular The Time Machine of H. G. Wells's novel. On film there were the modified Delorean of the ''Back to the Future'' trilogy, the telephone booth of ''Bill and Ted's Excellent Adventure'', the space-time portal of ''Army of Darkness'' and at least the first three ''Planet of the Apes'' movies. Other books, films and series which feature time travel are ''A Connecticut Yankee in King Arthur's Court'' by Mark Twain, ''Timequake'' by Kurt Vonnegut, ''Star Trek IV: The Voyage Home'', ''Star Trek: First Contact'', the Terminator series, ''La Jet�e'', ''12 Monkeys'', ''Primer (movie)'', and ''Quantum Leap''. Fictional time travel even exists in the medium of video games, such as Chrono Trigger, Blinx, Viewtiful Joe, Prince of Persia and Timesplitters. In physics, the concept of time travel has been often used to examine the consequences of physical theories such as special relativity, general relativity and quantum mechanics. There is no experimental evidence of time travel, and it is not even well understood whether (let alone how) the current physical theories permit any kind of time travel. Although theories do exist about the possibility of folding time to hop from one point to another. == Physics == Albert Einstein's Special relativity (and, by extension, the General relativity) very explicitly permits a kind of time dilation that would ordinarily be called time travel. The theory holds that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies: for example, a moving clock will appear to run slow; as a clock approaches the speed of light its hands will appear to nearly stop moving. Einstein referred to the effects of this sort of time dilation as the "twin paradox." However, this effect allows "time travel" only toward the future: never backward. It is not typical of science fiction, and there is little doubt surrounding its existence; "time travel" will hereafter refer to travel with some degree of freedom into the past ''or'' future. Many in the scientific community believe that time travel is highly unlikely. This belief is largely due to Occam's Razor. Any theory which would allow time travel would require that issues of causality be resolved. What happens if you try to go back in time and kill your grandfather?—see grandfather paradox. Also, in the absence of any experimental evidence that time travel exists, it is theoretically simpler to assume that it does not happen. Indeed, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel—a variant of the Fermi paradox, with time travellers instead of alien visitors. However, assuming that time travel cannot happen is also interesting to physicists because it opens up the question of why and what physical laws exist to prevent time travel from occurring. ===The equivalence of time travel and faster-than-light travel=== First of all, if one is able to move information from one point to another faster than light, then according to special relativity, there will be an observer who sees this information transfer as allowing information to travel into the past. The general theory of relativity extends the Special relativity to cover gravity. It does this by postulating that matter "curves" the space in its vicinity. But under relativity, properties of space are fairly interchangeable with properties of time, depending on one's perspective, so that a curved path through space can wind up being a curved path through time. In moderate degrees, this allows two straight lines of different length to connect the same points in space; in extreme degrees, theoretically, it could allow timelines to curve around in a circle and reconnect with their own past. General relativity describes the universe under a complex system of "field equations," and there exist solutions to these equations that permit what are called "closed time-like curves," and hence time travel into the past. The first and most famous of these was proposed by Kurt G�del, but all known current examples require the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown. Most physicists believe that it does, largely because assuming some principle against time travel prevents paradoxical situations from occurring. ===Using wormholes=== A proposed time-travel machine using a wormhole would (hypothetically) work something like this: A wormhole is created somehow. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now experienced less subjective passage of time than the stationary end. An object that goes into the stationary end would come out of the other end in the past relative to the time when it enters. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine; in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back in time. Creating a wormhole of a size useful for macroscopic spacecraft, keeping it stable, and moving one end of it around would require significant energy, many orders of magnitude more than the Sun can produce in its lifetime. Construction of a wormhole would also require the existence of a substance known as "exotic matter," or "negative matter", which, while not known to be impossible, is also not known to exist in forms useful for wormhole construction (but see for example the Casimir effect). Therefore it is unlikely such a device will ever be constructed, even with highly advanced technology. On the other hand, microscopic wormholes could still be useful for sending ''information'' back in time. Matt Visser argued in 1993 that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. [http://arxiv.org/abs/hep-th/9202090] Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesised that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely than not a flaw in classical quantum gravity theory rather than proof that causality violation is possible. [http://xxx.lanl.gov/abs/gr-qc/9702043] ===Using massive spinning cylinders=== Another approach, developed by Frank Tipler, involves a Tipler Cylinder. If a cylinder is long, and dense, and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string. Physicist Robert Forward noted that a na�ve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time. However, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense. ===Using Quantum Entanglement=== Quantum mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presumes that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance (physics)" by Einstein. Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information theory, and therefore do not appear to allow for time travel or superluminal communication. This misunderstanding seems to be widespread in popular press coverage of quantum teleportation experiments. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem, and how the rules of quantum mechanics work to preserve causality is an active area of research. ===The possibility of paradoxes=== The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are ''no'' initial conditions that lead to paradox once time travel is introduced. If his results can be generalised they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that ''any'' situation you can set up in a time travel story turns out to permit ''many'' consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange. Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel histories would form a branching tree symbolizing all possible outcomes of any interaction. Since all possibilities exist, any paradoxes can be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction. However, in actuality, physicists believe that such interaction or interference between these histories is not possible (see Chronology protection conjecture). ===Time travel and the anthropic principle=== It has been suggested by physicists such as Max Tegmark that the absence of time travel and the existence of causality may be due to the anthropic principle. The argument is that a universe which allows for time travel and closed time-like loops is one in which intelligence could not evolve because it would be impossible for a being to sort events into a past and future or to make predictions or comprehend the world around them. Note that this imposes no restriction on supernatural agents (e.g. God) which are not confined by the bounds of space-time. See the next section for details. ==Time travel and religion== ===Prophecy and theology=== It is interesting to note that any religion which postulates the existence of fulfilled Prophecy requires, at the very least, an agent which can move information from the future into the past. In Christian theology, for example, God is assumed to exist unbound by space or time. Doctrinally, God is held to be omniscient and omnipresent. Statements in the Bible such as Jesus's claim "before Abraham was born, I am" (John 8:58) and Peter's claim "[Jesus] was chosen before the creation of the world" (1 Peter 1:20) (assuming the creation of the world began at t = 0) imply that God does not occupy the same timeline that we do. This is further supported by the assertion "I the LORD do not change" (Malachi 3:6), since change requires movement along, and constrained by, a temporal continuum. Two popular interpretations of these statements are that God (1) exists outside the space-time continuum; or (2) exists at every point in space-time simultaneously. In either case, God can transfer information