This article details time travel itself. For other uses, see Time Traveler.

Time travel is the concept of moving between different moments in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to a moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate). This is a list of some of the major unsolved problems in Physics. A paradox is a true statement or group of statements that leads to a Contradiction or a situation which defies intuition; or inversely This article refers to the Time travel paradox In novels based on the television series Doctor Who, "Grandfather Paradox" is the semi-mythical For other uses see Time (disambiguation Time is a component of a measuring system used to sequence events to compare the durations of Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another Some interpretations of time travel also suggest that traveling backwards in time might take one to a parallel universe whose history could begin to diverge from the traveler's original history after the moment the traveler arrived in the past. The multiverse (or meta-universe) is the hypothetical set of multiple possible Universes (including our universe that together comprise all of Reality. ^[1] Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox) as well as gravitational time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel. A plot device is an element introduced into a story solely to advance or resolve the plot of the story Fiction is the telling of stories which are not real More specifically fiction is an imaginative form of Narrative, one of the four basic Rhetorical modes. This article discusses a concept in physics For the concept in sociology see Time displacement. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial In physics the twin paradox is a thought experiment in Special Relativity, in which a person who makes a journey into space in a high-speed rocket will return home to find he Gravitational time dilation is the effect of time passing at different rates in regions of different Gravitational potential; the higher the local distortion of Spacetime General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 A physical law or scientific law is a Scientific generalization based on empirical Observations of physical behavior (i Any technological device, whether fictional or hypothetical, that is used to achieve two-way time travel is known as a time machine.

Contents 1 Origins of the concept 2 Time travel in theory 2.1 Tourism in time 2.2 General relativity 3 Time travel to the past in physics 3.1 The equivalence of time travel and faster-than-light travel 3.2 Special spacetime geometries 3.3 Using wormholes 3.4 Other approaches based on general relativity 3.5 Time travel and the anthropic principle 3.6 Experiments carried out 3.6.1 Non-physics based experiments 4 Time travel to the future in physics 4.1 Time dilation 4.2 Time perception 5 Other ideas about time travel from mainstream physics 5.1 The possibility of paradoxes 5.2 Using quantum entanglement 6 Philosophic understandings of time travel 7 Ideas from fiction 7.1 Types of time travel 7.1.1 Immutable timelines 7.1.2 Mutable timelines 7.2 Gradual and instantaneous 7.3 Time travel, or space-time travel? 8 See also 8.1 Speculations 8.2 Claims of time travel 8.3 Fiction, humor 9 References 10 Further reading 11 External links

Origins of the concept

There is no widespread agreement as to which written work should be recognized as the earliest example of a time travel story, since a number of early works feature elements ambiguously suggestive of time travel. For example, Memoirs of the Twentieth Century (1733) by Samuel Madden is mainly a series of letters from English ambassadors in various countries to the British "Lord High Treasurer", along with a few replies from the British Foreign Office, all purportedly written in 1997 and 1998 and describing the conditions of that era. Samuel Madden (1686-1765 was an Irish author His works include Themistocles The Lover of His Country, Reflections and Resolutions Proper for the Gentlemen ^[2] However, the framing story is that these letters were actual documents given to the narrator by his guardian angel one night in 1728; for this reason, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel who returns with state documents from 1998 to the year 1728",^[3] although the book does not explicitly show how the angel obtained these documents. A guardian angel is an Angel who protects and guides a particular person Alkon later qualifies this by writing "It would be stretching our generosity to praise Madden for being the first to show a traveler arriving from the future", but also says that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backwards from the future to be discovered in the present. "^[2]

Louis-Sébastien Mercier's L'An 2440, rêve s'il en fut jamais ("The Year 2440: A Dream If Ever There Was One") is a utopian novel set in the year 2440. Louis-Sébastien Mercier ( 6 June 1740 - 25 April 1814) was a French Dramatist and Writer. An extremely popular work (it went through twenty-five editions after its first appearance in 1771), the work describes the adventures of an unnamed man, who, after engaging in a heated discussion with a philosopher friend about the injustices of Paris, falls asleep and finds himself in a Paris of the future. Robert Darnton writes that "despite its self-proclaimed character of fantasy. . . L'An 2440 demanded to be read as a serious guidebook to the future. " [Robert Darnton, The Forbidden Best-Sellers of Pre-Revolutionary France (New York: W. W. Norton, 1996), 120. ]