from one point in space-time to any other point without restriction. ===Transcending time through ancient wisdom=== The Yoga Sutras of Patanjali have been considered by some, such as physicist Fred Alan Wolf in his book, ''The Yoga of Time Travel'' to describe an inner process by which we can access knowledge of the past and future in the present. This form of time travel can be acquired by transcending the five earthly anchors of the ego mind which otherwise leave us locked into the illusory self. ==Time travel in fiction== ===Literature=== 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 demonstrations of supposed time travel paradoxes are in Robert A. Heinlein's "\"-All You Zombies-\"" and "By His Bootstraps." One very well known time travel fiction writer is Jack Finney. His novels include ''Time and Again'', ''From Time to Time'', ''The Third Level'', and others. ''Harry Potter and the Prisoner of Azkaban'' by J. K. Rowling features a time travel paradox as does Harry Potter and the Prisoner of Azkaban (movie) (see below.) Michael Crichton's ''Timeline (novel),'' in which characters travel to 14th century France, describes time travel in great detail, explaining the science of exactly how the time machine works. The book was made into a movie in 2004, with much of the science explanation missing. ===Film and television=== The idea of time travel in motion pictures and television is a theme that has run throughout entertainment history. Key examples of such recent films are: * the 1960 in film version of ''The Time Machine'', * ''Time After Time (1979 movie)'' (1979 in film) * ''Somewhere in Time (movie)'' (1980 in film) * ''The Final Countdown'' (1980 in film) * ''Time Bandits'' (1981 in film) * ''Timerider: The Adventure of Lyle Swan'' (1982 in film) * ''The Terminator'' (1984 in film) * ''Back To The Future'' (1985 in film) * ''Flight of the Navigator'' (1986 in film) * ''Star Trek IV: The Voyage Home'' (1986 in film) * ''Peggy Sue Got Married'' (1986 in film) * ''Bill and Ted's Excellent Adventure'' (1989 in film) * ''Groundhog Day (movie)'' (1993 in film) * ''Timecop'' (1994 in film) * ''Twelve Monkeys'' (1995 in film) * ''Donnie Darko'' (2001 in film) * ''Kate & Leopold'' (2001 in film) * ''The One (movie)'' (2001 in film) * ''The Time Machine'' (2002 in film version) * ''Harry Potter and the Prisoner of Azkaban (movie)'' (2004 in film) * ''Primer (movie)'' (2004 in film) * ''The Butterfly Effect'' (2004 in film) * ''Timeline (movie)'' (2004 in film) Time travel has been represented in television, with shows such as ''Rocky_and_bullwinkle's'' (1959–1964) "Wayback machine", ''Doctor Who'' (1963–1989, 2005–), ''The Time Tunnel'' (1966–1967), and ''Quantum Leap'' (1989–1993). Various episodes of ''Red Dwarf'', ''Star Trek'' and ''Stargate SG-1'' extensively featured time travel. ===Computer games=== The computer game series that began with Command & Conquer: Red Alert was based upon a postulated time travel technique and a particular event where Albert Einstein traveled back in time to chronoshift a young Adolph Hitler and thus altered the course of history with catastrophic results. In the computer game ''Fallout (computer game)'', there is a special encounter involving a gate-like stone structure which is a time portal. Stepping through it will transport the player back in time, to a period before the start of the first ''Fallout'' game, where they will find a computer with a water chip. Breaking the chip will ensure that the events of the first game will occur, as it involves the player of the first game seeking a replacement for the broken chip. This also ensures the ''Fallout 2'' player's own existence as a descendant of the first game's player—a causal loop known as a predestination paradox. The encounter is called "The Guardian of Forever", a reference to the ''Star Trek'' episode, ''The City on the Edge of Forever''. In the fantasy/role-playing game ''Chrono Trigger'', a group of heroes travel back and forth through time in an attempt to prevent the apocalypse. (The sequel to this game, known as Chrono Cross also involves dimensional travel, and references time travel.) The Playstation game ''Crash Bandicoot 3: The Wrath Of Cortex'' involves the use of time portals to travel to various points in time (both past and future) to collect crystals. The ''Legacy of Kain'' game series states that "History Abhors a Paradox". In the Kain series, the timeline, referred to as the "Timestream", is immutable. Changes made by individuals have no effect on the general flow of time, but major changes can be made by introducing a paradox. When a paradox is introduced, the Timestream is forced to reshuffle itself to accommodate the change in history. Furthermore, no one in the Kain series has free will save for the messianic figure known as the Scion of Balance. The game ''Where in Time is Carmen Sandiego?'' and two derivative televison series (''Where in Time is Carmen Sandiego?'' and ''Where on Earth is Carmen Sandiego?'') feature time travel extensively. The games ''Freedom Force'' and its sequel, ''Freedom Force vs. the Third Reich'', both feature a villainous character named Time Master who has absolute power over time. The timestream of the Freedom Force universe is dependent on a construction called the Celestial Clock, which Time Master attempts to destroy in the first game—the consequences of which would be, with some potential scientific accuracy, the end of all intelligent life in the universe. In ''The Legend of Zelda: Ocarina of Time'', the main character Link can travel back and forth through time via the Master Sword and the Temple of Time, but this ages him from child to adult and vice versa as he does so. An educational video game titled ''Mario's Time Machine'' involves Bowser stealing precious artifacts from history (such as Shakespeare's pen and Magellan's ship's steering wheel) and displaying them in his museum, which Mario must then go back in time to stop. The obvious flaw in Bowser's scheme, however, is that if he removes those artifacts and alters history (for example so Shakespeare cannot write his plays without his favorite pen) then the artifacts become worthless. ===Types of time travel=== Time travel themes in science fiction and the media can generally be grouped into two types (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''. ::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. ====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 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 #1: 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. Two excellent 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 (movie)'' (based on Richard Matheson's novel ''Bid Time Return''). :Example #2: In the case of ''Somewhere In Time'', the film deals with events that have already or about to happen which the lead character Richard Collier (played by Christopher Reeve) could not control. Here, Collier is given a watch by a lady he has not yet known (but who already knew him in the past). Sometime later, Collier is fascinated by a picture taken in 1912 of a young actress. Eventually he learns that the woman in the picture is the old lady who gave him the watch, and that he was actually there in 1912 to meet her. Collier chooses to willfully go back in time 68 years in the past to fulfill what was written in the history books. He meets her and falls in love with her, but one day finds a penny in his pocket that he had brought back in time accidentally; the minting date on it is 68 years in the future. Holding tangible proof that he does not "belong" in the past hurls him back to the present day, and so everything that will be/was written in history has happened and Collier could not do anything to change that history. Had he remained in 1912, history would have been altered, and everything that happened at the beginning of the film would not have come true. 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 Potter (character) and Hermione Granger 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 realising ''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 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 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), and the (1994) film ''Star Trek: Generations''. :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 1951 forward, and decides to use it for financial gain via time travel. Doc Brown forbids him to take the book with him, and inadvertently leaves it lying around for the aged Biff Tannen to take with him. That night, without McFly and Doc Brown knowing it, Tannen takes the time-traveling DeLorean with the book and goes back in time to change history (using the sports almanac for his own financial success). By the movie audience's point of view, Tannen shortly after returns to 2015 and leaves the DeLorean, and McFly and Doc Brown again use the car in an attempt to go back to 1985. But soon the two discover what Tannen had done: Tannen went back to a certain point in 1955, met up with his younger self, and gave the younger Tannen the almanac for him to use for personal and financial gain, so the 1985 that McFly and Brown returned to was the future of a tangent that started in the now alternate 1955, with Hill Valley now corrupt and its citizens' lives changed because of Tannen. McFly and Brown could not just go back to 2015-A ''(''A ''for alternate)'' to nab Tannen because whatever they would have done there would have been the future of that particular tangent. In simple words, once you go back in time to change history in this particular instance, whatever happens next will be the future of that particular tangent you just altered (so, for example, if you went back in time to prevent the assassination of John F. Kennedy in 1963, or, in the case of ''Star Trek: Generations'', change the fate of a planet and thus saving the crew of the ''Starship USS Enterprise (NCC-1701-D)'', the future after that will be the future based on whatever you altered). ====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 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 (writer). 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 travellers 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. ===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 the hand of a boy lifting a toy train from the rails with the wheels still turning, and putting it back at a different place. There is not even the beginning of a scientific explanation for this kind of time travel; its popularity is probably due to the fact that 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 travelling the other way. 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". ===Other approaches and examples=== In Bill Gaines, Al Feldstein and Joe Orlando's Weird Science comic tale ''Why Papa left Home'' (1952, based on Charles L. Harness's ''Child by Chronos'') a time travelling scientist is greatly shocked at realising he's become his own father. However, 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 cannot deal with. The big problem is grammar—the tense formation for time travellers. Another issue in the book is that time travel was so complex that in order to understand all the math involved one had to live a dozen lives. As that was possible only after time travel was invented, no one ever knew who was able to invent it. In the climactic scenes of ''Superman (movie)'' (1978), Lois Lane dies as a result of her car falling into a crevice created by an earthquake and being buried by falling dirt. Wracked with anguish, Superman decides to defy his Krypton (planet) father, Jor-El, by interfering with Earth's history. Superman accomplishes this by reversing the rotation of Earth, turning time back to the point where the earthquake began, then returning the planet to its proper rotation. As a result, Lois (and the entire population of California) is saved as if the event never happened in the first place. It is also implied that Superman somehow manages to stop the nuclear weapon that triggers the earthquake from detonating. In ''Timerider: The Adventure of Lyle Swan'', Lyle Swan (Fred Ward) is a cross country motorcyclist who goes off course and finds himself at the testing ground of a time travel device which sends him back to 1882. There, he rides across the American Old West, sleeps with a local Spanish woman and battles a gang of gunslingers before being rescued from the past, but not before discovering that he is his own great-grandfather. Robert Heinlein's story "\"—All You Zombies—\"" shows the possible results of taking this concept to its logical conclusion ''ad absurdum'': the time travelling protagonist is/was/becomes his/her own father, son, mother and daughter. 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 machine to go back to the days of the American Civil War and equip the dispirited Confederate States of America army with 20th century weapons such as the AK-47. They soon win every battle and gleefully march into Washington D.C. to capture Abraham Lincoln. The time machine, however, is arbitrarily limited—it can only take people back a set number of years, allowing Turtledove to prevent the white supremacists from making another trip to cure the ills of the first, which goes wrong at the end. In most science fiction books about time travel, there is a physical machine for transporting people through time but there are stories which involve time travel through mental discipline, or "psychic time travel". Jack Finney's ''Time and Again'' is an example of this, as is Matheson's ''Bid Time Return'' (the inspiration for the film ''Somewhere in Time''). In Daphne du Maurier's ''House on the Strand'' the protagonist uses Hallucinogenic drugs to experience travel in time although his physical body appears to stay in the present. Poul Anderson's ''There Will Be Time'' portrays time travel as an ability some are born with, as does the movie ''The Butterfly Effect''. The latter displays time travel as an inherited talent, where one's mind and/or spirit travels back into its past and the traveller is able to change history, returning to an altered present. Some people affiliated with the Unidentified flying object 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. Another common plot device in fiction involves the concept of altering history with malicious intent. In this sort of story, the villain attempts to change history and alter the present or future, and history must be put right by the protagonist. Sometimes, it is assumed that there is only a limited amount of time available to the hero before history is permanently altered. 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 the Evangelist (while on the Greece 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. In the tradition of Mark Twain's ''Connecticut Yankee at the Court of King Arthur'', there are also several children's books (for example ''Half Magic'' by Edward Eager, or ''Catweazle'' by Richard Carpenter (Film)) that exploit the amusing fish-out-of-water potential of time travel, without worrying too much about the philosophical consequences. ==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 an accelerating platform with a highly complicated trajectory! A vessel that moves "ahead" 5 seconds might materialise in the air, or inside solid rock, depending on where Earth was "before" and "after." If you moved "behind" a year, you'd end up in cold outer space, where 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 filmmakers make look so easy in films such as the ''Back to the Future'' series and ''The Time Machine'', a time machine might have to be a very powerful spacecraft that could move you large distances and that kept track of 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 Earth? Or does it decouple? But does not that bring back the idea of an absolute reference frame? 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 that the acceleration and deceleration in space-time would challenge the structural integrity not only of the vessel but also of the passengers' bodies. A theorist might even use this to argue in the style of Zeno's paradoxes, for the impossibility of time machines. A possible rebuttal to this criticism, of course, is the fact that cars and airplanes built by humans manage to move around the surface of the Earth with it, despite the surface itself moving with an astronomical speed. It is reasonable to assume that a time traveller experiences a combination of spatial temporal inertia that makes him move along with the Earth. In 1980 Robert Heinlein published a novel ''The Number of the Beast (novel)'' about a ship that lets you dial in the six (not four!) co-ordinates of space and time and it instantly moves you there—without explaining how such a device might work. The television series ''Seven Days'' also dealt with this problem; the chrononaut would pilot the time machine away from the earth's surface, and then back to it, by means of a joystick-like device. In her novel ''Harry Potter and the Prisoner of Azkaban'', J.K. Rowling states that time is not something to mess around with and that no one should change history, even if they are able to time travel. :''"But remember this, both of you. You must not be seen. Miss Granger, you know the law - you know what is at stake... you - must - not - be - seen."'' - Albus Dumbledore (''Harry Potter and the Prisoner of Azkaban'', Chapter 21, pg. 288 UK edition) :''"...You wouldn't understand, you might even attack yourself! Don't you see? Professor McGonagall told me what awful things have happened when wizards have meddled with time... loads of them ended up killing their past or future selves by mistake!"'' - Hermione Granger (''Harry Potter and the Prisoner of Azkaban'', Chapter 21, pg. 292 UK edition) =="Distance" of time travel== According to special relativity, the physical laws may be invariant over Lorentz transformations. This mixes time and space dimensions as time can be compared to a distance times the speed of light. So, the second is comparable to a unit of distance equal to 299,792.458 kilometres. Conversely, the distance of 1 metre is comparable to about 3.34 nanoseconds. You can also compare a "year" to a "light-year" (since the square of a distance has the opposite sign to the square of a time, time and space are not actually identical). Now, if we suppose that the same distances in space and time present the same level of technical difficulty, then moving in time for just one second, forward or backward, would be like flying to the Moon. Moving for a