In the science fiction anthology Far Boundaries (1951), the editor August Derleth identifies the short story "Missing One's Coach: An Anachronism", written for the Dublin Literary Magazine by an anonymous author in 1838, as a very early time travel story. August William Derleth ( February 24 1909 &ndash July 4 1971) was an American writer and anthologist Anonymity is derived from the Greek word ανωνυμία, meaning "without a Name " or "namelessness" ^[4] In this story, the narrator is waiting under a tree to be picked up by a coach which will take him out of Newcastle, when he suddenly finds himself transported back over a thousand years, where he encounters the Venerable Bede in a monastery, and gives him somewhat ironic explanations of the developments of the coming centuries. For other meanings see Stagecoach (disambiguation. A stagecoach (also called diligence) is a type of four-wheeled enclosed Newcastle upon Tyne ( (often shortened to Newcastle) is a city and Metropolitan borough of Tyne and Wear, England Bede (ˈbiːd (also Saint Bede, the Venerable Bede, or (from Latin Beda (beda (c This article concerns the buildings occupied by monastics. For the life inside monasteries and its historical roots see Monasticism. It is never entirely clear whether these events actually occurred or were merely a dream — the narrator says that when he initially found a comfortable-looking spot in the roots of the tree, he sat down, "and as my sceptical reader will tell me, nodded and slept", but then says that he is "resolved not to admit" this explanation. A number of dreamlike elements of the story may suggest otherwise to the reader, such as the fact that none of the members of the monastery seem to be able to see him at first, and the abrupt ending where Bede has been delayed talking to the narrator and so the other monks burst in thinking that some harm has come to him, and suddenly the narrator finds himself back under the tree in the present (August of 1837), with his coach having just passed his spot on the road, leaving him stranded in Newcastle for another night. ^[5]

Charles Dickens' 1843 book A Christmas Carol is considered by some^[6] to be one of the first depictions of time travel, as the main character, Ebenezer Scrooge, is transported to Christmases past, present and yet to come. A Christmas Carol in Prose Being a Ghost Story of Christmas (commonly known as A Christmas Carol) is a Novella by Charles Dickens These might be considered mere visions rather than actual time travel, though, since Scrooge only viewed each time period passively, unable to interact with them.

A clearer example of time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men), published posthumously by the French botanist and geologist Pierre Boitard. Pierre Boitard (1789 &ndash 1859 was a French Botanist and Geologist. In this story the main character is transported into the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters such extinct animals as a Plesiosaur, as well as Boitard's imagined version of an apelike human ancestor, and is able to actively interact with some of them. Plesiosaurs (ˈpliːziəˌsɔɹ ( Greek: plesios/πλησιος meaning 'near' or 'close to' and sauros/σαυρος meaning ^[7] Another clear early example of time travel in fiction is the short story The Clock That Went BackwardPDF (35. 7 KB) by Edward Page Mitchell, which appeared in the New York Sun in 1881. Edward Page Mitchell (b Bath Maine, March 24, 1852 - d New London Connecticut, 1927 was an American editorial and Short story The Sun was a New York newspaper that was published from 1833 until 1950 Mark Twain's A Connecticut Yankee in King Arthur's Court (1889), in which the protagonist finds himself in the time of King Arthur after a fight in which he is hit with a sledge hammer, was another early time travel story which helped bring the concept to a wide audience, and was also one of the first stories to show history being changed by the time traveler's actions. Samuel Langhorne Clemens (November 30 1835 – April 21 1910 better known by the Pen name Mark Twain, was an American Humorist, satirist A Connecticut Yankee in King Arthur's Court is an 1889 Novel by American Humorist and Writer Mark Twain. King Arthur is a legendary British leader who according to medieval histories and romances, led the defence of Britain against the Saxon invaders

The first time travel story to feature time travel by means of a time machine was Enrique Gaspar y Rimbau's 1887 book El Anacronópete. Enrique Lucio Eugenio Gaspar y Rimbau ( Madrid, March 2, 1842 – Olorón, September 7, 1902) was a Spanish ^[8] This idea gained popularity with the H. G. Wells story The Time Machine, published in 1895 (preceded by a less influential story of time travel Wells wrote in 1888, titled The Chronic Argonauts), which also featured a time machine and which is often seen as an inspiration for all later science fiction stories featuring time travel. Herbert George Wells (21 September 1866 &ndash 13 August 1946 He was an outspoken socialist and a pacifist, his later works becoming increasingly political The Time Machine is a novella by H G Wells, first published in 1895 and later directly adapted into at least two Feature films of the same name as " The Chronic Argonauts " is a Short story written by H

Since that time, both science and fiction (see Time travel in fiction) have expanded on the concept of time travel, but whether it could be possible in reality is still an open question. Time travel is a common theme in Science fiction and is depicted in a variety of media

Time travel in theory

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, might allow time travel into the past and future if these geometries or motions are possible. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS Space is the extent within which Matter is physically extended and objects and Events have positions relative to one another ^[9] In technical papers physicists generally avoid the commonplace language of "moving" or "traveling" through time ('movement' normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of closed timelike curves, which are worldlines that form closed loops in spacetime, allowing objects to return to their own past. In a Lorentzian manifold, a closed timelike curve (CTC is a Worldline of a material particle in Spacetime that is "closed" returning to its In physics the world line of an object is the unique path of that object as it travels through 4- Dimensional Spacetime. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, but the physical plausibility of these solutions is uncertain.

Physicists take for granted that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (although according to relativity there is no single objective answer to how much time has 'really' passed between the departure and the return). Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that issues of causality be resolved. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, either by appealing to the Novikov self-consistency principle or to the notion of branching parallel universes (see the possibility of paradoxes below). This article refers to the Time travel paradox In novels based on the television series Doctor Who, "Grandfather Paradox" is the semi-mythical The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a Principle developed by Dr The multiverse (or meta-universe) is the hypothetical set of multiple possible Universes (including our universe that together comprise all of Reality. This article details time travel itself For other uses see Time Traveler.

Tourism in time

Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel—a variant of the Fermi paradox. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. Tourism is Travel for Recreational or Leisure purposes The World Tourism Organization defines tourists as people who "travel The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for Of course this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never in fact developed (or was cautiously never used); and even if it is developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the right way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so "This picture would explain why we haven't been over run by tourists from the future. "^[10] Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers. Carl Edward Sagan ( November 9 1934 &ndash December 20 1996) was an American Astronomer, astrochemist, author ^[11]

General relativity

However, the theory of general relativity does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Semiclassical gravity is the approximation to the theory of Quantum gravity in which one treats Matter fields as being quantum and the gravitational field Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons ^[12] These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,^[13] but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory. The chronology protection conjecture is a Conjecture by the Physicist Professor Stephen Hawking that the Laws of physics are such as to prevent Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature ^[14]

Time travel to the past in physics

Time travel to the past is theoretically allowed using the following methods:^[15]

• Space traveling faster than the speed of light
• The use of cosmic strings and black holes
• Wormholes and Alcubierre 'warp' drive

The equivalence of time travel and faster-than-light travel

If one were able to move information or matter from one point to another faster than light, then according to special relativity, there would be some inertial frame of reference in which the signal or object was moving backwards in time. A cosmic string is a hypothetical 1-dimensional (spatially Topological defect in various fields A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time The Alcubierre metric, also known as the Alcubierre drive or Warp Drive, is a speculative mathematical model of a spacetime exhibiting features reminiscent of Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest This is a consequence of the relativity of simultaneity in special relativity, which says that in some cases different reference frames will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events (technically, these disagreements occur when spacetime interval between the events is 'space-like', meaning that neither event lies in the future light cone of the other). The relativity of simultaneity is the concept that simultaneity is not absolute but dependent on the observer SpaceTime is a patent-pending three dimensional graphical user interface that allows end users to search their content such as Google Google Images Yahoo! YouTube eBay Amazon and RSS In Special relativity, a light cone (or null cone) is the pattern describing the temporal evolution of a flash of Light in Minkowski spacetime ^[16] If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event. ^[16] However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backwards in time. And since one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backwards in time in any one frame, it must be possible in all frames. See also Special relativity Postulates of special relativity 1 This means that if observer A sends a signal to observer B which moves FTL (faster than light) in A's frame but backwards in time in B's frame, and then B sends a reply which moves FTL in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality in every frame. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in An illustration of such a scenario using spacetime diagrams can be found here. The Minkowski diagram was developed in 1908 by Herman Minkowski and provides an illustration of the properties of space and time in the Special theory of relativity

It should be noted that according to special relativity it would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light, and although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory it seems that it would not actually be possible to use them to transmit information faster than light,^[17] and there is no evidence for their existence. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial A tachyon (from the Greek, takhyónion, from, takhýs, ie swift fast is any hypothetical particle that travels at Faster-than-light In quantum field theory (QFT the forces between particles are mediated by other particles

Special spacetime geometries

The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity describes the universe under a 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 Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the In a Lorentzian manifold, a closed timelike curve (CTC is a Worldline of a material particle in Spacetime that is "closed" returning to its ^[9]The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have. Kurt Gödel (kʊɐ̯t ˈgøːdl̩ (April 28 1906 – January 14 1978 was an Austrian American Logician, Mathematician and Philosopher The Gödel metric is an exact solution of the Einstein field equations in which the Stress-energy tensor contains two terms the first representing the ^[9] Whether general relativity forbids closed time-like curves for all realistic conditions is unknown .

Using wormholes

Main article: Wormhole

Wormholes are a hypothetical warped spacetime which are also permitted by the Einstein field equations of general relativity,^[18] although it would be impossible to travel through a wormhole unless it was what is known as a traversable wormhole. In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time The Einstein field equations ( EFE) or Einstein's equations are a set of ten equations in Einstein 's theory of General relativity in which the In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time

A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less than the stationary end, as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around. This article discusses a concept in physics For the concept in sociology see Time displacement. ^[19] This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, a trip backwards in time as seen by other observers outside. 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;^[20] 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. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance known as "exotic matter" with negative energy. Exotic matter is a hypothetical concept of Particle physics. It covers any material which violates one or more classical conditions or is not made of known baryonic particles More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions. In relativistic classical field theories of Gravitation, particularly General relativity, an energy condition is one of various alternative conditions ^[21] However, it is known that quantum effects can lead to small measurable violations of the null energy condition,^[21] and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics. In Physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. ^[22] Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small. ^[23]

In 1993, Matt Visser argued 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. Matt Visser is a Mathematics Professor at Victoria University of Wellington. ^[24] Because of this, the two mouths could not be brought close enough for causality violation to take place. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in However, in a 1997 paper, Visser hypothesized 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 a flaw in classical quantum gravity theory rather than proof that causality violation is possible. A Roman ring, in theoretical Physics, is a configuration of Wormholes where for each individual wormhole the Time difference across its mouths is such that ^[25]

Other approaches based on general relativity

Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum^[26] in 1936 and Kornel Lanczos^[27] in 1924, but not recognized as allowing closed timelike curves^[28] until an analysis by Frank Tipler^[29] in 1974. A Tipler cylinder, also called a Tipler time machine, is a Hypothetical object theorized to be a potential mode of Time travel &mdashan Willem Jacob van Stockum ( November 20 1910 - June 10 1944) was a physicist who made an important contribution to the early development of Cornelius Lanczos ( Lánczos Kornél) (approximate pronunciation 'LAHNT-sawsh') born Löwy Kornél ( February 2, 1893 Frank Jennings Tipler III (born February 1, 1947 in Andalusia Alabama Prof 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. A cosmic string is a hypothetical 1-dimensional (spatially Topological defect in various fields

Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. This is about the physicist and science fiction writer You may be looking for his son Robert D 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. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges 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.