few years would be like flying to some of the nearest stars. And if you want to go visiting dinosaurs, perhaps it would be like flying to a far-off galaxy. On the basis of the above argument, some people think that time travel will require a lot of energy (unless we use something like teleportation). Alternately, remote viewers suggest that all space-time is connected, perhaps through quantum properties of the Bose-Einstein Condensate, and that we may access any point instantaneously through directed consciousness. ==References== ===Scientific references=== *Paul Davies, ''About Time'' ISBN 0684818221 *::''How to Build a Time Machine'' ISBN 0142001864 *J. Richard Gott, ''Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time'' ISBN 0618257357 *Paul J. Nahin, ''Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction'' ISBN 0387985719 *Clifford A. Pickover, ''Time: A Traveler's Guide'' ISBN 0195130960 *Frank J. Tipler, ''Rotating Cylinders and the Possibility of Global Causality Violation'', Physical Review D 9 (1974), 2203 ===Literary references=== *''A Christmas Carol'' by Charles Dickens *''A Connecticut Yankee in King Arthur's Court'' by Mark Twain *''\"All You Zombies\"'', ''By His Bootstraps,'' ''The Cat Who Walks Through Walls'', ''Farnham's Freehold'', and ''Time Enough For Love'' by Robert A. Heinlein *''The Time Machine'' by H. G. Wells *''A Bridge of Years'' and ''The Chronoliths'' by Robert Charles Wilson *''Bones of the Earth'' by Michael Swanwick *''Harry Potter and the Prisoner of Azkaban'' and ''Harry Potter and the Order of Phoenix'' by J.K. Rowling *''Rotating Cylinders and the Possibility of Global Causality Violation'' by Larry Niven *''The End of Eternity'' by Isaac Asimov *''The Time Traveler's Wife'' by Audrey Niffenegger *''Twice Upon a Time'' by Allen Appel *''Doomsday Book'' (ISBN: 0553562738) and ''To Say Nothing of the Dog'' by Connie Willis *''Lost in a Good Book'' by Jasper Fforde *''The House on the Strand'' by Daphne du Maurier *''Catweazle'' by Richard Carpenter (Film) *''Household Gods'' (1999) by Judith Tarr and Harry Turtledove *"Thus We Frustrate Charlemagne" by R. A. Lafferty published in Galaxy in 1966 and also appeared in ''Alpha 1'' (edited by Robert Silverberg) and ''World's Best SF 1968'' (edited by Donald A. Wollheim and Terry Carr). ===Philosophical references=== *Fred Alan Wolf, ''The Yoga of Time Travel'' (2004) ISBN 083560828X ==See also== *anachronism and time travel *anachronism *chronovisor *grandfather paradox *predestination paradox *Ronald Mallett *thiotimoline *John Titor *Montauk Project *Time Traveler Convention ==External links== *[http://www.anomalies.net/cgi-bin/bbs/ultimatebb.cgi?ubb=forum;f=9 Time Travel's message forum] *[http://timetravelportal.com/viewtopic.php?t=293 Time, Time Travel & Traversable Wormholes] and other time travel related [http://timetravelportal.com/viewtopic.php?t=300 science & technology] topics *[http://www.xibalba.demon.co.uk/jbr/chrono.html SF Chronophysics], a discussion of Time Travel as it relates to science fiction *[http://plato.stanford.edu/entries/time-travel-phys Entry on Time Travel] in the Stanford Encyclopedia of Philosophy *[http://www.asimovs.com/_issue_0407/onthenet2.shtml On the Net: Time Travel] by James Patrick Kelly in ''Asimov's Science Fiction'' *[http://science.howstuffworks.com/time-travel.htm Howstuffworks' article on "How Time Travel Will Work"] *[http://www.theory.caltech.edu/people/patricia/lctoc.html Time Travel in Flatland?] *[http://www.pbs.org/wgbh/nova/time NOVA Online: Time Travel] *[http://web.mit.edu/adorai/timetraveler Time Traveler Convention], at MIT - "Technically, you would only need one..." Science fiction themes Time travel General relativity Special relativity Forteana

Time travel

Okay, I've just redesigned the entire talk page here Please try to keep the talk page readable in this (or a similar manner) (Way too many Horizontal Rules "----") == Faster than Light == === and causality loops === This brings another objection. According to special relativity, traveling faster than light is equivalent to traveling backwards in time, according to some observers. In particular, if faster than light travel is possible without too many arbitrary restrictions, it is possible to have events in the future cause events in the past. This is called a ''causality loop''. In relativity there are two kinds of intervals, time-like and space-like, the former corresponding to sublight speeds and that latter to supralight speeds. The two are non-equivalent so faster than light objects won't have a rest frame. This is not the same thing as traveling backwards through time, but I'm not really sure how this paragraph should be edited, without trying to explain the whole theory here. :There are also light-like intervals, which correspond to (!) light speed. ---- For anything travelling faster than light, there exists a reference frame in which it goes back in time. This does not cause paradox unless it or a signal from it returns to the starting point before the start of travel, thereby creating a ''causality loop''. === Difficulty of traveling FTL === Not sure about this. First, exceeding the speed of light wouldn't result in a reversal of time. Second, this isn't true if you have an object of a rest mass of zero. :This would lead one to posit that exceeding the speed of light would result in a reversal of time. While this is mathematically sound, as one approaches the speed of light, exponentially more force must be applied to accelerate, until finally an infinite amount of energy must be applied to reach the speed of light exactly. The only people who suggest that we must only develop technology to exceed the speed of light to develop time travel are people who do not understand the concept of mathematical limits. ::But that's exactly what it is: a mathematical limit, in a mathematical model of reality. Mathematical models are constrained by what we know of the universe. Reality isn't. :) :::Re Mathematical limit. It was shown (I believe proven mathematically) that we would never be able to exceed the speed of sound because of the pressure built up in front of the craft. As speed increases toward the speed of sound, this buildup increases. There was a large portion of the mathematical community convinced of this fact. We were able to overcome it and exceed that barrier (several times over now). While I admit that the concepts involving increasing math is substantially different, I do not underestimate the human mind. User:Mckaysalisbury 00:27, 27 Oct 2004 (UTC) Objects of zero mass have to travel at the speed of light. There can be no exception. Objects that go faster than the speed of light to travel backwards in time. And while the reasoning above is valid, it doesn't deal with the possibility of something that comes into existence travelling faster than the speed of light. What is the difference between the Revelation of John and the Book of Revelation? - User:Zoe :Stuffed if I know, Zoe. I was just trying to remove the preaching implicit in the last version. -User:Robert Merkel 07:18 May 7, 2003 (UTC) An important issue for time travel is the movement of information, and here it crosses over with prophecy. User:Harry Potter === Why Faster than light is Time Travel === ====Question==== This is an interesting topic that I never really did understand much. The part that I am still a bit confused about is how it is believed that travelling faster than the speed of light will actually result in reversal of time. Is it because we mark time based on what we are able to see? I imagine not, since, if we were all blind, then sound would be our measure, and clearly going faster than the speed of sound doesn't cause time to reverse. It seems clear that light cannot travel faster than the speed of light (although I've recently heard tell that the speed of light may not actually be as constant as once thought), and clearly there are other small particles that travel close to the speed of light that seem to slow down timewise. However, I can still see that it has clearly taken light a long time to reach from the outside of the galaxy back to earth, and we are seeing events that happened billions of years ago, but if something was able to go faster than the speed of light from the edge of the galaxy to reach here (say a billion times faster) it seems to me that it would still be years behind in reaching us, and not ahead of us. Some of the arguments that I have heard (I apologize for not having any references here) seem to be similar to the arguments that we should not be able to move, since in order to reach a destination, we would have to get half way there first, and in order to get half way there, we would have to get half way to the half way point ad infinitum. We would have to traverse an infinite distance just to move. Clearly we are capable of moving over an infinite number of points in space, so could it be possible that if we found a way to travel faster than light, it would not necessarily cause a regression of time? I have other thoughts on this matter, but very little factual information to back them up, so I will quit while I'm behind. (I'm still a bit new to wiki, so if this conversation seems very out of place, please let me know if there is a more appropriate place to move it to.) Thank you --User:Chris Strolia-Davis ====Answer==== It all comes down to the speed fo light being a constant. Understanding why time dilation occurs because both moving and stationary obsesrvers both have to clock the speed of light at c is the first part understanding this. Second imagine a traveler exceeding the speed of light (from a rest frame). In order for both a rest frame observer and the traveling observer to observer the light traveling at the speed of light. The light would actually have to travel backwards to "catch up" to where it needs to be. User:Mckaysalisbury 06:24, 27 Oct 2004 (UTC) :As an added note, the relationship between time and the speed of light in Special Relativity is expressed as a formula which shows that the observed time of an object (''t'') slows down as it approaches the speed of light (''c'') - time dilation. When the velocity is equal to ''c'', ''t'' equals zero. If you plug in a velocity faster than ''c'', ''t'' goes into negative figures. -User:Khaosworks 08:11, 27 Oct 2004 (UTC) ::Khaos, What equation are you referring to? You aren't referring to the Lorentz equation are you? Can you provide the equation here? User:Mckaysalisbury 20:23, 29 Oct 2004 (UTC) :::It's Special Relativity's equation for time dilation, $t\text{'}\; =\; \{t\backslash over1-\backslash sqrt\{(v^2/c^2)\}\}$, where $t\text{'}$ is the fraction of $t$ that is dilated, $v$ is velocity and $c$ is the speed of light. :::So, if an object is at rest, $v\; =\; 0$ and time is moving at a normal pace, $t\; =\; 1$, then time dilation is $0$. Keeping the flow of time constant at $1$, as $v$ increases, $t\text{'}$ decreases accordingly, until finally, when $v$ is equal to $c$, then $t\text{'}\; =\; \{1\backslash over0\}\; =\; 0$. So if $v$ exceeds $c$, then $t\text{'}$ becomes negative, indicating that the object is actually observed going backwards in time. -User:Khaosworks 00:31, 30 Oct 2004 (UTC) ::::While I see what you are doing, I wonder where you got that equation. That looks similar to the Lorentz Factor, but is actually quite different. the lorents factor shows time dilation as $t\text{'}\; =\; \{t\backslash over\backslash sqrt\{1-(v^2/c^2)\}\}$ Unless I'm mistaken. In this case, plugging numbers for $v$ higher than speed_of_light return an undefined solution (in real space). Am I missing something somewhere.User:Mckaysalisbury 05:34, 30 Oct 2004 (UTC) ::::Well $t\text{'}\; =\; \{1\backslash over0\}$ does not equal $0$ anyway. That's undefined. User:PaulHammond 20:07, 6 Jan 2005 (UTC) :::::Hmm. A bit of research shows two versions of the time dilation equation. I'm not sure which one is correct now. -User:Khaosworks 06:31, 30 Oct 2004 (UTC) ::::::Well, I can prove mine mathematically, and wouldn't mind doing so, (want me to do it here?) so where did you find the other equation, somewhere in wikipedia? I'd like to follow their math. :::::::I'm not a physicist, and to be honest my math is pretty basic - I'm just a relatively well-read amateur. You might want to Google for it and have a look yourself. -User:Khaosworks 21:32, 30 Oct 2004 (UTC) ::::::::I decided to provide a proof at Lorentz factor feel free to take a look, I'd be happy to provide any questions if you want.User:Mckaysalisbury 07:35, 31 Oct 2004 (UTC) :::::::::Thanks. Looking at it, it seems to me that either way, $t\text{'}$ appears to go into negative figures for velocities exceeding $c$ -User:Khaosworks 08:54, 31 Oct 2004 (UTC) ::::::::::Not quite. In the formula that I provided, velocities cannot exceed $c$, because this provides a value less than zero under a radical with an even root, so the time dilation according to the Lorentz factor is undefined, not negative.User:Mckaysalisbury 18:58, 31 Oct 2004 (UTC) == Wormhole Time Machine == ===not just winding back clock? === In your article on time travel you cover the proposal to use of a wormhole for the creation of a time machine. The basic idea is to take one end of a wormhole through a relativistic journey - using either Special Relativity i.e. using pure speed to slow down time or General Relativity i.e. using a strong gravitation field to slow down time. This article ( and other like it in popular scientific magazines ) generally dismiss this approach on purely practical grounds - such as the amount of energy required to create such a wormhole; but I would like to question the whole theoretical basis of the argument. To make things simpler I would like to start by considering a far more practical experiment as a first step. Instead of one end of a wormhole I would like to use a camera with a built-in clock ( the sort of digital camera that can put a timestamp on the picture ). Take this camera on the sort of relativistic journey as proposed above. To make the discussion comprehensible let's assume on 1st January 2000 we set of on a ten year ( Earth time ) journey in such a way that the camera only experiences nine years elapsed time. When the camera arrives back on Earth all the local calendars will be showing 1st Jan 2010 but the internal clock for the camera will show 1st January 2009. Now the big question. If I take a photograph of the calendar on my wall do I get a picture of the 2009 calendar or the 2010 calendar? As I see it the picture must be of the 2010 calendar - even though the camera will give it a 2009 timestamp. Now simply replace the camera with the end of the wormhole and attach a clock to the exit of the wormhole. When the end of the wormhole returns to Earth the very fabric of the space in the wormhole may have only aged by nine years and the clock will read 2009 but anyone looking out will see the 2010 calendar on the wall and if you step out of the hole you will be stepping into 2010 and not 2009. Time will then continue to progress for the exit of the wormhole and the outside world at the same rate so if you step into the entrance in 2020 you will see the wormhole exit clock reading 2019 but you will step out into 2020 - back when you started. So as a means of time travel it is no more effective that winding your wristwatch back. RLS, 2004-08-04 : Hah, I figured it out. This one had me stumped for a few weeks. I tried explaining it in my head, but couldn't quite get the words out right. As I was explaining a couple of TT concepts to a roomate, I figured out how to describe it. :Draw two paralell lines on a sheet of paper (or just visualize yourself doing it). Place a straightedge perpendicular to both parallel lines. This straightedge represents the wormhole. Currently, the two different lines represent the time lines for the two locations in space where the ends of the wormhole are (seperated by however much 3-D distance you wish). Under normal circumstances, the ruler will move forward, torque free, with a vector of motion paralell to the two lines. As one end of the wormhole is accelerated to a fraction of the speed of light and experiences time dilation, the other end of the wormhole does not. The "stationary" end of the wormhole continues to move forward along its timeline at the same rate as before, while the "moving" end of the wormhole doesn't move forward as fast as it used to. If under the same speed and time conditions you presented above, imagine you are the one moving the end of the wormhole in your spaceship. Leaving in 2000, making what appears to you to be a 9 year journey, the rest of the world experienced ten years. Your calendar will read 2009, and the worlds will read 2010. Being someone who doesn't live in the past, you move your calendar forward a year to match theirs, but you are now biologically one year younger than your twin brother. But where is the other end of the wormhole? If you were still moving it forward at the same rate, and you quickly enter the end of the wormhole that was accelerated to a fraction of the speed of light, the other end is now in 2011.111111... (just after noon on Feb 9 if I did my math right), so your calendar is now 1.111 years behind normal earth time, and your brother is 2.11111 years older than you are. :Does this all make sense? Should this be included in the main article to alleviate questions like yours (and mine ;) )? Thankyou for your response. The part that troubles me is the claim that the entrance end ( the one that remained on Earth all the time ) will be in 2011. The journey for the exit end ( the one that did the travelling ) was specified as lasting ten years as measured by Earth time. As the entrance end remains stationary relative to the Earth it will have aged ten years and the entrance clock will read 2010 and any camera attached to the entrance will photograph the 2010 calendar and give it a 2010 timestamp, essentially the entrance will be in 2010. When the exit end returns to Earth in 2010 ( Earth time ) the wormhole exit camera will be able to photograph both the laboratory calendar and the wormhole entrance clock and both will show 2010 but be given a timestamp of 2009. Let us take the case of a subject who travelled with the moving end. He will have aged only nine years and his own watch will read 2009 but he will be back on earth in 2010. If he immediately goes through the wormhole in a reverse direction ( i.e. enters the exit end and emerges from the entrance end ) he will come out in 2010. The