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine in any finite region that satisfies the weak energy condition, meaning that the region contains no exotic matter with negative energy. In relativistic classical field theories of Gravitation, particularly General relativity, an energy condition is one of various alternative conditions Exotic matter is a hypothetical concept of Particle physics. It covers any material which violates one or more classical conditions or is not made of known baryonic particles Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough,^[30] he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy. "^[31] This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "[t]here will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. The chronology protection conjecture is a Conjecture by the Physicist Professor Stephen Hawking that the Laws of physics are such as to prevent In Physics, a Cauchy horizon is a Light-like boundary of the domain of validity of a Cauchy problem (a particular Boundary value problem of the One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon. "^[32] However, this theorem does not rule out the possibility of time travel in regions which contain exotic matter with negative energy (which would be necessary for traversable wormholes or the Alcubierre drive), and because the theorem is based on general relativity, it is also conceivable a future theory of quantum gravity which replaced general relativity would allow time travel even without exotic matter (though it is also possible such a theory would place even more restrictions on time travel, or rule it out completely). The Alcubierre metric, also known as the Alcubierre drive or Warp Drive, is a speculative mathematical model of a spacetime exhibiting features reminiscent of

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 might be due to the anthropic principle. Max Tegmark (born 5 May 1967) is a Swedish - American cosmologist. Causality describes the relationship between Causes and Effects is fundamental to all natural Science, especially Physics, and has a basis in In Physics and Cosmology, the anthropic principle states that humans should take into account the constraints that human existence imposes on the kind of theoretical 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 (at least, not if the time travel occurs in such a way that it disrupts that evolutionary process).

Experiments carried out

Certain experiments carried out during the last ten years give the impression of reversed causality but are interpreted in a different way by the scientific community. Causality (but not causation) denotes a necessary relationship between one event (called cause and another event (called effect) which is the direct consequence For example, in the delayed choice quantum eraser experiment performed by Marlan Scully, pairs of entangled photons are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double slit experiment, and depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. A delayed choice quantum eraser is a cross between a Quantum eraser experiment and Wheeler's delayed choice experiment. Marlan Orvil Scully is a physicist best known for his work in theoretical quantum optics Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, and under most interpretations of quantum mechanics the results can be explained in a way that does not violate causality.

The experiment of Lijun Wang might also give the appearance of causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. But a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backwards in time even if none of the pure waves in the sum do so. In mathematics Fourier analysis is a subject area which grew out of the study of Fourier series This effect cannot be used to send any matter, energy, or information backwards in time, so this experiment is understood not to violate causality either.

The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. Günter Nimtz (1936 is a German physicist Working at the 2nd Physics Institute at the University of Cologne ( Universität zu Köln) in Germany he has been conducting They say they have conducted an experiment in which microwave photons - energetic packets of light - traveled "instantaneously" between a pair of prisms that had been moved up to 3 ft (0. 91 m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of. " However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the center of the train exceeds the speed of any of the individual cars. ^[33]

Some physicists have attempted to perform experiments which would show genuine causality violations, but so far without success. The Space-time Twisting by Light (STL) experiment run by physicist Ronald Mallett is attempting to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal. Ronald L Mallett, PhD is a Professor of Physics in the University of Connecticut. Photonic crystals are periodic Optical (nanostructures that are designed to affect the motion of Photons in a similar way that periodicity of a Semiconductor Mallett has some physical arguments which suggest that closed timelike curves would become possible through the center of a laser which has been twisted into a loop. However, other physicists dispute his arguments (see objections). Ronald L Mallett, PhD is a Professor of Physics in the University of Connecticut.

Non-physics based experiments

Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day or MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. The traditions and student activities at the Massachusetts Institute of Technology encompasses hundreds of student activities organizations and athletics that contribute to MIT's distinct These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far—no time travelers are known to have attended either event. Although it is theoretically possible that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe. The multiverse (or meta-universe) is the hypothetical set of multiple possible Universes (including our universe that together comprise all of Reality. ^[34] Another factor is that for all the time travel devices considered under current physics (such as those that operate using wormholes), it is impossible to travel back to before the time machine was actually made. In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time ^[35][36]

Time travel to the future in physics

There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth. In physics the twin paradox is a thought experiment in Special Relativity, in which a person who makes a journey into space in a high-speed rocket will return home to find he For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on your choice of reference frame. See also Inertial frame A frame of reference in Physics, may refer to a Coordinate system or set of axes within which to

This form of "travel into the future" is theoretically allowed using the following methods:^[15]