fact that his watch still read 2009 does not mean he travelled back a year, simply that his watch was wrong as a result of his original relativistic journey. To make this clearer assume that when he first arrived back on Earth, after his trip with the wormhole exit, he meets his brother and takes him on the reverse wormhole trip. The brother would have been on Earth for the ten year duration of the original trip; his watch would say 2010; he would see the calendar and wormhole entrance clocks both saying 2010; he would see the wormhole exit clock saying 2009. After his reverse trip through the wormhole his watch would still say 2010 and he would still see the laboratory calendar saying 2010. RLS, 2004-11-20 :I think you're getting a little confused ebcause you are switching reference frames when you shouldn't. I'm not sure though. i'm going to explain what each of the perspectives, Wormhole, and stationary and moving twin. I will do all of this relative to the earth (stationary) time. *Moving Twin. He's the guy who gets accelerated to the fraction of the speed of light, with the exit of the wormhole. For simplicity, assume the acceleration is very fast, and the time spent doing such is negligible. On January 1st 2000, he begins his trip beginning with an acceleration to the fraction of the speed of light (0.19$c$ I believe), and remaining at that speed for some time. While on the ship, he practices StarCraft for the matchup with his brother when he returns. When his calendar says (is about to say) January 1st 2009, he begins the rapid deceleration and lands just about where he left. His clock says Jan 1st 2009, but the world clock says January 1st 2010. *Stationary Twin. He's the guy who just sits around playing StarCraft the entire time. He's watching the clock when his twin leaves at 1 January 2000, and watches him come back on 1 January 1 2010. He then proceeds to kick his brother's trash because he's got an extra year of practice on him. *Wormhole. Now I think its important to realize that a wormhole isn't really space, nor does it "age" per se. The wormhole exists out of space-time. For our purposes, it was created|discovered some time ago, and was prepared for this jorney. Initially, it maps between two points at the same time. Uhh, one end at Blizzard's headquarters (very close to the twin's house), and the other has been placed in a special device that allows it to move when the spacecraft moves, but has access ports that allow anyone to go through it still. This is where the straightedge example comes in. before 2000, both ends of the wormhole were moving through the timelines of each of their respective associated points in space at the same rate. When one end is acclerated, that end experiences time slower. In this example, after 1 Jan 2000, and before 1 Jan 2010 (remember we're using Earth Time, because the wormhole itself doesn't have frame of reference within a time dimension) 9/10 as fast. The one end of the wormhole that was accelerated to the fraction of the speed of light, is now 10 years in the future compared to when it began it's journey. The other end, must be 10/9 times as far into the future, because it was stationary, so that would be 100/9 years, or 11.1111111... years. The wormhole doesn't have an age. Had an observer (a long lost triplet perhaps) entered the stationary end of the wormhole on 1 Jan 2010, he would be put on the ship, (9/10 * 10 years) 9 earth years into the journey, which would put him in the spaceships time of (9/10 * 9 years), 8.1 years past the departure, or 2008 (feb ish) One minor point. I do not agree that the wormhole exist outside spacetime - it is part of spacetime. In general I still thing you are confusing local aging with passage through time as measured by a reference frame. To try to make my ideas clearer I have created a web page with a detailed presentation of my argument. See :- http://homepage.ntlworld.com/rlsinclair/physics/wormtime.htm my webpage I hope this will either convince you I am right or allow you to pinpoint exactly where I went wrong. RLS, 2004-11-27 :I read your paper, and I think I understand what you are saying now, but I still am not sure I entirely agree. Lemme make a couple of assumptions, before I make my statement, you let me know where I go wrong. *Wormholes are essentially a mapping between two points in normal space-time (actually it's an einstein-rosen bridge, but it wouldn't be hard to mathematically state the 4 dimensional coordinates of both locations. *Wormholes can exist that travel through time. (Whether such procedure stated by Kip could do such a thing, is yet to be determined) *Moving ends of a wormhole is concievable *Moving ends of a wormhole is difficult, because this is not just the motion of matter, but the alteration of space/time. :Having said all of that, my conclusion is that while devices that move wormholes are very difficult, and might exhibit the exact behaviour that you describe, it could also be concievable that the device moves the wormhole through time as well as through space. I don't think that Kip's method would prevent it, as such a device might bind the exit to matter, which could be modified in such a way. :Minor comments on your paper: **typos all over the place. **you mention only moving one end of the wormhole, but between figures 1 and 2, you show both ends moving. User:Mckaysalisbury 04:30, 28 Nov 2004 (UTC) I am not sure I agree with the first assumption. Normal space is continuous through the wormhole ( I could conceive of a situation where the universe looks like an erythrocyte with the two faces touching, but this is not how the Thorne wormhole is configured ). Personally I doubt that wormholes can travel through time but I am sure that my argument shows that the Thorne method won’t produce them. Sorry about all the typos, I have removed a lot of them but every time I look more appear. As for the figure; only one end is shown, the other is at some indeterminate point of the diagram - I have changed the caption to make this ( slightly ) less confusing. Early next week I intend to float the idea on the sci.physics.relativity newsgroup; so you can have the fun of seeing me being savaged by the mob. RLS, 2004-12-04 == Vehicles and positioning == In science fiction and elsewhere, I've encountered 3 different types of time machine. Please improve the suggested names of each of these types. * Vehicular type This time machine travels with the traveller(s) to the destination time and place (e.g. the :TARDIS of :Doctor Who). * External type This time machine does not travel with the traveller(s). The travellers arrive at the destination without the time machine (e.g the :Time Tunnel or the time machine of ''Quantum Jump''). * History Type This time machine does not travel in time with the traveller(s), but the traveller(s) always emerge from the time machine at some time in its history (e.g. wormhole time machine). The travellers can not go further back in time than the earliest time in the time machine's history or go anywhere where the time machine was not located. li An interesting case is the :H. G. Wells :The Time Machine. It looks like a ''History type'' machine, because it does not move, during the time travel. However, it is however moved in the far future and upon return to 1900, it arrives outside the house, rather than inside the house where it was in 1900. This makes it a ''vehicular type''. Someonewhat unrelated, there is an Isaac Asimov story (I think) about a murder committed with a type of antigravity machine that decouples the movement of an object from all the forces acting on it - the assumption in the story is that a billiard ball on the table (for that is what the story revolves around) will slowly lift off the table as forces around it stop acting on it, but, of course, what happens is that it shoots off at high speed and kills someone. It is stationary relative the the movement of the sun, earth etc. --- User:Karl Palmen :Taking in account Earth, Sun, and Milky Way's movement don't make much sense for me. In physics, position in space (and time) it's only a relative measure. Taking something off the "coordinate system" makes as sense as taking it off the Universe. The "Earth movement" problem is the same as teleporting: the travelling device needs physical means to transmit matter, energy and information (first of all, information) in this universe (as a Clifford D. Simak book does with teleportation, with a web of machines through the galaxy which send information about the traveller's particles, so they can be destroyed at departure and reconstructed at arrival, making a bit of creepy kind of voyage). If you give means to do it (as did Wells, who nailed it), it necessary will provide a clear arriving point in space and time. There is nothing as dissapearing from here and pop there, whithout nothing underlying doing the work to link one event to another. If it's something magic, unespecified, or through another universe, it could be anything. That would be more of fantasy than science (or science-fiction). One way to have a "paradoxical" universe is to have two time dimensions. For example, model the entire present universe as the internal state of a computer. In that case, when someone goes into the "past", that just tells the computer to rewind and alter a backup copy of the universe's state. In this case, there's the time of the computer and the time inside of the computer. And of course, this is exactly how Star Trek and other such shows model time travel. ==Screwball theories== ===TMaxine?