• Using time dilation under the Theory of Special Relativity, for instance:
  • Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth^[37] (see the Twin paradox)
• Using time dilation under the Theory of General Relativity, for instance:
  • Residing inside of a hollow, high-mass object;
  • Residing just outside of the event horizon of a black hole, or on the surface of a larger-than-earth mass object. Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 In physics the twin paradox is a thought experiment in Special Relativity, in which a person who makes a journey into space in a high-speed rocket will return home to find he General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e

Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":

• Hibernation
• Suspended animation

Time dilation

Main article: Time dilation

Time dilation is permitted by Albert Einstein's special and general theories of relativity. For the ability of certain operating systems see Hibernate (OS feature Hibernation is a state of inactivity and metabolic depression Suspended animation is the slowing of life processes by external means without termination This article discusses a concept in physics For the concept in sociology see Time displacement. This article discusses a concept in physics For the concept in sociology see Time displacement. Albert Einstein ( German: ˈalbɐt ˈaɪ̯nʃtaɪ̯n; English: ˈælbɝt ˈaɪnstaɪn (14 March 1879 – 18 April 1955 was a German -born theoretical Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well. In Physics, a gravity well is the Gravitational potential field around a massive body (a particular kind of Potential well) ^[38] For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. In Special relativity the rest frame of a particle is the coordinate system ( Frame of reference) in which the particle is at rest For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less. In physics the twin paradox is a thought experiment in Special Relativity, in which a person who makes a journey into space in a high-speed rocket will return home to find he General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a black hole. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e

It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing at the bottom of a 5 meter tall funnel with the mass of Jupiter. ^[15] For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into a non-spherical object five meters in length is not expected to be within our technological capabilities in the near future.

Time perception

Time perception can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. For the ability of certain operating systems see Hibernate (OS feature Hibernation is a state of inactivity and metabolic depression Thermoregulation is the ability of an Organism to keep its body temperature within certain boundaries even when temperature surrounding is very different Metabolism is the set of Chemical reactions that occur in living Organisms in order to maintain Life. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced. Suspended animation is the slowing of life processes by external means without termination

Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and arguably should not be considered time travel. Causality (but not causation) denotes a necessary relationship between one event (called cause and another event (called effect) which is the direct consequence

Other ideas about time travel from mainstream physics

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. The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a Principle developed by Dr Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist, known for his prolific contributions in gravitation physics 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 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. Hugh Everett III ( November 11, 1930 – July 19, 1982) was an American physicist who first proposed the Many-worlds interpretation The many-worlds interpretation or MWI (also known as relative state formulation, theory of the universal wavefunction, parallel universes, ^[39] These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that if time travel is possible and the many-worlds interpretation is correct, then a time traveler should indeed end up in a different history than the one he started from. David Elieser Deutsch FRS (born 1953 in Haifa, Israel) is a Physicist at the University of Oxford. ^[1] On the other hand, Stephen Hawking has argued that even if the many-worlds interpretation is correct, we should expect each time traveler to experience a single self-consistent timeline, so that time travelers remain within their own world rather than traveling to a different one. Stephen William Hawking CH, CBE, FRS, FRSA (born 8 January 1942 is a British theoretical physicist. ^[10]

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel without paradoxes. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons ^[40] In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

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 presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. Quantum teleportation, or entanglement-assisted teleportation, is a technique used to transfer information on a Quantum level usually from one Particle In Quantum mechanics, the EPR paradox is a Thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities Quantum entanglement is a quantum mechanical Phenomenon in which the Quantum states of two or more objects are linked together so that one object The Bohm interpretation of Quantum mechanics, sometimes called Bohmian mechanics, the ontological interpretation, or the causal interpretation ^[41] This effect was referred to as "spooky action at a distance" by Einstein. In Physics, action at a distance is the Interaction of two objects which are separated in Space with no known mediator of the interaction

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In quantum field theory (QFT the forces between particles are mediated by other particles Superluminal communication is the term used to describe the hypothetical process by which one might send Information at Faster-than-light (FTL speeds In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. In Quantum information theory, a no-communication theorem is a result which gives conditions under which instantaneous transfer of information between two observers is impossible The fact that these quantum phenomena apparently do not allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments. How the rules of quantum mechanics work to preserve causality is an active area of research.

Philosophic understandings of time travel

The relativity of simultaneity in modern physics favors the philosophic view known as four dimensionalism in which past, present and future events all coexist in a single spacetime. The relativity of simultaneity is the concept that simultaneity is not absolute but dependent on the observer Philosophy is the study of general problems concerning matters such as existence knowledge truth beauty justice validity mind and language In the philosophy of time, four dimensionalism is a term sometimes used to refer to the view that the past present and future are all equally real and that (tenselessly Block Universe theory, or eternalism, builds on a standard method of modeling time as a dimension in physics, to give time a similar ontology to that of space. Eternalism is a philosophical approach to the ontological nature of time. This would mean that time is just another dimension, that future events are "already there", and that there is no objective flow of time.

Presentism is a school of philosophy that holds that neither the future nor the past exist, and there are no non-present objects. In the Philosophy of time, presentism is the Belief that only the present exists and the Future and the Past are unreal The future is commonly understood to contain all events that have yet to occur PAST (short for Polska Akcyjna Spółka Telefoniczna, Polish Telephone Joint-stock Company) was a Polish telephone operator in the period between World War I In this view, time travel is impossible because there is no future or past to travel to. However, some 21st century presentists have argued that although past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to appear in the present could explain the time traveler's actual existence in the present. ^[42][43]

Ideas from fiction

Further information: 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. Time travel is a common theme in Science fiction and is depicted in a variety of media These 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. The Morphail Effect is a fictitious notion about Time travel from Michael Moorcock 's Science fiction series The Dancers at the End This concept of time can be referred to as circular causation. For examples of circular causation, see Robert A. Heinlein's story By His Bootstraps. Robert Anson Heinlein (July 7 1907 – May 8 1988 was an American Novelist and Science fiction Writer. " By His Bootstraps " is a Science fiction Short story by Robert A

1. 2 The Novikov self-consistency principle applies (named after Dr. The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a Principle developed by Dr Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University). Igor Dmitriyevich Novikov (И́горь Дми́триевич Но́виков (born November 10, 1935 in Moscow) is a Russian (and former The University of Copenhagen (Københavns Universitet is the oldest and largest University and research institution in Denmark. 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. It has been suggested that travel to the past would create an entire new parallel universe where the traveler would be free from paradoxes since he/she is not from that universe^[44].

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 might 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. Jesus of Nazareth (7–2 BC / BCE —26–36 AD / CE) 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. (Example: The Saga of Darren Shan, where major events in the past cannot be changed, but minor events can be affected. The Saga of Darren Shan (known as Cirque Du Freak The Saga of Darren Shan in the US is a young adult 12 book series written by Darren Shan about the struggle This is explained as if you went back in time and killed Hitler, another Nazi would take his place and commit his same actions. Hi and welcome to Wikipedia! Please understand that this article is frequently vandalized and vandalism is reverted immediately Nazism, which was a short name for National Socialism (Nationalsozialismus refers primarily to the Ideology and practices of the National Socialist German )

2. 2 The time line is easily changed. (Example: Doctor Who, where the time line is fluid and changes often naturally; even changes to the traveler's own timeline are possible, though it is suggested such an act would destroy most of the universe. Doctor Who is a British science fiction television programme produced by the BBC. )

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. The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a Principle developed by Dr 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 traveler (e. This article refers to the Time travel paradox In novels based on the television series Doctor Who, "Grandfather Paradox" is the semi-mythical 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. A paradox is a true statement or group of statements that leads to a Contradiction or a situation which defies intuition; or inversely 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. Michael John Moorcock (born 18 December 1939, in London) is an English writer primarily of Science fiction and fantasy who has also 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. Constance Elaine Trimmer Willis (born 31 December 1945) is an American Science fiction writer

Example: A man who travels into the past with intentions to kill Hitler finds himself on a Montana farm in late April 1945.

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). Jeannot Szwarc (born 21 November, 1939 in Paris) is a French Film director. Somewhere in Time is a 1980 Time travel Romance film directed by Jeannot Szwarc, written by Richard Matheson Richard Matheson (born February 20, 1926) is an American Author and Screenwriter, typically of fantasy, horror Bid Time Return is a 1975 Science fiction novel by Richard Matheson.

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 went back in time with Hermione to change history. Hermione Jean Granger 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. A predestination paradox, also called either a causal loop, or a causality loop and (less frequently either a closed loop or closed time loop 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 traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated 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 and The Time Ships by Stephen Baxter, plus several episodes of the TV show Star Trek: The Next Generation. David Gerrold, born Jerrold David Friedman ( January 24, 1944) in Chicago Illinois, is a Science fiction author who started his career The Man Who Folded Himself is a 1973 Science fiction Novel by David Gerrold that deals with Time travel. The Time Ships is a 1995 Science fiction novel by Stephen Baxter. Stephen Baxter (born 13 November 1957 is a British Hard science fiction Author.

Mutable timelines

Time travel in a Type 2 universe is much more complex. 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. Thrice Upon A Time is a Science fiction Novel by James P Hogan, first published in 1980. James Patrick Hogan (born June 27, 1941) is a British Science fiction Author. The Back to the Future trilogy films also seem to feature a single mutable timeline (see the Back to the Future FAQ for details on how the writers imagined time travel worked in the movies' world). The Back to the Future trilogy is a comedic Science fiction Film Trilogy written by Bob Gale and Robert Zemeckis By contrast, the short story Brooklyn Project by William Tenn provides a sketch of life in a Type 2 world where no one even notices as the timeline changes repeatedly. William Tenn is the Pseudonym for the Science fiction work of Philip Klass.

In type 2. 1, attempts are being made at changing the timeline, however, all that is accomplished in the first tries is that the way of how decisive events happen is changed; final conclusions in the bigger scheme cannot be brought to a different outcome. Example: In the Movie Deja Vu a paper note is being sent to the past with vital information to prevent the main plot incident. Déjà Vu is a Science fiction Crime thriller directed by Tony Scott, produced by Jerry Bruckheimer, and starring All that happens, though, is that an ATF agent gets killed, with the final disaster still not being prevented; also, the very same agent died in the previous version of the timeline as well, albeit under different circumstances. Finally though, the timeline is changed (Claire Kuchever is being saved from murder) by sending a human back into the past in order to prevent the murder of Claire and the main incident (a terrorist attack), which is arguably a "stronger" measure than simply sending back a paper note.