=== I moved the following from the article as it did not fit into the article where it was posted. I could not find any google hits for it either, so either it is nonsense or it needs rewording (context? Maybe a SF story I don't know) to be understandable by a normal reader. :''The TMaxine Project is designing a console and Multidimensional Training in order to develop experience in the practicalities.'' User:Ahoerstemeier _____________________________________ Who is John Titor? ---- The purveyor of the Tmxxine stuff, deleted last month, has reposted it; I've reverted that edit. This month's newcomer from the realm of ideas that, to put it charitably, don't have general acceptance, is the "Chronovisor" stuff. My impression is that this one actually has a few adherents, with a book published about it, so I'm reluctantly leaving it in, the way we leave in other pseudoscientific drivel. The Chronovisor article to which it links does say "still unconfirmed," "alleged existence," etc. User:JamesMLane 08:52, 2 Jul 2004 (UTC) == Recent edits == ===Religious traditions=== From opening paragraph of article: :"...the religious traditions of Jesus, descended from the Kingdom of God, which exists out-of-time-and-space..." Can anyone give some more specific reference for this? I am not aware of any traditions of time travel episodes involving Jesus, certainly not comparable to the explicit Mohammad story. There are some (not particularly systematic) modern metaphysical constructions about god's kingdom being outside time, but I don't think an off-hand reference like this is justified. As far as Jesus is concerned the nearest would probably be mystical statements in John's Gospel such as "before Abraham was born, 'I am'" (Jn. 8.58) but that doesn't seem to amount to the same thing. User:FrankP 10:21, 17 Nov 2004 (UTC) :I think that they really meant to link to Kingdom of Heaven, which doesn't explain it either. I'm not a believer in this particular trait of heaven's "location", but some believe that heaven exists outside of time and space, I think the necian creed, or the trenton creed or something may mention something like this. I don't belive it, so I wouldn't know where to look. I don't like the implication, so I'm not going to go to all the trouble to find the correct info and fix it either, if someone does, fine by me though. You are correct that nothing states this directly in the scriptures AFAIK. User:Mckaysalisbury 13:34, 17 Nov 2004 (UTC) Let's leave it out then, unless somebody else can justify it. I don't believe religious stories either (I'm an atheist with an unhealthy tendency towards Biblical scholarship), but it seems to me there's enough fiction in the Bible already without inventing more that isn't there. There's some airborne transport (Elijah, Ezekiel) and some flying by Jesus (Acts 1:9), also predicted for his followers on his return (1 Thess 4:17), but no time travel. I'd have to check the wording for the Nicene and Tridentine creeds, but from memory there is no very specific church teaching on spatial or temporal whereabouts of heaven. User:FrankP 15:25, 17 Nov 2004 (UTC) ===Anon Technobabble=== An anon recently added this paragraph: Consider this,We all know the fundamental cells are made of atoms.SO inorder to travel time too we need time molecule.But time molecule is not actually a molecule,but an assumption of a great energy.Consider Einstein's theory of relativity M/[t*(1-(v^2/c^2))^1/2],where M=mass,v=velocity,c=velocity of light,t=time. here if the body travels equal to the speed of light then its mass will be at infinity.But if it travels faster than speed of light then its mass decreases.Therefore an utmost potential form of energy is created in itself.It also posses enormous kinetic energy.This energy if harnessed could provide either necessary time or dimension travel. NOTE:-This is just one of the visions for time travel.There may be a slight truth in this theory. So if this is true when you travel through time you travel through each and every second of a day.So if you travel through a YEAR PAST or FUTURE you are crossing each and every day at unimaginable(may be reality in future)speeds.SO time travel is not safe even if you think of wormholes. Obviously, it has to be cleaned up, but I have no idea if the information is correct or not. Can someone who understands this decide whether we should renovate it/keep it/etc.? --User:Pie4all88 :It sounds like psuedo-scientific technobabble to me. What the passage is describing is the tachyon, but the inference that the tachyon possesses enormous kinetic energy is patent nonsense. -User:Khaosworks 05:48, 3 Oct 2004 (UTC) ::Ok, I think I'll remove the paragraph. If someone wants to put it back in, they can just go to the article's history and clean it all up. Paragraphs with this many errors reflect badly on Wikipedia anyways. --User:Pie4all88 18:42, 3 Oct 2004 (UTC) Going faster than light does not take you backward in time. Plug in such a velocity in the time dilation expression and see that it results in an imaginary temporal displacement. Going backward in time requires a timelike catastrophe into the nonprincipal conjugate, at any velocity.User:Lysdexia 02:57, 7 Oct 2004 (UTC) ===Cernobov=== It appears there was a revert war about this Cernobov paragraph. The paragraph sounds like crackpotism to me and I've deleted it, not doing a simple revert because some normal changes were made in the interim. User:JamesMLane 20:04, 4 Apr 2004 (UTC) ==Help on Edits== ===Possibility of paradoxes=== The article states under "The possibility of paradoxes" If his results can be generalized they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that ''any'' situation you can set up in a time travel story turns out to permit of ''many'' consistent solutions. The last part of the sentence, "turns out to permit of ''many'' consistent solutions" has me baffled--the "of" shouldn't appear here. I don't know, however, if it's supposed to permit one of many or just many. Can someone who knows help me out here? ===Examples of Time Travel Vehicles=== When I rewrote the part involving the name of those two popular time travel vehicles, I wrote it in such a way that didn't require usage of the name of the time travel device. That's because in Wells's novel, the ship isn't given a name, so saying "Wells's novel of the same name" doesn't work.User:Mckaysalisbury 06:24, 27 Oct 2004 (UTC) ===Heinlein/Paradox reference isn't=== "Probably the most elaborate demonstrations of supposed time travel paradoxes are in Robert A. Heinlein's All You Zombies... and By His Bootstraps" Actually, these are examples of a lack of paradox, as is the movie Twelve Monkeys. In 'All You Zombies' Heinlein describes a character who is a natural and functional hermaphrodite, raised as an orphan, impregnated by a mmysterious man, who disappears. The female reproductive organs are destroyed in the one birth. Later the child is stolen from the new mother-father. The person grows older and meets up with an older man who introduces them to the ability to travel in time, whereupon they travel back to confront the mysterious man and, instead, are the man, then travel forward again to retrieve the child and drop it off back in time as an orphan, then travel forward in time to instroduce their younger self to time travel. Likewise, 'Byt his Bootstraps' describes a similar closed loop. Mind boggling as the story may be, NO paradox is encountered. Compare this to the movie 'Twelve Monkeys' wherein Bruce Willis's character is sent back time to prevent a plague that wipes out most of humanity and not only does he completely fail to stop it, he inadvertently causes it (indirectly) AND sees his younger self from the opposite end of a suppressed memory in his own childhood of seeing a man who would later be him. These three stories are not demonstrations of paradox but, quite the contrary, demonstrations of The Novikov self-consistency principle. :See predestination paradox. -User:Khaosworks 20:47, 5 Dec 2004 (UTC) == Parallel universes need clarification == ''If we make an assumption that each of these parallel universes is a dimension'' :Universe is not a dimension, at least not in the physical sense (only in the sense of "another dimension"). Thus I don't think that this paragraph actually makes sense even gramatically, much less physically. Please, someone more familiar with the history of this article, clear this up. User:Paranoid 22:51, 18 Dec 2004 (UTC) == Deleted reference to Superman == ''Superman (movie)'' (1978), Because I don't think it's a "Key example" of a film involving time travel concepts User:PaulHammond 19:57, 6 Jan 2005 (UTC) In reference to the film, it is mentioned that film implied that Supermen prevented bombs from exploding, which is incorrect - bomb exploded in space, and that was important part of the plot for Superman II, IIRC. saigon_from_europe :The bomb that exploded in space from Superman II and released the Phantom Zone criminals was in the elevator of