Similar to the Back to the Future movie trilogy, there seems to be a ripple effect (changes from the past 'propagate' into the present, and people in the present have altered memory of events occurred after the changes made to the timeline)

The type of timetravel in Deja Vu fits the 2. The Back to the Future trilogy is a comedic Science fiction Film Trilogy written by Bob Gale and Robert Zemeckis The ripple effect is an education-related term associated with the studies of Jacob Kounin. 1 Type very well: Sending the paper note seems to be too "weak" a measure to cause any permanent effect, but agent Carlin going back into the past has a final decisive impact.

The science fiction writer Larry Niven suggests in his essay The Theory and Practice of Time Travel that in a type 2. Laurence van Cott Niven (born April 30, 1938 Los Angeles California) is a US Science fiction author. 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 might 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 Dilution Effect. "

Few if any physicists or philosophers have taken seriously the possibility of "changing" the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent,^[45] so the mutable timeline idea is rarely considered outside of science fiction.

Also, deciding whether a given universe is of Type 2. 1 or 2. 2 can not be done objectively, as the categorization of timeline-invasive measures as "strong" or "weak" is arbitrary, and up to interpretation: An observer can disagree about a measure being "weak", and might, in the lack of context, argue instead that simply a mishap occurred which then led to no effective change.

An example would be the papernote sent back to the past in the film Deja Vu, as described above: Was it a too "weak" change, or was it after all just (time-local; that is, in the past) bad circumstance which made it have no effect, but it might have worked if the paper note would have been sent back 1 hour earlier, or 1 hour later into the past? As the universe in Deja Vu seems to be not entirely self-preserving from paradoxes (some, arguably minute, paradoxes, do occur), both versions seem to be equally probable, to which the film gives no further clarification.

Gradual and instantaneous

In literature, there are two 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, though in most cases, there is a machine of some sort, and some energy expended in order to make this happen (Like the time-traveling De Lorean in Back to the Future or the phonebooth which traveled through the 'circuits of history' in Bill and Ted's Excellent Adventure). A Compact Disc player (often written as compact disc player) or CD player, is an electronic device which plays audio Compact Discs CD players are often Back to the Future is a 1985 science fiction Comedy film directed by Robert Zemeckis and produced by Steven Spielberg. A telephone booth, telephone kiosk, (or telephone box in the British Isles) is a small structure furnished with a Payphone and designed Bill & Ted's Excellent Adventure ( 1989) is an American comedy / Science fiction Movie in which two Slackers In some cases, 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. The "Universal Remote" used by Adam Sandler in the movie Click works in the same manner, although only in one direction, the future. Adam Richard Sandler (born September 9 1966 is an American Comedian, Click is an Academy Award-nominated 2006 comedy/drama / Science fiction / fantasy Film directed by Frank While his character Michael Newman can travel back to a previous point it is merely a playback which he cannot interact with.

2. In The Time Machine, H.G. Wells explains that we are moving through time with a constant speed. The Time Machine is a novella by H G Wells, first published in 1895 and later directly adapted into at least two Feature films of the same name as Herbert George Wells (21 September 1866 &ndash 13 August 1946 He was an outspoken socialist and a pacifist, his later works becoming increasingly political 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 is not as 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. Charles Lutwidge Dodgson (ˈdɒdsən (27 January 1832 &ndash 14 January 1898 better known by the Pen name Lewis Carroll (/ˈkærəl/ was an English Through the Looking-Glass and What Alice Found There ( 1871) is a work of Children's literature by Lewis Carroll (Charles Lutwidge Dodgson Her kind of time travel is uncontrolled: she moves through time with a constant speed of -1 and she cannot change it. 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. Terence Hanbury White ( 29 May 1906 &ndash 17 January 1964) was an English Author best known for his sequence of Arthurian The Once and Future King is an Arthurian Fantasy novel written by T The Sword in the Stone is a novel by T H White, published in 1938, initially a stand-alone work but now the first part of a tetralogy The Once "Some people call it having second sight", he says.

Time travel, or space-time 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, according to the theory of relativity, this argument is based on a false premise. This page is about the scientific concept of relativity for philosophical or sociological theories about relativity see Relativism. Relativity rejects the idea of absolute time and space; there can be no universal truth about the spatial distance between events which occurred at different times^[46] (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 Physics, the concept of absolute time and absolute space are Hypothetical models in which time either runs at the same rate for all the observers in 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". Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial In Physics, an inertial frame of reference is a Frame of reference which belongs to a set of frames in which Physical laws hold in the same and simplest 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"^[47] (also called general covariance). General relativity or the general theory of relativity is the geometric theory of Gravitation published by Albert Einstein in 1916 In Theoretical physics, general covariance (also known as Diffeomorphism covariance or general invariance) is the Invariance of the

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-materialize in space. 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. The Door into Summer is a Science fiction Novel by Robert A Heinlein, originally serialized in The Magazine of Fantasy & Science The term handwaving is an informal term that describes either the Debate technique of failing to Rigorously address an Argument in an attempt to bypass the In physics the world line of an object is the unique path of that object as it travels through 4- Dimensional Spacetime. " 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 Number of the Beast is a Science fiction Novel by Robert A 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. Seven Days (also written as 7 Days) is a Science fiction television series based around the premise of Time travel. 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. Piers Anthony Dillingham Jacob (born August 6, 1934 in Oxford England) is an English American writer in the Science fiction Incarnations of Immortality is the name of an eight-book Fantasy series by Piers Anthony. This article contains brief biographies for major characters from Piers Anthony 's Incarnations of Immortality series ; 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 cleverly 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. Doctor Who is a British science fiction television programme produced by the BBC. The TARDIS ( T ime A nd R elative D imension(s I n S pace is a time machine and Spacecraft in the

See also

Speculations

• Grandfather paradox
• Predestination paradox
• Temporal paradox
• Tipler Cylinder
• Ronald Mallett

Claims of time travel

• Philadelphia Experiment
• Chronovisor
• Billy Meier
• Darren Daulton
• John Titor
• Moberly-Jourdain incident
• Montauk Project
• Time slip

Fiction, humor

• Andrew Carlssin
• Time travel in fiction
• Thiotimoline
• Time loop
• Chronodynamics

References

Further reading

• Davies, Paul (1996). Paul Charles William Davies (born April 22, 1946) is a British-born Physicist, Writer and broadcaster, presently a professor About Time. Pocket Books. ISBN 0-684-81822-1.  
• Davies, Paul (2002). Paul Charles William Davies (born April 22, 1946) is a British-born Physicist, Writer and broadcaster, presently a professor How to Build a Time Machine. Penguin Books Ltd. ISBN 0-14-100534-3.  
• Gale, Richard M (1968). The Philosophy of Time. Palgrave Macmillan. ISBN 0-333-00042-0.  
• Gott, J. Richard. Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time. ISBN 0-618-25735-7.  
• Gribbin, John (1985). John R Gribbin (born 1946 is a British science writer and a visiting Fellow in astronomy at the University of Sussex. In Search of Schrödinger's Cat. Corgi Adult. ISBN 0-552-12555-5.  
• Miller, Kristie (2005). "Time travel and the open future". Disputatio 1 (19): 223-232.  
• Nahin, Paul J. (2001). Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction. Springer-Verlag New York Inc. . ISBN 0-387-98571-9.  
• Nikolic, H. "Causal paradoxes: a conflict between relativity and the arrow of time".   arXiv:gr-qc/0403121
• Pagels, Heinz (1985). The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic Heinz Rudolf Pagels ( February 19, 1939 &ndash July 23, 1988) was an American Physicist, an adjunct professor of physics Perfect Symmetry, the Search for the Beginning of Time. Simon & Schuster. ISBN 0-671-46548-1.  
• Pickover, Clifford (1999). Clifford A Pickover is an American author editor and columnist in the fields of Science, Mathematics, and Science fiction, and is employed at the Time: A Traveler's Guide. Oxford University Press Inc, USA. ISBN 0-19-513096-0.  
• Randles, Jenny (2005). "Oz Factor" redirects here For the album by Unwritten Law see Oz Factor (album. Breaking the Time Barrier. Simon & Schuster Ltd. ISBN 0-7434-9259-5.  
• Shore, Graham M. "Constructing Time Machines". Int. J. Mod. Phys. A, Theoretical.   arXiv:gr-qc/0210048
• Toomey, David (2007). The arXiv ( pronounced " Archive " as if the "X" were the Greek letter Chi, χ is an Archive for electronic The New Time Travelers: A Journey to the Frontiers of Physics. W. W. Norton & Company. ISBN 978-0-393-06013-3.  

External links

• Black holes, Wormholes and Time Travel Freeview Lecture. A Royal Society Lecture by Paul Davies provided by the Vega Science Trust
• SF Chronophysics, a discussion of Time Travel as it relates to science fiction
• On the Net: Time Travel by James Patrick Kelly in Asimov's Science Fiction
• Howstuffworks' article on "How Time Travel Will Work"
• Time Travel in Flatland?
• NOVA Online: Time Travel
• Professor Predicts Human Time Travel This Century Ronald Mallett, Professor at the University of Connecticut, has used Einstein’s equations to design an experiment to observe a time traveling neutron in a circulating light beam. Paul Charles William Davies (born April 22, 1946) is a British-born Physicist, Writer and broadcaster, presently a professor Ronald L Mallett, PhD is a Professor of Physics in the University of Connecticut. He published his research in Physics Letters.
• Through The Looking Glass: Time-Travel in Brane Theory An interview with a University of Hawaii research team seeking reverse-time communications using sterile neutrinos
• Time Traveler Convention, at MIT - "Technically, you would only need one. . . "
• Time Machines in Physics - almost 200 citations from 1937 through 2001
• Stanford Encyclopedia of Philosophy:
  • Time Machines
  • Time Travel and Modern Physics
• Internet Encyclopedia of Philosophy:
  • Time
  • Time Travel
• Aparta Krystian. Conventional Models of Time and Their Extensions in Science Fiction A master's thesis exploring conceptual blending in time travel. Conceptual Blending (aka Conceptual Integration) is a general theory of Cognition.
• Time travellers from the future 'could be here in weeks' Two mathematicians suggest that the Large Hadron Collider might create tiny wormholes that could allow time travel

In Physics, a wormhole is a hypothetical topological feature of Spacetime that is fundamentally a 'shortcut' through Space and Time

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