the Eiffel Tower. Different nuclear device entirely. --User:Khaosworks 21:51, 14 Mar 2005 (UTC) ==Religious edit== :"Prophecy as decision ::The most simple explanation is that prophecy is merely the announcement of future events that will happen because god chooses to make them happen and the fulfillment is merely the event taking place, which was announced earlier. This is the most likely explanation and meets Einstein's demand that explanations should be as simple as possible, but not simpler." :::Attributed to 213.23.248.77 on April 8, 2005. Comments: The first sentence this seems to be a re-statement of the material right after it. The confused reference to Occam's Razor - which was not in any way "Einstein's demand" - does not make sense. Applying Occam's Razor would ''in my opinion'' result in the explanation that observers choose to remember prophecies that appear to have been fulfilled over prophecies that were not fulfilled. This sentence also fails to be neutral POV, especially with the blanket statement: "This is the most likely explanation". ::::User:WCFrancis 20:02, 8 Apr 2005 (UTC) :::::(Reverted while I was writing the above. Not the first time this has happened and I doubt that it will be the last.) User:213.23.248.77 is at it again. This nonregistered user at same address as above changed "It is interesting to note that any religion which postulates the existence of fulfilled Prophecy requires, at the very least, an agent which can move information from the future into the past." by changing to "It is not interesting...., because that is a logical fallacy which ignores that god may just be deciding on a specific event and announce it ahead of time, which leads to the more interesting question if he should be doing this at all and what the ethical foundation for this is." Already reverted. This begins to appear as an agenda well beyond any reasonable neutral point of view. Please note: it is my understanding (limited though it may be due to my "newby" status) that an article is a totally inappropriate place for philosophical discussion and/or argument. That's what Talk is for. ::::User:WCFrancis 03:09, 9 Apr 2005 (UTC) :Actually, it does raise an interesting point; if a sufficiently powerful entity issues the statement "In X number of years event Y will happen," and then X years later it ''makes'' event Y happen, or if a sufficiently clever entity figures out that event Y is almost certain to happen in X years and predicts it, that would have the appearance of prophecy without requiring any time travel to happen. We therefore can't say that prophecy ''requires'' time travel. The anon is wording it poorly, however. User:Bryan Derksen 18:13, 9 Apr 2005 (UTC) == Can this be expressed w/o treating time as medium? == Intro now says :It has been confirmed that travelling at speeds approaching the speed of light can cause time dilation, the effects of which cause the individual travelling to pass through time more slowly. From the perspective of the traveller, external time would be going much faster, causing the traveller, upon stopping, to arrive at a place farther in the future than their subjective elapsed time would indicate. *pass through time more slowly suggests a preferred frame of reference *instead of external time & subjective time why not just talk about frames of reference? *subjective time suggests it is just a matter of personal ''perception'' rather than a ''measurement'' that even tools would make Can this idea be expressed w/o talking about time as though it were a medium (like water or the aether)? --User:JimWae 20:27, 2005 Jun 11 (UTC) Can length (space) be measured apart from the length of something? Can time be measured apart from measuring the time for an event(s)? Can either space or time be measured, or are they not conceptual constructs (perhaps unavoidable, perhaps because of how our brains work) with which we measure events & objects?--User:JimWae 20:27, 2005 Jun 11 (UTC) :The difficulty Jim sees seems to me to be the result of not paying attention to the maths. The contrast between length and time is misguided - it should be duration vs. length; time vs distance. Creeping idealism, perhaps - is Jim having trouble understanding time travel from an idealist viewpoint? User:Banno 21:35, Jun 11, 2005 (UTC) You persist in making this personal, don't you Banno? You want to solve the problem by Banno's fiat of what "it should be"? Perhaps instead you could explain on what basis you claim it is improper to treat length & time as 2 of 4 dimensions. Your dodge does not affect the relevance of my point - my point remains releavent whether you talk about measuring length or measuring distance. We do not measure distance itself either, we measure the distance between 2 "points". If you think YOU do understand time travel, Banno, I suggest you are fooling yourself. --User:JimWae 21:56, 2005 Jun 11 (UTC) ---- The wording of the time dilation remarks in the introductory paragraph was indeed unclear (though not as bad as the rest of the paragraph used to be). I've reworded it into something more appropriate. My main concern is with going into too much detail in the introduction (that's what the "physics" section of the page is for). --User:Christopher Thomas 22:53, 11 Jun 2005 (UTC) == Info Box == As an anonymous user said; the info box at the top of the article makes it look like a magazine. The user removed it and it was almost immediately replaced by someone else. Well IT DOES make this article look like a magazine article! It's completely out of place in an encyclopaedia. It looks stupid; it's banal. I vote it be removed. User:Arcturus 10:58, 12 Jun 2005 (UTC) :As I said when I reverted the anon's edit, I think it is fine. It is no completely out of place - other articles have similar boxes; I don't see why why "looking like a magazine" is a ''bad'' thing; It is eye-catching. So I say keep it. But this is a trivial issue. User:Banno 21:34, Jun 12, 2005 (UTC) ::Yes, I didn't realise it, but this template is used in numerous articles concerning unsolved problems in physics. However, I still maintain that its use trivialises these important topics. It might be eye catching, but it serves no useful purpose here. I hope other people contribute to this discussion. User:Arcturus 21:45, 12 Jun 2005 (UTC) :::I think that, far from trivializing it, it directs the lay reader's mind as to why the subject is worthy of contemplation. It would be a boring article indeed if it were only a dry, academic treatise — Wikipedia should be useful, and articles should inform not just the interesed, but also interest the random reader. As Plutarch said, the mind is not a vessel to be filled, but a fire to be lighted. --User:Khaosworks 21:53, Jun 12, 2005 (UTC) ==Place and time== if time travel was possible, would you end up in the same place you started. say you were in london england in 2005, and wanted to time travel to the year 3000 a.d., if you started your journey in england would u end up in england? or could you end up in another country? ''(--user:nicksinif, 21 June 2005)'' : The short answer is that you could end up just about anywhere you wanted to. What constitutes "the same place" depends on your reference frame, and picking that is pretty arbitrary. A reasonable constraint to keep movement in space and time of the same order would be to say you couldn't teleport more than 10 light-years if you were travelling forward or backwards 10 years in time (as the two represent comparable distances in spacetime from the point of view of someone stationary in your original reference frame), but that's an arbitrary restriction. Another way of looking at it is to realize that by performing time travel, you're already moving in space from the point of view of someone in a different reference frame (this is covered in more detail at faster than light and special relativity; you actually look like you're doing a FTL teleport from some reference frames).--User:Christopher Thomas 07:42, 21 Jun 2005 (UTC) in back to the future 3, marty was living in a 1985 la suburb, but somehow ended up 6 hrs north of la in jamestown ca when he went back to 1885. is that possible? he starts out in la and ends up in jamestown ca? ''(--user:nicksinif, 21 June 2005)'' : Helpful note - you can sign your name (and a datestamp) by putting four ~ signs at the end of your comment. This makes it less confusing for others to read talk threads. --User:Christopher Thomas 07:45, 21 Jun 2005 (UTC) * The dates changed, so maybe the Earth orbits the sun in a very slightly different plane than space-time does? ;)Still the sun is not where it was 100 years ago either. Or maybe it's a work of fiction ;) --User:JimWae 07:46, 2005 Jun 21 (UTC) ** Spacetime doesn't actually orbit anything; it's what makes things orbit :). More seriously, part of the principles behind relativity are that spacetime doesn't impose a preferred reference frame, so whether you tag along with the earth or follow a different path is mostly arbitrary (it just takes work to change which inertial frame you're in). A time machine is also very handy as a distance-teleporting machine. --User:Christopher Thomas 07:51, 21 Jun 2005 (UTC)

Time travel

Science fiction themes Time

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